Copernicus: Sentinel-3 — Global Sea/Land Monitoring Mission including Altimetry
The Sentinel-3 (S3) mission of ESA and the EC is one of the elements of the GMES (Global Monitoring for Environment and Security) program, which responds to the requirements for operational and near-real-time monitoring of ocean, land and ice surfaces over a period of 20 years. The topography element of this mission will serve primarily the marine operational users but will also allow the monitoring of sea ice and land ice, as well as inland water surfaces, using novel observation techniques.The Sentinel-3 mission is designed as a constellation of two identical polar orbiting satellites, separated by 180º, for the provision of long-term operational marine and land monitoring services. The operational character of this mission implies a high level of availability of the data products and fast delivery time, which have been important design drivers for the mission. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)
The Sentinel-3 program represents a series of operational spacecraft over the envisioned service period to guarantee access to an uninterrupted flow of robust global data products.
Table 1: Copernicus is the new name of the former GMES program 15)
The main observation objectives of the mission are summarized in the following list:
• Ocean and land color observation data, free from sun-glint, shall have a revisit time of 4 days (2 days goal) and a quality at least equivalent to that of Meris instrument on Envisat. The actual revisit obtained over ocean at the equator (worst case) is less than 3.8 days with a single satellite and drops below 1.9 days with 2 satellites, phased 180° on the same orbital plane.
• Ocean and land surface temperature shall be acquired with at least the level of quality of AATSR on Envisat, and shall have a maximum revisit time of 4 days with dual view (high accuracy) observations and 1 day with single view. Achieved performance is shown to be significantly better, even with a single satellite (dual view: 3.5 days max, 1.8 days average).
• Surface topography observations shall primarily cover the global ocean and provide sea surface height (SSH) and significant wave height (SWH) to an accuracy and precision at least equivalent to that of RA-2 on Envisat. Additionally, Sentinel-3 shall provide surface elevation measurements -in continuity to CryoSat-2 - over ice regions covered by the selected orbit, as well as measurements of in-land water surfaces (rivers and lakes).
In addition, Sentinel-3 will provide surface vegetation products derived from synergistic and co-located measurements of optical instruments, similar to those obtained from the Vegetation instrument on SPOT, and with complete Earth coverage in 1 to 2 days.
The EU Marine Core Service (MCS) and the Land Monitoring Core Service (LMCS), together with the ESA GMES Service Element (GSE), have been consolidating those services where continuity and success depends on operational data flowing from the Sentinels.
The operational character of the mission implies a high level of availability of the data products and fast delivery time, which have been important design drivers for the mission.
Figure 1: Artist's rendition of the deployed Sentinel-3 spacecraft (image credit: ESA/ATG medialab) 16)
Legend to Figure 1: Sentinel-3 is arguably the most comprehensive of all the Sentinel missions for Europe’s Copernicus programme. Carrying a suite of state-of-the-art instruments, it provides systematic measurements of Earth’s oceans, land, ice and atmosphere to monitor and understand large-scale global dynamics and provide critical information for ocean and weather forecasting.
The Sentinel-3 spacecraft is being built by TAS-F (Thales Alenia Space-France). A contract to this effect was signed on April 14, 2008. The spacecraft is 3-axis stabilized, with nominal pointing towards the local normal and yaw steering to compensate for the Earth rotation affecting the optical observations. The spacecraft has a launch mass of about 1150 kg, the height dimension is about 3.9 m. The overall power consumption is 1100 W. The design life is 7.5 years, with ~100 kg of hydrazine propellant for 12 years of operations, including deorbiting at the end.
AOCS (Attitude and Orbit Control Subsystem): The spacecraft is 3-axis stabilized based on the new generation of avionics for the TAS-F LEO (Low Earth Orbit) platform. The AOCS software of the GMES/Sentinel-3 project is of PROBA program heritage. NGC Aerospace Ltd (NGC) of Sherbrooke, (Québec), Canada was responsible for the design, implementation and validation of the autonomous GNC (Guidance, Navigation and Control) algorithms implemented as part of the AOCS software of PROBA-1, PROBA-2, and PROBA-V. 17)
Table 2: Overview of Sentinel-3 spacecraft parameters
Data handling architecture: The requirements for the Sentinel-3 data handling architecture call for: a) minimized development risks, b) system at minimum cost, c) operational system over 20 years. This has led to design architecture as robust as possible using a single SMU (Satellite Management Unit) computer as the platform controller, a single PDHU (Payload Data-Handling Unit) for mission data management, and to reuse existing qualified heritage. 18)
The payload accommodates 6 instruments, sources of mission data. The 3 high rate instruments provide mission data directly collected through the SpaceWire network, while the low rate instruments are acquired by the central computer for distribution through the SpaceWire network to the mass memory. The PDHU acquires and stores all mission data for latter multiplexing, formatting, encryption and encoding for download to the ground.
The payload architecture is built-up over a SpaceWire network (Figure 2) for direct collection of high rate SLSTR, OLCI and SRAL instruments and indirect collection of low rate MWR, GNSS and DORIS instrument data plus house-keeping data through the Mil-Std-1553 bus by the SMU, all data being acquired from SpaceWire links and managed by the PDHU.
The mission data budget is easily accommodated thanks to the SpaceWire performance. Each SpaceWire link being dedicated to point-to-point communication without interaction on the other links (no routing), the frequency is set according to the need plus a significant margin. The PDHU is able to handle the 4 SpaceWire sources at up to 100 Mbit/s.
All mission data sources (OLCI, SLSTR, SRAL and SMU) provide data through two cold redundant interfaces and harnesses. The PDHU, being critical as the central point of the mission data management, implements a full cross-strapping between nominal and redundant sources interfaces and its nominal and redundant sides.
The PDHU SpaceWire interfaces are performed thanks to a specific FPGA, the instrument’s ones are based on the ESA Atmel SMCS-332, while the SMU interfaces are implemented by an EPICA ASIC circuit developed by Thales Alenia Space.
Figure 3: Schematic view of a full cross-strap redundancy within the PDHU (image credit: TAS-F, Ref. 18)
RF communications: The S-band is used for TT&C transmissions The S-band downlink rate is 123 kbit/s or 2 Mbit/s, the uplink data rate is 64 kbit/s. The X-band provide the payload data downlink at a rate of 520 Mbit/s. An onboard data storage capacity of 300 Gbit (EOL) is provided for payload data.
Four categories of data products will be delivered: ocean color, surface topography, surface temperature (land and sea) and land. The surface topography products will be delivered with three timeliness levels: NRT (Near-Real Time, 3 hours), STC (Standard Time Critical, 1-2 days) and NTC (Non-Time Critical, 1 month). Slower products allow more accurate processing and better quality. NRT products are ingested into numerical weather prediction and seastate prediction models for quick, short term forecasts. STC products are ingested into ocean models for accurate present state estimates and forecasts. NTC products are used in all high-precision climatological applications, such as sealevel estimates.
The resulting analysis and forecast products and predictions from ocean and atmosphere adding data from other missions and in situ observations, are the key products delivered to users. They provide a robust basis for downstream value-added products and specialized user services.
Introduction of new technology: A newly developed MEMS rate sensor (gyroscope), under the name of SiREUS, will be demonstrated on the AOCS of Sentinel-3. The gyros will be used for identifying satellite motion and also to place it into a preset attitude in association with optical sensors after its separation from the launcher, for Sun and Earth acquisition. Three of the devices will fly inside an integrated gyro unit, each measuring a different axis of motion, with a backup unit ensuring system redundancy. Each unit measures 11 cm x 11 cm x 7 cm, with an overall mass of 750 grams. 19)
The SiREUS device is of SiRRS-01 heritage, a single-axis rate sensor built by AIS (Atlantic Inertial Systems Ltd., UK), which is using a ’vibrating structure gyro’, with a silicon ring fixed to a silicon structure and set vibrating by a small electric current. The SiRRS-01 MEMS gyro has been used in the automobile industry. These devices are embedded throughout modern cars: MEMS accelerometers trigger airbags, MEMS pressure sensors check tires and MEMS gyros help to prevent brakes locking and maintain traction during skids. - In a special project, ESA selected the silicon-based SiRRS-01 to have it modified for space use (and under the new name of SiREUS).
Figure 4: Photo of the MEMS rate sensor (image credit: ESA)
Figure 5: Alternate view of the Sentinel-3 spacecraft and the accommodation of the payload (image credit: ESA)
Figure 6: Photo of the Sentinel-3A spacecraft in the cleanroom of Thales Alenia Space in Cannes, France with the solar wings attached (image credit: ESA, A. Le Floc’h) 20)
Status of project development:
• April 13, 2018: The team of propulsion experts has spent two days carrying out the tricky task of fuelling the Copernicus Sentinel-3B satellite with 130 kg of hydrazine and pressurizing the tank for its life in orbit. 21) 22)
- Since hydrazine is extremely toxic, only specialists remained in the cleanroom for the duration. A doctor and security staff waited nearby with an ambulance and fire engine ready to respond to any problems.
- The satellite is scheduled for liftoff on 25 April from Russia’s Plesetsk Cosmodrome at 17:57 GMT (19:57 CEST).
- In orbit it will join its identical twin, Sentinel-3A, which was launched in 2016. This pairing of satellites provides the best coverage and data delivery for Copernicus.
- Sentinel-3B is the seventh Sentinel satellite to be launched for Copernicus. Its launch will complete the constellation of the first set of Sentinel missions for Europe’s Copernicus program.
Figure 7: Fuelling of the Sentinel-3B spacecraft (image credit: Thales Alenia Space)
• March 23, 2018: With the Sentinel-3B satellite now at the Plesetsk launch site in Russia and liftoff set for 25 April, engineers are steaming ahead with the task of getting Europe’s next Copernicus satellite ready for its journey into orbit. 23)
- After arriving at the launch site on 18 March, the satellite has been taken out of its transport container and is being set up for testing. Kristof Gantois, ESA’s Sentinel-3 engineering manager, said, “The satellite’s journey from France was hampered slightly by the freezing winter weather here in Russia, but it’s now safe in the milder cleanroom environment.
- Sentinel-3B will join its twin, Sentinel-3A, in orbit. The pairing of identical satellites provides the best coverage and data delivery for Europe’s Copernicus program – the largest environmental monitoring program in the world.
Figure 8: Following its arrival at Russia’s Plesetsk launch site, the Copernicus Sentinel-3B satellite has been removed from its transport container. The satellite will now be prepared for liftoff, scheduled for 25 April 2018. Its identical twin, Sentinel-3A, has been in orbit since February 2016. The two-satellite constellation offers optimum global coverage and data delivery. The mission has been designed to measure systematically Earth’s oceans, land, ice and atmosphere to monitor and understand large-scale global dynamics. It will provide essential information in near-realtime for ocean and weather forecasting (image credit: ESA)
• February 2, 2018: After being put through its paces to make sure it is fit for life in orbit around Earth, the Copernicus Sentinel-3B satellite is ready to be packed up and shipped to Russia for liftoff. 24)
- Its twin, Sentinel-3A, has been in orbit since February 2016, systematically measuring our oceans, land, ice and atmosphere. The information feeds a range of practical applications and is used for monitoring and understanding large-scale global dynamics.
- The pairing of identical satellites provides the best coverage and data delivery for Europe’s Copernicus program – the largest environmental monitoring program in the world.
- Sentinel-3B has spent the last year at Thales Alenia Space’s premises in Cannes, France, being assembled and tested, and now it is fit and ready for its journey to the Plesetsk launch site in northern Russia.
- This included putting it in a vacuum chamber, exposing it to extreme temperatures, and we have also simulated the vibrations it will be subjected to during launch. - With liftoff expected to be confirmed for the end of April, the satellite will start its journey to Russia in March.
- Both Sentinel-3 satellites carry a suite of cutting-edge instruments to supply a new generation of data products, which are particularly useful for marine applications. For example, they monitor ocean-surface temperatures for ocean and weather forecasting services, aquatic biological productivity, ocean pollution and sea-level change. — Sentinel-3B also marks a milestone in Europe’s Copernicus program.
- With the Sentinel-1 and Sentinel-2 pairs already in orbit monitoring our environment, the launch of Sentinel-3B means that three mission constellations will be complete. In addition, Sentinel-5P, a single-satellite mission to monitor air pollution, has been in orbit since October 2017.
- While the Sentinel-1 and Sentinel-2 satellites circle Earth 180° apart, the configuration for Sentinel-3 will be slightly different: the 140° separation will help to measure ocean features such as eddies as accurately as possible.
- Prior to this, however, they will fly just 223 km apart, which means that Sentinel-3B will be a mere 30 seconds behind Sentinel-3A.
- Flying in tandem like this for around four months is designed to understand any subtle differences between the two sets of instruments – measurements should be almost the same given their brief separation.
- ESA’s ocean scientist, Craig Donlon, explains, “Our Sentinel-3 ocean climate record will eventually be derived from four satellites because we will be launching two further Sentinel-3s in the future.
- “We need to understand the small differences between each successive satellite instrument as these influence our ability to determine accurate climate trends. The Sentinel-3 tandem phase is a fantastic opportunity to do this and will provide results so that climate scientists can use all Sentinel-3 data with confidence.”
• December 5, 2017: EUMETSAT has confirmed the readiness of its teams and the new version of its ground segment to support the launch and commissioning of the Copernicus Sentinel-3B satellite in a two-satellite configuration with Sentinel-3A. 25)
- The new version of the ground segment includes enhancements and upgrades necessary to exploit a dual Sentinel-3 system. Its acceptance follows a comprehensive campaign of verification and validation tests.
- During the commissioning of Sentinel-3B, the two Sentinel-3 satellites will fly in close formation, 30 seconds apart. In this phase, ESA will manage Sentinel-3B flight operations, and EUMETSAT will be progressively ramping up its flight control activity to prepare the hand-over, while continuing to perform flight operations of Sentinel-3A.
- The close formation flight will allow to compare thoroughly the measurements from all instruments aboard Sentinel-3A and –B, ensuring the best consistency between the products from the two satellites.
- The completion of commissioning will lead to a handover of the Sentinel-3B satellite from ESA to EUMETSAT once the latter has been moved to it final orbital position, at a 140º phasing from Sentinel-3A, to form the full Sentinel-3 constellation. The 140° phasing was chosen to optimize global coverage and ensure optimized sampling of ocean currents by the combined altimeters on board Sentinel-3A and -3B.
- Thus the Sentinel-3 constellation will also realize the best possible synergy with the cooperative Jason-3 high precision ocean altimeter mission, another Copernicus marine and climate mission exploited by EUMETSAT on behalf of the European Union.
- Under the Copernicus data policy, all Sentinel-3 marine data and products are available on a full, free and open basis to all users through EUMETSAT’s Near Real Time dissemination channels EUMETCast, the Copernicus Online Data Access and EUMETview.
• June 1, 2017: While the Copernicus Sentinel-3A satellite is in orbit delivering a wealth of information about our home planet, engineers are putting its twin, Sentinel-3B satellite through a series of vigorous tests before it is shipped to the launch site next year. It is now in the thermal–vacuum chamber at Thales Alenia Space’s facilities in Cannes, France. This huge chamber simulates the huge swings in temperature facing the satellite in space. Once this is over, the satellite will be put through other tests to prepare it for liftoff in the spring 2018. Both Sentinel-3 satellites carry the same suite of cutting-edge instruments to measure oceans, land, ice and atmosphere. 26)
Figure 9: Sentinel-3B being placed in the thermal/vacuum chamber in Cannes, France (image credit: Thales Alenia Space)
• January 14, 2016: Following the Christmas break, the Sentinel-3A satellite has been taken out of its storage container and woken up as the campaign to prepare it for launch resumes at the Russian Plesetsk Cosmodrome. Liftoff is set for 4 February. 27)
• Nov. 20, 2015: The Sentinel-3A spacecraft has left France bound for the Plesetsk launch site in Russia and launch in late December. An Antonov aircraft carries the precious cargo to Arkhangelsk in Russia after a stopover in Moscow to clear paperwork. 28)
• Oct. 15, 2015: Before the latest satellite for Copernicus is packed up and shipped to the Plesetsk Cosmodrome in Russia for launch at the end of the year, the media and specialists were given the chance to see this next-generation mission center-stage in the cleanroom. The event was hosted by Thales Alenia Space in Cannes, France, where engineers have spent the last few years building and testing Sentinel-3A. 29)
• In December 2014, the Sentinel-3A spacecraft is now fully integrated, hosting a package of different instruments to monitor Earth’s oceans and land. After spending many months carefully piecing the satellite together, it is now being tested in preparation for launch towards the end of 2015. 30)
- Environmental tests will start in early 2015.
• In July 2014, the OLCI instrument was delivered and mounted onto the satellite.
Launch: The Sentinel-3A spacecraft was launched on February 16, 2016 (17.57 GMT) on a Rockot/Briz-KM vehicle of Eurockot Launch Services (a joint venture between Astrium, Bremen and the Khrunichev Space Center, Moscow). The launch site was the Plesetsk Cosmodrome in northern Russia. The satellite separated 79 minutes into the flight. 31) 32)
ESA awarded the contract to Eurockot Launch Services on Feb. 9, 2012. 33)
There are three spacecraft in this series: Sentinel-3A, -3B, and -3C. The second satellite is expected to be launched ~18 months after the first one.
Orbit: Frozen sun-synchronous orbit (14 +7/27 rev./day), mean altitude = 815 km, inclination = 98.6º, LTDN (Local Time on Descending Node) is at 10:00 hours. The revisit time is 27 days providing a global coverage of topography data at mesoscale.
With 1 satellite, the ground inter-track spacing at the equator is 2810 km after 1 day, 750 km after four days, and 104 km after 27 days.
For the altimetry mission, simulations show that this orbit provides an optimal compromise between spatial and temporal sampling for capturing mesoscale ocean structures, offering an improvement on SSH mapping error of up to 44% over Jason - due to improved spatial sampling (Figure )- and 8% over the Envisat 35-day orbit - due to better temporal sampling. After a complete cycle, the track spacing at the equator is approximately 100 km.
The Sentinel-3 mission poses the most demanding POD (Precise Orbit Determination) requirements, specially in the radial component, not only in post-processing on-ground, but also in real-time. This level of accuracy requires dual-frequency receivers. The main objective of the mission is the observation with a radar altimeter of sea surface topography and sea ice measurements (see columns 3, 4, 5 in Table 3).
Table 3: Error budget requirements in Sentinel-3 as a function of time wrt measurement 34)
The second satellite will be placed in the same orbit with an offset of 140º; this phasing improves interleave between S-3A and S-3B for better SRAL meso-scale sampling of 4-7 days. 37)
Commissioning will include a 4-5 month tandem flight. A tandem phase operation of the A/B pair with ~30 s separation in time between satellites on near identical ground-track for ~4-5 months will be flown during Phase E1.
Figure 10: Tandem phase operations overview (EUMETSAT, ESA, Ref. 37)
With two satellites flying simultaneously, the following coverage will be achieved (Ref. 11):
- Global Ocean color data is recorded with OLCI and SLSTR in less than 1.9 days at the equator, and in less than 1.4 days at latitudes higher than 30º, ignoring cloud effects.
- Global Land color data is recorded with OLCI and SLSTR in less than 1.1 days at the equator, and less than 0.9 days in latitudes higher than 30º.
- Global Surface temperature data is recorded in less than 0.9 days at the equator and in less than 0.8 days in latitudes higher than 30º.
- Continuous altimetry observations where global coverage is achieved after completion of the reference ground track of 27 days.
Status of the Sentinel-3 mission
• July 30, 2019: Hundreds of wildfires have broken out in Siberia, some of which can be seen in this image captured from space on 28 July 2019. Almost three million hectares of land are estimated to have been affected, according to Russia’s Federal Forestry Agency. 38)
Figure 11: This Copernicus Sentinel-3 image shows a number of fires, producing plumes of smoke. The smoke has carried air pollution into the Kemerovo, Tomsk, Novosibirsk, and Altai regions (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)
- An unprecedented amount of wildfires have been raging in various regions of the Arctic, including Greenland and Alaska in the US. They have been caused by record-breaking temperatures and lightning, fuelled by strong winds.
- Wildfires release harmful pollutants and toxic gases into the atmosphere. According to the WMO (World Meteorological Organization), fires in the Arctic released around 50 megatons of carbon dioxide in June alone – equivalent to Sweden’s total annual emissions.
• July 25, 2019: An extreme heatwave has hit Europe once again this week, following extreme weather in June. High temperatures are expected to peak today, reaching as high as 39—40°C, with Netherlands, Belgium and Germany recording their highest ever temperatures. Paris reached a sweltering 41°C, breaking its previous record in 1947. 39)
Figure 12: This animation of two images shows the land surface temperature from today 25 July, compared to data recorded during the previous heatwave on 26 June 2019 (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)
- The map has been generated using the Copernicus Sentinel-3’s SLSTR (Sea and Land Surface Temperature Radiometer). Whereas weather forecasts use predicted air temperatures, the satellite measures the real amount of energy radiating from Earth – therefore this map better represents the real temperature of the land surface. Clouds are visible in white in the image, while the light blue represent snow-covered areas.
- The heatwave in June broke several records for many countries, with France reaching over 45°C for the first time. Germany, Hungary, Poland, Austria, Czech Republic, Slovakia also reached peak temperatures.
- In many countries, red heat warnings have been issued, including Italy, Spain and France and civilians are advised to avoid travelling and stay hydrated.
• June 27, 2019: With some places expecting to be hit with air temperatures of over 40°C in the next days, much of Europe is in the grip of a heatwave – and one that is setting record highs for June. According to meteorologists this current bout of sweltering weather is down to hot air being drawn from north Africa. 40)
- Countries worst hit by this unusual June weather include Spain, France, Germany, Italy and Poland. In many places heat warnings have been issued and cities such as Paris have connected fountains and sprinklers to hydrants to help give people some relief. Wildfires in Catalonia, said to be the worst in two decades, have already ripped across 5000 hectares of land and are being blamed on the heat and strong winds.
Figure 13: This map shows the temperature of the land on 26 June. It has been generated using information from the Copernicus Sentinel-3’s SLSTR (Sea and Land Surface Temperature Radiometer), which measures energy radiating from Earth’s surface in nine spectral bands – the map therefore represents temperature of the land surface, not air temperature which is normally used in forecasts. The white areas in the image are where cloud obscured readings of land temperature and the light blue patches are snow-covered areas (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)
• June 24, 2019: An unexpected and powerful eruption started at Raikoke volcano in the Kuril Islands on 21 June 2019. 41)
- The Kuril Islands are an island chain, located in the Pacific Ocean between northern Japan (i.e. Hokkaido) and the Kamchatka Peninsula. The Kuril Islands are claimed by Russia.
Figure 14: This image, which was captured on 22 June, shows the brown ash plumes rising high above the dense clouds – drifting eastwards over the North Pacific Ocean (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)
- According to the Volcanic Ash Advisories Center (VAAC) Tokyo, the thick ash plumes rose to approximately 13 km above sea level. Weather officials warned aircraft flying over the area to be careful of any volcanic ash following the eruption.
- The majority of the plume is now drifting over the Bering Sea. Raikoke is a circular stratovolcano located on an inhabited island. Its last eruption was in 1924.
• May 29, 2019: Most of us probably wouldn’t think of describing snow in terms of its grain size. However, grain size is fundamental to the amount of sunlight that snow reflects back into space – its albedo. With both snow and albedo part of the climate system, scientists are applying a novel analytical theory to Copernicus Sentinel-3 data and shedding new light on Greenland’s changing albedo. 42)
Figure 15: Greenland snow grain and albedo. A sequence of snow grain size and albedo from the Copernicus Sentinel-3 satellites' OLCI (Ocean Land and Color Instruments). The animation illustrates a view through clear skies to the surface of the Greenland ice sheet where warming causes snow grain growth and reduced albedo. The darkest albedo areas are where snow melt gives way to bare glacier ice that melts even faster than snow cover, highlighting the fact that snow and ice are sensitive responders to weather and climate (image credit: GEUS–J. Box/ESA)
- The amount of sunlight absorbed or reflected by Earth’s surface drives our climate and weather. About one-third of the sunlight that hits Earth is reflected back into space and the other two-thirds is absorbed by the land, oceans and atmosphere. This ratio is governed by the reflectivity, or albedo, of the surface that the sunlight hits.
- Surfaces with lighter colors reflect more sunlight than darker surfaces. An everyday example of this is the difference we feel on a hot sunny day when wearing black clothes compared to wearing white. Earth is affected in the same way.
- So hypothetically, if the planet were completely covered in ice, it would reflect over 80% of incident sunlight back into space. On the other hand, if it were covered by dark green forest, it would only reflect about 10%.
- The albedo of Earth’s surface varies naturally according to the changing colors of the season, but long-term trends in changing snow and ice cover, as well as changing vegetation cover and air pollution, are having an impact on the overall balance of Earth’s albedo – and, hence, on how much heat it absorbs.
- The Global Climate Observing System lists both albedo and snow as essential climate variables, which when measured and studied over time are used to understand, monitor and predict climate change.
- Ice and snow are often cited as the first causalities of climate change, and are measured and monitored from space in a variety of ways. However, while ice and snow may be present, the melting process affects its albedo.
- Snow grain size is a fundamental property of snow and is directly proportional to its surface area. Fresh dry snow tends to have a small grain size (under 0.5 mm in diameter), but as it melts the grain size grows and the larger grains reflect less sunlight.
- Thanks to Alex Kokhanovsky from Vitrociset who, along with several authors, published an elegant analytical theory, scientists have a fast new way of retrieving snow grain size from satellite images.
- Scientists from the Geological Survey of Denmark and Greenland (GEUS) in Copenhagen are coupling this theory with data from the Copernicus Sentinel-3 satellites’ Ocean Land and Color Instruments – as the animation above shows.
- Jason Box, from GEUS, explains, “One way of measuring the albedo of snow is to monitor how the surface color changes because of pollution such as from wildfire soot. But this doesn’t give us the whole story. Remarkably, this exciting new theory allows us to retrieve snow grain size from satellite optical images.
- “Through ESA’s Earth Observation Science for Society program, we have been able to demonstrate this over Greenland. We have found that pulses of warm air cause dark blemishes far inland on the ice sheet, contributing to increased climate sensitivity.”
- In fact, the Copernicus Sentinel-3 satellite constellation can now take the relay in maintaining the climate record on snow albedo, which was first provided by the AVHRR (Advanced Very High Resolution Radiometer) instruments on the US NOAA and Europe’s MetOp satellites, and then the MODIS (Moderate Resolution Imaging Spectroradiometer) on the US Terra and Aqua satellites.
- In the future, the method will be extended and applied to areas with more complex terrain than Greenland. Furthermore, grain size data is now on the horizon for being used operationally to improve weather, hydrological and hazards forecasts, in service to society.
Figure 16: Grainy nature of snow. Most of us probably wouldn’t think of describing snow in terms of its grain size. However, grain size is fundamental to the amount of sunlight that snow reflects back into space, its albedo (image credit: H. C. Steen Larsen)
Figure 17: Polluted snow and ice on Greenland. Dark and colored impurities resurface from melting snow and lie atop water-saturated glacier ice on Greenland. Much of the colored material is biological in origin (image credit: GEUS–J. Box)
• May 02, 2019: This Copernicus Sentinel-3 image, captured just yesterday on 1 May 2019, shows Cyclone Fani. Brewed in the Bay of Bengal and heading westwards, the cyclone is expected to make landfall on India’s east coast on Friday 3 May. 43)
Figure 18: With wind speeds of up to 200 km per hour, heavy rainfall and flooding have been forecast along the Odisha coast, and has led to the evacuation of around 800 000 people from the nearby low-lying areas. In the image, the width of the storm is estimated to be around 700-800 km. Once Cyclone Fani makes landfall, it is expected to move north-east, hitting Bangladesh and Bhutan on Saturday 4 May (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)
- On the island’s east coast, bright turquoise colors in the Pacific Ocean suggest the presence of sediment being carried into the ocean by river discharge as well as algal blooms.
- Algal blooms occur when there is a rapid increase in the number of algae in water, and are usually a result of slow water circulation and high water temperatures, they can be toxic and potentially dangerous to both fish and humans.
- The emerald green color of the coastal Lake Ellesmere (Te Waihora), below the circular peninsula jutting out, is most likely because of a high concentration of chlorophyll. This brackish lake is home to over 150 species of birds and more than 40 species of fish thanks to the influx of both freshwater and marine species migrating in and out of the lake.
- Across the Cook Strait, nestling on the southern tip of the North Island, the image shows a body of water called Lake Wairarapa. It is yellow-ochre in color owing to high concentrations of sediment. This shallow lake, which is surrounded by wetlands and farms, drains into the smaller Lake Onoke, further south.
- Tongariro National Park, in the center of the North Island, is a UNESCO World Heritage Site owing to its natural and cultural significance. The park has three active volcanoes. At 2797 m high, the snow-covered Ruapehu – a majestic stratovolcano – is the most visible in the image. The area’s rugged terrain and jagged rocks made it the ideal location for filming the Lord of the Rings trilogy.
- On the far west, the snow-capped cone of Mount Taranaki is in the middle of Egmont National Park. The mountain is surrounded by dark-colored dense forest that is in contrast to the unprotected pasture outside of the park’s circular boundary. It is considered one of the most symmetrical volcano cones in the world.
Figure 19: This true color image of New Zealand, captured on 22 August 2018 with Sentinel-3A, shows the snow-covered Southern Alps stretching 500 km across the west coast of the South Island. This image is also featured on the Earth from Space video program (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by ESA, CC BY-SA 3.0 IGO)
• March 06, 2019: A new study of the Sentinel-3 mission shows its great potential for precision elevation data observations of the Antarctic ice sheet proving a valuable addition to monitoring efforts in the region, according to work published this week in The Cryosphere. The study, led by researchers from the new joint Lancaster University (UK)-CEH Centre of Excellence in Environmental Data Science (CEEDS), alongside ESA (European Space Agency) and industry partners, shows the potential of Sentinel-3 — one of the EU Copernicus satellite missions — to contribute towards long-term ice sheet monitoring programs. 45) 46)
- The scientists also found that Sentinel-3 could detect areas where the ice surface was rapidly lowering, thereby establishing the satellite's credentials as a new platform which can help to monitor Antarctica's contribution to sea level rise.
- Determining how well Sentinel-3 functions over ice sheets is particularly important given that CryoSat-2, ESA's dedicated polar mission, is already operating well beyond its planned lifetime.
- CryoSat-2 was designed to fly in a unique orbit, to maximize coverage of coastal areas of the ice sheet, and to map the regions close to the North and South Poles that were beyond the reach of previous satellites.
- Although Sentinel-3 — which has to balance many applications — cannot match this coverage, it still holds potential as a valuable long-term monitoring platform for decades to come.
- Dr Mal McMillan, lead author and co-Director of CEEDS, said: "Although the Sentinel-3 altimeter was primarily designed to monitor the oceans, we wanted to test how well it works over ice, and to see whether it is able to detect signs of glaciological change. Through the support offered by ESA's Scientific Exploitation of Operational Missions element, we have been able to study the performance of the Sentinel-3 mission for several years now, and we are pleased to be able to publish these results."
- He added: "From what we can see here, with just two years' worth of data, Sentinel-3 is going to be a very useful tool for surveying the Antarctic ice sheet."
- Sentinel-3 uses a radar technique called Delay-Doppler altimetry [use of SRAL (SAR Radar Altimeter) instrument] to make high resolution measurements of the height of the ice sheet.
- Where the ice is relatively flat, Sentinel-3 could map its height to within 10 cm of measurements taken by aircraft, as part of NASA's Operation Icebridge campaign.
- Dr McMillan explained: "This level of accuracy means that we can also use Sentinel-3 to track important features on the ice surface, like the imprint of active subglacial lakes draining and refilling beneath several kilometers of ice."
- Using radar satellites like Sentinel-3 over ice nonetheless has its challenges. For example, measurements over Antarctica's steeper, craggy coastal areas were less accurate because of how the rough landscape affects the radar signal.
- Future research into Sentinel-3's performance, as well as further improvements to data processing, will help take these effects into account. In the meantime, Sentinel-3 has already shown its value as a new tool for detecting ice sheet change.
- Co-author Jérôme Benveniste of the European Space Agency summarized: "We are delighted with the early promise shown by Sentinel-3 for ice sheet monitoring, and are increasingly confident that it will be a long-term asset to climate science."
Figure 20: Sentinel-3, a workhorse mission for Copernicus. Following its launch in February 2016 and subsequent commissioning phase, the Copernicus Sentinel-3A satellite has been systematically measuring our oceans, land, ice and atmosphere. The information feeds a range of practical applications and is used for monitoring and understanding large-scale global dynamics. Sentinel-3A will soon be joined in orbit by its identical twin, Sentinel-3B. Both satellites carry a suite of cutting-edge instruments to supply a new generation of data products, which are particularly useful for marine applications. For example, they monitor ocean-surface temperatures for ocean and weather forecasting services, aquatic biological productivity, ocean pollution and sea-level change. The mission also delivers unique and timely information about changing land cover, vegetation, urban heat islands, and for tracking wildfires. With the two satellites in orbit, global coverage and data delivery will be optimized (video credit: ESA, published 6 March 2019)
• March 01, 2019: The Alps extend 1200 km through eight different countries: France, Monaco, Italy, Switzerland, Liechtenstein, Germany, Austria and Slovenia. This mountain range, which is inhabited by some 20 million people, covers an area of approximately 200,000 km2. 47)
- The Copernicus Sentinel-3A satellite takes us over the high, snow-studded Alps under clear skies (Figure 21). Patches of snow are visible on the island of Corsica, to the left of mainland Italy, Croatia, to the right, and at the bottom of the Apennines in central Italy. Most of Italy’s rivers find their source in the Apennines, including the Tiber and the Arno.
- The Adriatic Sea to the east of Italy is visible in turquoise, particularly the coastal area surrounding the Gargano National Park, jutting out. This light-green color of the sea along the coast is likely to be caused by sediment carried into the sea by river discharge.
- Directly to the right of the Alps, the image shows a pale-green Lake Neusiedl straddling the Austrian-Hungarian border. Neusiedl, meaning ‘swamp’ in Hungarian, is the largest endorheic lake in central Europe, meaning water flows into but not out of the lake, hence its size and level frequently fluctuates. It is a popular area for windsurfing, sailing and spotting the woolly Mangalica pig.
- To the right, the freshwater Lake Balaton is visible, it is the largest lake in central Europe. It stretches for over 75 km in the southern foothills of Hungary. Its striking emerald-green color is probably due to the presence of algae that grow in the shallow waters.
Figure 21: Captured on 16 February 2019 with Sentinel-3A, this true-color image shows little clouds, particularly over the Alps and the surrounding flatter lands in southern France. There is an interesting contrast between this and the haze hanging over the Po valley in Italy, directly south of the Alps. The haze is most likely to be a mix of both fog and smog, trapped at the base of the Alps owing to both its topography and atmospheric conditions. This image is also featured on the Earth from Space video program (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by ESA, CC BY-SA 3.0 IGO)
• December 6, 2018: From 12 December 2018, Sentinel-3B OLCI Level-1 Products will be available at Near Real Time (NRT) and Non Time Critical (NTC) timeliness. These data are the first Sentinel-3B OLCI data to be released after the successful completion of the Sentinel-3B commissioning phase. 48)
- The Sentinel-3 Product Notice for OLCI Level 1 provides information on the OLCI current processing baseline, product quality, known limitations, and product availability status. It is relevant for both Sentinel-3A and Sentinel-3B OLCI products.
- To receive Sentinel-3B UNS (User Notification Service) alerts by email, make sure you update your UNS subscriptions via our Earth Observation Portal (EOP).
Figure 22: Sentinel-3B OLCI L2 algal pigment concentration (image credit: EUMETSAT)
• December 3, 2018: From 6 December 2018, Sentinel-3B STM Level-1 and Marine Level-2 Products will be available at Near Real Time (NRT), Short Time Critical (STC) and Non Time Critical (NTC) timeliness. 49)
- These data are the first Sentinel-3B STM (Surface Topography Mission) data products to be released after the successful completion of the Sentinel-3B commissioning phase.
- The Sentinel-3 Product Notices for STM Level-1 and for Level-2 Marine provide information on the SRAL/MWR current processing baseline, product quality, known limitations, and product availability status. They are relevant for both Sentinel-3A and Sentinel-3B STM data products.
Figure 23: Example of the Sentinel 3A and B SRAL Significant Wave Height product (image credit: EUMETSAT)
• November 23, 2018: The Copernicus Sentinel-3A satellite takes us over the Gulf. Also known as the Arabian Gulf and the Persian Gulf, this marginal sea of the Indian Ocean is just under 1000 km long and covers an area of around 250,000 km2 (Figure 24) . It is bordered by eight nations shown counter-clockwise from the top of the image – Iran, Iraq, Kuwait, Saudi Arabia, Bahrain, Qatar, United Arab Emirates and Oman. 50)
- Most of these countries are shown in a warm shade of orange in the lower part of the image. The variations in color in this area represent the different surface types and compositions. Some areas are purely sand, whilst others are a mixture of rocks and sand, or salt mineral deposits. Sand dunes are prevalent in the central area, depicting one of the largest and driest sand deserts in the world – the Rubh al Khali or the Empty Quarter.
- The satellite image shows in particular the significance of the Strait of Hormuz, the narrowest passage in the Gulf, a strait between the Persian Gulf and the Gulf of Oman. It provides the only sea passage from the Persian Gulf to the open ocean and is one of the world's most strategically important choke points. On the north coast lies Iran, and on the south coast the United Arab Emirates and Musandam, an exclave of Oman. At its narrowest, the strait has a width of 21 nautical miles (39 km). - About 20% of the world's petroleum (about 35% of the petroleum traded by sea) passes through the strait, making it a highly important strategic location for international trade.
- To reduce the risk of collision, ships moving through the Strait follow a Traffic Separation Scheme (TSS): inbound ships use one lane, outbound ships another, each lane being two miles wide. The lanes are separated by a two-mile-wide "median". To traverse the Strait, ships pass through the territorial waters of Iran and Oman under the transit passage provisions of the United Nations Convention on the Law of the Sea. The Strait of Hormuz is of vital importance of these waterways – not only economically, but also politically.
Figure 24: This image of Sentinel-3A, which was captured on 30 September 2018, is also featured on the Earth from Space video program (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by ESA, CC BY-SA 3.0 IGO)
• November 9, 2018: The Copernicus Sentinel-3 mission takes us over Madagascar. This huge island nation, located off the east coast of Africa and seen in the left of the image, has a population of around 25 million. More than half of the country’s inhabitants are aged under 25. The island is also home to rare flora and fauna, having developed its own ecosystems and wildlife since splitting from the African continent some 160 million years ago. 51)
- Preserving its impressive biodiversity is an ongoing challenge for the country. With an area of almost 600,000 km2, Madagascar is the fourth largest island in the world. Some of the world’s most extensive coral reef systems, huge mangrove areas, and a vast array of birds, lemurs, and many other species can be found here. Deforestation poses a serious threat to the island’s habitats, with illegal wildlife trade representing a further challenge.
- In this true-color image from Copernicus Sentinel-3’s (OLCI (Ocean and Land Color Instrument), we can see sediment in the water along the coast. The island appears in green on the east coast, where the prevailing trade winds bring clouds and up to 3.5 m of rain per year to the low-lying coast.
- The central and western highlands and coast appear in brown and are much dryer, especially between May to October. The volcanic mountainous area of Ankaratra can be found in the central highlands. In the north of the island, the highest peak of Maromokotro stands almost 2900 m above sea level.
- The Mozambique Channel, which separates Madagascar from the continent, is an important shipping route for east Africa and home to significant tuna reserves. Efforts are underway to ensure that sustainability is considered in local development. Identifying and protecting critical habitats, such as migratory corridors, is part of this work.
- Sentinel-3 is a two-satellite mission to supply the coverage and data delivery needed for Europe’s Copernicus environmental monitoring program. It provides critical information for a range of applications from marine observation to large-area vegetation monitoring.
Figure 25: Sentinel-3 captured this image on 7 August 2018. The image is also featured in the Earth from Space video program (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by ESA, CC BY-SA 3.0 IGO)
• October 5, 2018: The Copernicus Sentinel-3 satellite takes us over eastern US. Spanning a huge area, including the states of Ohio, Maryland, West Virginia and Delaware, a number of major cities can be seen in this true-color image. The megacity of New York is visible in the top right. A megacity is defined by the United Nations as a city with a population of over ten million. According to the latest estimates there will be 43 megacities across the globe by 2030. 52) The image of Figure 26 is also featured on the Earth from Space video program
- Further down the coast, the US capital of Washington, D.C. can be seen in the upper-central part of the image. Washington, D.C. is a territory, not a state. The first part of the capital’s name is in honor of the first president of the US, George Washington, and D.C. stands for District of Columbia, derived from Christopher Columbus.
- The brown color, that dominates the central part of the image, represents mountainous areas and forests, running through West Virginia and beyond. Known as the Mountain State, this is the only state completely within the Appalachian Mountain region. At around 460 m, its average elevation is higher than any other state east of the Mississippi River.
- To the north of West Virginia, Pennsylvania, which takes its name from a combination of Latin words, meaning ‘Penn’s woods’, stretches up towards New York. Half of this state is covered by forests, including the Allegheny National Forest, which can be seen in the top-center of the image.
Figure 26: This true-color image from Sentinel-3’s OLCI (Ocean and Land Color Instrument), captured on 1 May 2018, shows sediment being carried into the North Atlantic Ocean along the coast. Sediment and potentially algae can also be seen in Lake Erie in the top left. This lake is the fourth-largest of the five Great Lakes of North America. It has a surface area of over 25,000 km2. Around five million tons of a type of rock salt called halite is mined from beneath the lake every year. The state of Ohio is also known for its fertile soil, coal, and natural gas reserves (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by ESA, CC BY-SA 3.0 IGO)
• September 7, 2018: The Copernicus Sentinel-3A satellite takes us over the North Sea, revealing a significant algae bloom covering most of the southern part. One of Europe's most productive fisheries, the North Sea covers an area of 570, 000 km2 and is linked to the Atlantic by one of the world’s busiest shipping regions – the English Channel. 53)
- The image of Figure 27 covers a large section of Scandinavia, including Norway, the south of Sweden, and Denmark, stretching down to Germany and the Netherlands in the bottom right. On the left of the image we can see the east coast of Scotland and the Northern Isles, comprising two archipelagos – Orkney and Shetland. This image is also featured on the Earth from Space video program. 54)
- Harmful algal blooms caused by excessive growth of marine algae have occurred in the North Sea and the English Channel area in recent years, with satellite data being used to track their growth and spread. These data can then be used to help develop alert systems to mitigate against damaging impacts for tourism and fishing industries.
- Harmful blooms, which pose a threat to various forms of water life, are thought to carry an annual cost of over 900 million euros to these industries in the EU.
- Helping to map algal blooms and providing critical information for marine operations are just some of the ways that the two-satellite Sentinel-3 is used for Europe’s Copernicus environmental monitoring program. Since 2016, Sentinel-3A has been measuring our oceans, land, ice and atmosphere to monitor and understand large-scale global dynamics. In April 2018, it was joined by its twin satellite Sentinel-3B.
Figure 27: This true-color image taken using Sentinel-3’s OLCI (Ocean and Land Color Instrument) shows a significant algae bloom. The image was captured on 27 May 2017 (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by ESA, CC BY-SA 3.0 IGO)
• August 24, 2018: The Copernicus Sentinel-3A satellite took the temperature at the top of Hurricane Lane as it headed towards Hawaii’s Big Island on 22 August 2018. Lane weakened to a Category 3 storm on 23 August, just before it hit Hawaii. Still a powerful storm, it has brought torrential rain to the Big Island. The island is still reeling from months of devastating lava flows from the Kilauea volcano and is now coping with heavy rain and flooding. The brightness temperature of the clouds at the top of the storm, some 12–15 km above the ocean, range from about –80°C near the eye of the storm to about 15°C at the edges (Figure 28). 55)
- Hurricanes are one of the forces of nature that can be tracked only by satellites, providing up-to-date imagery so that authorities know when to take precautionary measures. Satellites deliver information on a storm’s extent, wind speed and path, and on key features such as cloud thickness, temperature, and water and ice content. Sentinel-3’s Sea and Land Surface Temperature Radiometer measures energy radiating from Earth’s surface in nine spectral bands.
Figure 28: The Copernicus Sentinel-3 mission takes the temperature of Hurricane Lane on 22 August as it headed for Hawaii (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by ESA, CC BY-SA 3.0 IGO)
• July 2018: Full coverage and revisit performance are achieved with both Sentinel-3A and Sentinel-3B in orbit. In order to guarantee long-term continuity of the Sentinel-3 mission, additional satellites (Sentinel-3C and Sentinel-3D) are under development as replacements to the initial satellite pair (S-3A and S-3B). In this way the series of Sentinel-3 satellites will ensure global, frequent and near-realtime ocean, ice and land monitoring, with the provision of observation data in routine, long term (up to 20 years of operations) and continuous fashion, with a consistent quality and a high level of reliability and availability. 56)
Table 4: Sentinel-3 mission activities
Figure 29: Schematic overview of planned Sentinel-3A and Sentinel-3B tandem mission activities identifying key elements (image credit: ESA, Ref. 56)
• July 20, 2018: With Europe in the grip of a heatwave and little rain, the scorched ground and dry vegetation is succumbing to fire. Fires have now broken out as far as the Arctic Circle, in Sweden. This animation shows images from the Copernicus Sentinel-2 (Figure 31) and Sentinel-3 (Figure 30) missions. Sentinel-2 carries a high-resolution MSI (Multispectral Imager) and Sentinel-3 carries a suite of instruments including an OLCI (Ocean and Land Color Instrument). The missions offer complementary views of Earth to help monitor environmental change and events such as these fires. 57)
- The fires have led to the authorities evacuating some villages and appealing for help from other countries, and concerns that the current record temperatures and drought are linked to climate change.
- There are said to be around 50 fires now burning in the country. Through July there have been three times as many fires as there were last year.
- Jonas Olsson from the Swedish Meteorological and Hydrological Institute is reported to have said, “It’s very, very dry in most of Sweden. The flows in the rivers and lakes are exceptionally low, except in the very northern part of the country. We have water shortages. Rainfall has only been around a seventh of the normal amount, the lowest since record-keeping began in the late 19th century.”
- While the focus is clearly on trying to extinguish these fires, the Copernicus Sentinel missions offer an eye in the sky to identify where fires have broken out. This is especially valuable in areas that are sparsely populated.
Figure 30: The wider view, which stretches from the west coast of Norway to central Sweden, was captured by Sentinel-3 on 17 July 2018. Here, smoke can be seen billowing from several fires (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by ESA)
Figure 31: The closer view was captured on the same day (17 July) by Sentinel-2. Here, flames and smoke from two of the fires can be seen clearly, along with smoke from other fires in the vicinity (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by ESA)
• July 13, 2018: The Copernicus Sentinel-3A satellite takes us over Shanghai, China. One of the most populous cities in the world and home to over 24 million people, the city is visible in the lower right of the image just above the Yangtze River mouth. As a significant global financial center it is also the site of the world’s busiest container ports because of its strategic location on the Yangtze River delta. 58)
- The Taihu Lake is shown in green in the lower right part of the image. In 2007, an algal bloom on the lake caused major problems with water supplies in the neighboring city of Wuxi. Such algal blooms may well be linked to the discharge of phosphates found in fertilizers used in industry and agriculture into the water.
- Steps have been taken to limit the use of such fertilizers in a bid to reduce algal blooms, which can significantly alter the ecology of the environment below the surface and pose a threat to various forms of water life.
Figure 32: This true color image taken using Sentinel-3A’s OLCI (Ocean and Land Color Instrument) shows the huge amount of sediment carried into the ocean along the coast. The image covers an area of over 1200 km side length, showing Beijing at the center-top, the salt flats close to the Mongolian border in the top left, and North Korea, with its capital, Pyongyang, just visible in the top right of the image. A large number of urban settlements represented as grey flecks are interspersed with agricultural fields, dominating the central part of the image (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by ESA, CC BY-SA 3.0 IGO)
• June 19,2018: The key to monitoring Earth’s changing environment and to guaranteeing a consistent stream of satellite data to improve our daily lives is to take the same measurements over the course of decades. But how do you know that measurements from successive satellites, even though identical in build, are like for like? The answer, for the Copernicus Sentinel-3 mission, is to engage in some nifty orbital flying. 59)
- Sentinel-3 is a two-satellite mission to supply the coverage and data delivery needed for Europe’s Copernicus environmental monitoring program.
- Launched in 2016, Sentinel-3A has been measuring our oceans, land, ice and atmosphere to monitor and understand large-scale global dynamics and to provide critical information for marine operations, and more.
- Its twin, Sentinel-3B, was launched in April 2018 and is having its instruments calibrated and being commissioned for service. Once Sentinel-3B is operational, the two satellites will orbit Earth 140° apart.
- Now, however, the satellites have been positioned much closer together, flying a mere 30 seconds apart. Travelling at 7.4 km/s, the separation equates to a distance of 223 km. - The reason for this is to see how their instruments compare.
- Even though the two Sentinel-3 satellites are identical, each carrying a radar altimeter, a radiometer and an imaging spectrometer, there’s a chance that their instruments could behave slightly differently. It is important that any differences are carefully accounted for otherwise the information they deliver could be misinterpreted as changes happening on Earth’s surface.
- Given the satellites’ current brief separation, their measurements should be virtually the same. This tandem phase is also important for the future Sentinel-3 satellites.
- ESA’s ocean scientist, Craig Donlon, explains, “Our Sentinel-3 ocean climate record will eventually be derived from four satellites because we will be launching two further Sentinel-3s in the future. We need to understand the small differences between each successive satellite instrument as these influence our ability to determine accurate climate trends. The four-month Sentinel-3 tandem phase is a fantastic opportunity to do this and will provide results so that climate scientists can use all Sentinel-3 data with confidence.”
- ESA’s Sentinel-3 project manager, Bruno Berruti, said, “Following liftoff and the usual checks, the operations team has been expertly flying Sentinel-3B so that it gradually flies closer to Sentinel-3A. We recently reached the magic separation of 30 seconds and I am happy to say that we are now officially in the tandem phase. This will last around four months, after which the two satellites will be gently moved apart until they reach their operational separation of 140°. This is different to the other Sentinel missions, but for our mission it is better to measure ocean features such as eddies as accurately as possible.”
- ESA’s Sentinel-3 mission manager, Susanne Mecklenburg, added, “So far, we are really happy with the results of the tandem phase. Measurements from the satellites’ instrument packages seem to be very much aligned, but we will be analyzing the results very carefully over the next months to make sure we account for any minor differences.”
Figure 33: Sentinel-3 tandem mission comparison (image credit: ESA, the images contain modified Copernicus Sentinel data (2018), processed by ESA, CC BY-SA 3.0 IGO)
• June 8, 2018: For World Oceans Day, the Copernicus Sentinel-3A satellite takes us over the Atlantic Ocean and the Republic of Cabo Verde. Several of the small islands that make up the archipelago of Cabo Verde can be seen peeking out from beneath the clouds (Figure 34). These 10 volcanic islands, with a total area of about 4000 km 2, lie in the Atlantic Ocean about 570 km off the west coast of Senegal and Mauritania, which frame the image on the right. 60) 61)
- The most striking thing about this image, however, is the dust and sand being carried by the wind towards Cabo Verde from Africa. The sand comes mainly from the Sahara and Sahel region. Owing to Cabo Verde’s position and the trade winds, these storms are not uncommon and can disrupt air traffic.
- However, this sand also fertilizes the ocean with nutrients and promotes the growth of phytoplankton, which are microscopic plants that sustain the marine food web. The iron in the dust is particularly important. Without iron mammals cannot make haemoglobin to transport oxygen around the bloodstream and plants cannot make chlorophyll to photosynthesize. Research has shown that around 80% of iron in samples of water taken across the North Atlantic originates from the Sahara. It can be assumed, therefore, that life in the deep ocean depends on this delivery of fertilizer from one of the world’s most parched regions.
- World Oceans Day takes place on 8 June each year and celebrates the ocean, its importance in all our lives, and how we can protect it.
- Historically, the name "Cape Verde" has been used in English for the archipelago and, since independence in 1975, for the country. In 2013, the Cape Verdean government determined that the Portuguese designation Cabo Verde would henceforth be used for official purposes, such as at the United Nations, even in English contexts. Cape Verde is a member of the African Union.
• June 5, 2018: Launched on 25 April 2018, Sentinel-3B has already delivered some impressive first images from its ocean and land color instrument, from its altimeter and from the optical channels of its radiometer. With the radiometer’s thermal-infrared channels now turned on, the satellite completes its set of firsts with an image that depicts thermal signatures over southern Italy, the Mediterranean Sea and Sicily – with the hotspot of Mount Etna clearly visible. This image shows the ‘brightness temperature’, which corresponds to radiation emitted from the surface. Further processing is needed to turn this into an actual temperature map. The land surface is shown in red–orange colors, corresponding to a brightness temperature range of 296–320K. The blue colors over the ocean correspond to a range of 290–295K. The dark blue–black areas correspond to clouds, which are opaque to thermal-infrared radiation and so prevent a view of the ocean or land surface. 62)
- Mount Etna, Europe’s largest and most active volcano in Europe, appears much hotter than the surrounding land. In a state of almost continuous unrest, the volcano is currently classified as having ‘minor activity’.
- Over the oceans, the Sentinel-3 radiometer is important for providing SST (Sea Surface Temperature) measurements for oceanographic and weather forecasting centers. Over land, the instrument can be used, in particular, to monitor urban heat islands and wildfires.
- Sentinel-3B joins its identical twin, Sentinel-3A, in orbit. This pairing of satellites increases coverage and data delivery for the European Union’s Copernicus environment program.
Figure 35: The infrared channels of Sentinel-3B’s radiometer have been turned on and reveal the hotspot of Mount Etna in Sicily (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by ESA, CC BY-SA 3.0 IGO)
• May 11, 2018: Sentinel-3B has also delivered first data from its altimeter, SRAL (SAR Radar Altimeter), – which means that all of the instruments are working well. 63)
Figure 36: The altimeter has measured the height of Lake Van in east Turkey, close to the boarder of Iran. This brackish lake is the largest body of water in Turkey. The image shows two tracks, about 7 km apart, across the lake: one from the Sentinel-3A satellite on 26 April 2018 and one from Sentinel-3B on 9 May. They measure the lake to be 1647 m above sea level and clearly show the transition from land to water (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by ESA)
- Sentinel-3B will not be fully commissioned for service for another five months or so, but even at this early stage in its life in orbit, these data compare well with those of its fully-operational twin, Sentinel-3A. The altimeter is particularly important for measuring changes in sea level, but as this example shows, the instrument also contributes to monitoring the height of lake water through the Copernicus Global Land Service. It is also used to measure the height of ice.
• May 11, 2018: The high-precision sea level anomaly measurements from SRAL (Synthetic Aperture Radar Altimeter) on Sentinel-3B enhance the Copernicus altimeter constellation of Sentinel-3A and Jason-3, along with Jason-2. 64)
Figure 37: The altimeter measurements, all acquired on 8 May 2018 over the Atlantic Ocean and the north American coastline, clearly match well at crossing points and the map of sea level anomalies produced by the Copernicus Marine Environment Monitoring Service (CMEMS), image credit: EUMETSAT, CMENS
- Pierre Yves Le Traon, Scientific Director at Mercator Ocean, which operates the Copernicus Marine Environment Monitoring Service (CMEMS), said receiving data from altimeters on two Sentinel-3 satellites will bring benefits to marine safety and industries.
- “The joint use of Sentinel-3A and Sentinel-3B high precision altimeters will improve capabilities of CMEMS to forecast waves and ocean currents at fine scale,” Le Traon said. “This is highly needed for the applications we service, such as marine safety, maritime transport and offshore operations.”
• May 10, 2018: The SLSTR (Sea and Land Surface Temperature Radiometer) is particularly sophisticated, measuring energy radiating from Earth’s surface in nine spectral bands, including visible and infrared. It also includes dedicated channels for measuring fires. This early image came from its optical channels. 65)
- SLSTR measures the energy radiating from the Earth’s surface in nine spectral bands, including visible and infrared. The thermal channels will be switched on once the instrument has completed outgassing water vapor, and the infrared channels have been cooled to the operating temperatures. 66)
- Sentinel-3B is currently flying in tandem with its twin, Sentinel-A, which was launched in 2016, so that their observations can be cross calibrated. The spacecraft are part of a series of Sentinel satellites under the umbrella of the EU’s Copernicus program, which takes a continuous “health check” of our planet.
Figure 38: The SLSTR instrument on Sentinel-3B captured this image on 9 May 2018. It shows a low pressure system over the UK and Ireland, France, the Bay of Biscay, Spain and part of north Africa. Vegetation appears in red (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by EUMETSAT)
• May 9, 2018: Less than two weeks after it was launched, the Copernicus Sentinel-3B satellite has delivered its first images of Earth. Exceeding expectations, this first set of images include the sunset over Antarctica, sea ice in the Arctic and a view of northern Europe. 67)
- The very first image, captured on 7 May at 10:33 GMT , shows the transition between day and night over the Weddell Sea in Antarctica. The satellite also captured swirls of sea ice off Greenland on the same day. Another in this first set of images offers a rare cloud-free view of northern Europe. They were taken by OLCI (Ocean and Land Color Instrument) on Sentinel-3B.
Figure 39: The first image of OLCI on Sentinel-3B was captured on 7 May 2018 (10:33 GMT). The image shows the sunset over the Weddell Sea off the coast of Antarctica. While the line between day and night is clearly visible, bright streaks glint on the clouds from the sunset. OLCI, features 21 distinct bands, a resolution of 300 m and a swath width of 1270 km. The instrument can be used to monitor aquatic biological productivity and marine pollution, and over land it can be used to monitor the health of vegetation (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by EUMETSAT, CC BY-SA 3.0 IGO)
Figure 40: One of the Copernicus Sentinel-3B’s first images of OLCI features Greenland. Captured on 7 May 2018 at 13:20 GMT, the image shows sea ice swirled into eddies caused by the wind and ocean currents (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by EUMETSAT, CC BY-SA 3.0 IGO)
- Josef Aschbacher, ESA’s Director of Earth Observation Programs, said, “The launch of Sentinel-3B completed the first batch of Sentinels that we are delivering for Copernicus. We finished the launch and early orbit phase in a record time and we are now getting on with the task of commissioning the satellite for service. These first images from the ocean and land color instrument already show how the satellite is set to play its role in providing a stream of high-quality environmental data to improve lives, boost the economy and protect our world.”
- Over oceans, OLCI measures the temperature, color and height of the sea surface as well as the thickness of sea ice. These measurements are used, for example, to monitor changes in Earth’s climate and for more hands-on applications such as for monitoring marine pollution. Over land, this innovative mission monitors wildfires, maps the way land is used, checks vegetation health and measures the height of rivers and lakes.
Figure 41: This OLCI image of cloud-free Northern Europe on Sentinel-3B was captured on 8 May 2018 at 09:33 GMT. Features over land and water can been seen clearly such as different types of land cover, snow and also a plume of phytoplankton in the North Sea (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by EUMETSAT, CC BY-SA 3.0 IGO)
• April 27, 2018: This mosaic of cloud-free images from the Copernicus Sentinel-3A satellite spans the entire continent of Europe, and more. The view stretches from Iceland in the northwest across to Scandinavia and Russia in the northeast, and from the northern tips of Norway and Finland to as far south as Algeria, Libya and Egypt. 68)
- While the satellite’s OLCI (Ocean and Land Color Instrument) depicts the green of summer in many parts of Europe, the dryness that summer brings, particularly to the south, can also be seen in parts of Spain, Italy and Turkey, for example.
- This week, aerospace fans have had their eyes firmly set on the ILA Berlin Air Show in Germany. Berlin lies in the center of the image (Figure 42). Here, participants have been learning about new space technologies as well as being treated to latest results from satellite missions such as ESA’s Gaia, which has been used to chart the position, brightness and motion of more than a billion stars. With the second Sentinel-3 satellite, Sentinel-3B, lifting off from Russia this week, the focus has also been this latest Copernicus mission.
- Like Gaia maps stars thousands of light-years away to understand the Universe, the Sentinel-3 mission observes our home planet to understand large-scale environmental dynamics. Based on a constellation of two identical satellites, the Sentinel-3 mission carries a suite of instruments to measure our oceans, land and ice.
- Over land, this innovative mission is being used to map the way land is used, provide indices of vegetation, monitor wildfires and measure the height of rivers and lakes. Over oceans it measures the temperature, color and height of the sea surface as well as the thickness of sea ice.
Figure 42: The image, which is made up of scenes captured between 1 March 2017 and 30 July 2017, is also featured on the Earth from Space video program (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by Sinergise/ESA)
• April 26, 2018: With the launch of Sentinel-3B, the first set of Sentinels for Copernicus is now in orbit delivering an almost mind-boggling amount of data. While this wealth of information advances environmental monitoring, it also opens up huge business opportunities. To help bring these data to market, ESA and SAP have joined forces to create the World Space Alliance. SAP (Systems, Applications & Products in Data Processing) is a German-based (HQs in Walldorf) European multinational software corporation that makes enterprise software to manage business operations and customer relations. 69)
- ESA and SAP first teamed up two years ago to close the gap between traditional Earth observation and the digitalized business world. Building on this fruitful partnership, the two organizations today signed another agreement to strengthen the use of digital technologies and Earth observation data for a range of business sectors.
- Josef Aschbacher, Director of Earth Observation Programs at ESA, said, “Data from the Sentinels are, of course, open and freely available to everybody, but through our partnership with SAP we are working on making data products that businesses can easily tap into.”
- The World Space Alliance will be powered by the SAP Cloud Platform. In turn, ESA also benefits from the platform’s big-data handling and dissemination capabilities. The data it uses will come primarily from the Copernicus Sentinel missions, but also from other satellite missions that contribute to Copernicus.
- The agreement was signed at the ILA Berlin Air Show on 26 April 2018, this new agreement paves the way to strengthen the use of digital technologies and Earth observation data for a range of business sectors.
• April 26, 2018: ESA’s mission control team keeping a close watch on the launch and progress of Sentinel-3B as it joined its identical twin, Sentinel-3A, to complete a constellation that will provide global monitoring of Earth’s oceans, land, ice and atmosphere for Europe’s Copernicus environmental monitoring program. 70)
- Europe’s Copernicus program now has three complete two-satellite constellations in orbit plus an additional single satellite, Sentinel-5P, a mission to monitor air pollution.
- At the end of the commissioning phase, encompassing the first five months in orbit to test the functionality of the satellite, ESA will hand over operations of the satellite to EUMETSAT. ESA and EUMETSAT manage the mission jointly with ESA producing land products and EUMETSAT marine products for application through the Copernicus services.
Table 5: About the Tandem Phase of the Sentinel-3 mission (Craig Donlon, the Mission Scientist for the Copernicus Sentinel-3 and Sentinel-6 Satellites at ESA) 71)
- In two years since its launch, the Sentinel-3A satellite has delivered over 500 Terabytes of data about the Earth’s oceans, land, ice and atmosphere. Its free and open data has allowed us to improve our understanding of large-scale global dynamics beyond expectations. Now, after two years of a solo journey, the Sentinel-3A finally has a twin – Sentinel-3B (Ref. 71).
• April 25, 2018: Just 92 minutes after liftoff, Sentinel-3B sent its first signals to the Kiruna station in Sweden. Data links were quickly established by teams at ESA’s operations center in Darmstadt, Germany, allowing them to assume control of the satellite (Ref. 35).
- During the three-day launch and the early orbit phase, controllers will check that all the satellite’s systems are working and begin calibrating the instruments to commission the satellite. The mission is expected to begin routine operations after five months.
- “This is the seventh launch of a Sentinel satellite in the last four years. It is a clear demonstration of what European cooperation can achieve and it is another piece to operating the largest Earth observation program in the world, together with our partners from the European Commission and EUMETSAT,” said ESA Director General Jan Wörner.
- With this launch, the first set of Sentinel missions for the European Union’s Copernicus environmental monitoring network are in orbit, carrying a range of technologies to monitor Earth’s land, oceans and atmosphere.
- ESA’s Director of Earth Observation Programs, Josef Aschbacher, said, “With Sentinel-3B, Europe has put the first constellation of Sentinel missions into orbit – this is no small job and has required strong support by all involved. It allows us to get a very detailed picture of our planet on a daily basis and provides crucial information for policy makers. “It also offers lots of opportunities for commercial companies to develop new innovative services. And, the free and open data policy allows every citizen to have updates for their own use. When we designed such a satellite constellation 20 years ago not everyone was convinced Europe could do that. I am glad to see this has become reality and that it is now a large European success story.”
• April 13, 2018: EUMETSAT collects and disseminates data from three instruments carried by the Sentinel-3 satellites. They are: OLCI (Ocean and Land Color Instrument), SLSTR (Sea and Land Surface Temperature Radiometer), and SRAL (Synthetic Aperture Radar Altimeter). The SRAL instrument has been designed to deliver accurate measurements of sea surface height, significant wave height and surface wind speeds over the world’s oceans. 72)
- Sea level rise is an important indicator of climate change. Globally, the sea level has been rising by an average of just over 3mm per year for the past 20 years. However, the rise is not uniform – it varies considerably around the world. Additionally, the rate of sea level rise is increasing by about 1mm per year each decade. This suggests that by the end of the century the sea level could have risen by as much as 65cm more than current projections.
- EUMETSAT Project Scientist and Altimetry Expert Remko Scharroo said the SRAL measures an array of important data in addition to sea level. “Sea surface height data from the satellite’s altimeter have significantly improved our capability to analyze and forecast ocean currents,” Scharroo said. “This is essential for the applications we serve such as marine safety, ship routing and predicting the fate of marine pollution events. Another set of data maps significant wave height, which again is important information needed for ship safety. Finally, the SRAL also provides accurate topography measurements over sea ice, ice sheets, rivers and lakes.”
- The successful launch of Sentinel-3B at the end of April will represent the full deployment of the Sentinel-3 mission. Sentinel-3A was launched in February 2016 and has been delivering observations for two years. The two satellites are needed to provide the full set of measurements required by users.
- This includes for use in ocean meteorology, which is all about capturing significant wave height, wind speed and sea level measurements. Altimetry ensures the monitoring of ocean currents and eddies which carry a lot of energy throughout the ocean and interact with the atmosphere, thereby influencing the weather.
- An important example is the forecasting of hurricanes. Altimeters can help derive the latent heat in oceans. “You might argue that the SLSTR instrument does the same thing, that it measures heat in the ocean,” Scharroo said. “But these are two different types of measurements entirely. “While sea surface temperature covers the total top layer of the ocean, it does not tell you much about the structure below. For this, you need altimeter measurements. Water expands when it’s hot. The more heat penetrate the depth of the oceans, the higher the sea level rises and that will give you vital information about the column of water underneath. In short, the SLSTR provides information about the surface but not about the column of water underneath or the latent heat.”
- Scharroo said both measurements were valid and important. “There is no competition between the instruments, believe me,” he said. “It is in synergy that they work best.”
Figure 43: Typical measurements of the instruments SRAL and SLSTR (image credit: EUMETSAT)
• April 9, 2018: EUMETSAT operates the Sentinel-3 satellites on behalf of the EU and delivers the marine mission. 73)
- Ocean Color data can be used for a wide variety of purposes. They can help to track and forecast harmful algal blooms, which can endanger humans, marine or freshwater life and aquaculture. These data also support the monitoring of coastal water quality, by tracking eutrophication, nuisance blooms, sedimentation and erosion.
- Ocean color data are also of a truly global importance as they help monitor climate change: ocean color is one of the so-called Essential Climate Variables (ECV) listed by the WMO (World Meteorological Organization) that help detect changes in biological activity in the ocean’s surface layer.
- Ewa Kwiatkowska, EUMETSAT Remote Sensing Scientist explains: “Phytoplankton take up carbon dioxide (CO2) during photosynthesis, making the ocean the most important carbon sink. Alternatively, ocean color data can be used to monitor the annual global uptake of CO2 by phytoplankton on a global scale.”
- Observations of ocean color support many industries, including fisheries and aquaculture, because of the phytoplankton role at the base of the aquatic food chain. They also support studies of the Earth system, for instance through monitoring El Niño/La Niña events and their impacts on ocean living ecosystems.
- She added: “Ocean color data can aid reporting obligations of the European Union's legislation within Marine Strategy Framework Directive and Water Framework Directive, the goal of which is to achieve or maintain Good Environmental Status of the seas by the year 2020.”
- To ensure a steady flow of these vital data, two Sentinels are a must as Kwiatkowska explains. “Harmful outbreaks of algal bloom, require constant and consistent monitoring and forecasting from the OLCI instrument. With a cloud cover and gaps in coverage between successive orbits, there could be a discontinuity in the observations – with possibly detrimental effects.
- She continued “Also, there is a need to time phytoplankton blooms for ecosystem forecasts and to detect trends in eutrophication, where water body gets over-enriched with an excess amount of nutrients. This process induces growth of plants and algae and due to the biomass load, may result in oxygen depletion of the water body.”
Figure 44: A typical OLCI ocean color image of Sentinel-3A (image credit: NASA Earth Observatory, image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response)
• April 6, 2018: The Copernicus Sentinel-3A satellite takes us over southern Siberia and the world’s largest freshwater lake: Lake Baikal. Imaged on 14 March 2017, this deep lake is covered by ice. The entire lake is typically covered between January and May and in some places the ice can be more than 2 m thick. 74) 75)
- Holding around 23 000 km3 of water, Lake Baikal is the largest freshwater lake by volume in the world. It contains about 20% of the world’s fresh surface water, which is more than all of the North American Great Lakes put together. Baikal water is extraordinarily clean, transparent and saturated with oxygen. The high transparency is thanks to numerous aquatic organisms purifying the water and making it similar to distilled water.
- At 25 million years, this remarkable lake is also the oldest in the world. It is known as the Galapagos of Russia because its age and isolation have produced rich and unusual water wildlife, which is of exceptional value to evolutionary science. Occasionally, new species are discovered and it has been estimated that we know of only 70–80% of all the species inhabiting the lake. For these reasons, in 1996 it was listed as a UNESCO World Heritage Site. The lake is surrounded by mountain-taiga landscapes, which are also protected to preserve their natural state.
Figure 45: This false-color image was acquired on 14 March 2018 with Sentinel-3A (image credit: ESA, this image contains modified Copernicus Sentinel data (2017), processed by ESA, CC BY-SA 3.0 IGO)
• March 9, 2018: The Copernicus Sentinel-3A satellite carries a suite of state-of-the art sensors that deliver a wealth of information to monitor our changing world, but this image was captured with its OLCI (Ocean and Land Color Instrument) . With a swath-width of 1270 km, this instrument delivers images that can span several countries, as we see here. 76)
- From east to west, the image features the islands of Corsica and Sardinia in the Mediterranean Sea, Italy and across the Adriatic Sea to Croatia, Bosnia and Herzegovina, Serbia, and to the western edges of Romania. To the north and partly obscured by clouds, lie Germany, Switzerland, Austria and the Alps.
- South of the Alps, haze hovers over Italy’s Po Valley. Following the Po River to the east, the sediment it carries can be seen entering the Adriatic Sea. In fact, sediments line most of the eastern coast of Italy, giving it a greenish blue frame, while the western coast is mostly sediment-free.
- As the colors in this image suggest, the camera can be used to monitor ocean ecosystems and vegetation on land – all of which will bring significant benefits to society through more informed decision-making.
- Sentinel-3A will soon be joined in orbit by its twin Sentinel-3B, which is scheduled for liftoff from Russia on 25 April. The pairing of identical satellites provides the best coverage and data delivery for Europe’s Copernicus program – the largest environmental monitoring program in the world.
Figure 46: This image was captured by Sentinel-3A on 28 September 2016 (image credit: ESA, the image contains modified Copernicus Sentinel data (2016), processed by ESA, CC BY-SA 3.0 IGO)
• February 28, 2018: Italy is usually associated with relatively warm weather, but this week it, too, has fallen victim to the cold snap nicknamed the Beast from the East. 77)
- Freezing temperatures carried on winds from Siberia have brought snow to much of Europe, causing widespread disruption. As this image captured on 27 February by the Copernicus Sentinel-3A satellite shows, Italy in southern Europe was not spared. Temperatures in Rome (bottom right of the image) are normally between 6°C and 14°C, but this week it has been down to –5°C during the night, and it is the first time the city has seen snowfall in six years. Naples also had its heaviest snowfall in decades.
Figure 47: Sentinel-3A image of northern Italy, acquired on 27 Feb. 2018, showing in partucular the snow-capped Apennine mountain range (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by ESA, CC BY-SA 3.0 IGO)
• February 9, 2018: The Copernicus Sentinel-3A satellite takes us over the Atlantic Ocean close to Spain and Portugal where the sky not only features clouds but also crisscross tracks from maritime vessels. 78)
- The familiar condensation trails – or contrails – we see in the sky usually come from aircraft, so it might seem strange that ships can also occasionally leave their mark in the sky. This rarely seen maritime twist on aircraft contrails was captured by Sentinel-3A on 16 January 2018. Known as ship tracks, these narrow cloud streaks form when water vapor condenses around small particles that ships emit in their exhaust fumes. They typically form when low-lying stratus and cumulus clouds are present and when the air surrounding the ship is calm.
- Like aircraft contrails, ship tracks may also play a role in our climate by reducing the amount of sunlight that reaches Earth’s surface or conversely by trapping the Sun’s radiation in our atmosphere – but this remains an uncertain aspect of climate science.
- The Copernicus Sentinel-3A satellite carries a suite of sensors including OLCI (Ocean and Land Color Instrument), which was used to capture this image of Figure 48.
Figure 48: Sentinel-3A image of the Atlantic Ocean off the coast of Portugal and Spain, captured on 16 January 2018, showing ship track contrails in the clouds. Although the Strait of Gibraltar is a busy shipping lane, with numerous ships travelling in and out of the Mediterranean Sea, there are no ship tracks visible here in the image. Most tracks are several hundred of kilometers off shore (image credit: ESA, the image contains modified Copernicus Sentinel data (2018), processed by ESA, CC BY-SA 3.0 IGO)
• January 11, 2018: Wave information is crucial for people working at sea, to be able to navigate and operate safely. A new product based on satellite altimeter data detailing ‘Significant Wave Height' now enables this. 79)
- High waves are not only dangerous but can threaten delicate procedures at sea, so wave information is paramount for operating safely and efficiently. For instance, in oil and gas offshore platform operations, historic data and forecasts of wave heights are vital for the safety of personnel, equipment and the environment.
- Marine renewable energy operations and site studies require similar information on waves and ship routing can also be improved by such forecasts.
- In physical oceanography, the SWH (Significant Wave Height) is defined traditionally as the mean wave height (trough to crest) of the highest third of the waves. This mathematical definition of ocean wave height is intended to express the height that would be estimated by a trained observer, capturing the most significant waves over the water surface.
- Satellite wave measurements come from two main sources: altimetry and SAR (Synthetic Aperture Radar). The SWH can be obtained through altimetry and directional and spectral information with SAR.
- CMEMS (Copernicus Marine Environment Monitoring Service) released the first realtime global wave product based on satellite data, broadening its offer—previously based on numerical wave forecast models. Released in the summer of 2017, this new product from satellite altimeter data contains the Significant Wave Height from Jason-3 and from the Copernicus Sentinel-3A satellite altimeter data, provided within three hours after data acquisition.
- It provides quality-filtered and inter-calibrated along-track high-resolution SWH (one measurement every 0.7 km, or every second). These measurements contribute to global ocean coverage along the satellite ground tracks with 0.7 km resolution.
- Such satellite wave products represent actual measurements of the waves, covering the entire Earth, regularly and homogeneously over several years. They often offer a better portrayal of extreme events, which numerical models tend to under estimate.
- In-situ wave data, typically provided by buoys, are similarly very helpful but in many open-water areas such moored buoys are not available, mainly due to the technical difficulty and cost of installing and maintaining them in deep ocean, far from the coast (Figure 49).
Figure 49: In-situ wave data, typically provided by buoys, are very helpful to validate satellite wave products but in many areas of open water such buoys are not available, because of the difficulty and costliness of installation and maintenance (image credit: INSITU TAC /CMEMS)
- Sentinel-3A's wave data are also assimilated into numerical realtime wave models to provide wave forecasts with better accuracy. For example, assimilation into the CMEMS global wave forecast model has a strong impact in the north-west of the Pacific Ocean related to the typhoon season and in the Gulf of Mexico after Hurricane Harvey (Figure 50).
- Dr Romain Husson, responsible for wave products at CLS for CMEMS, says, "In the first quarter of 2018, CMEMS will also deliver wave products derived from Sentinel-1A and -1B's SAR instruments. With respect to altimetry, SAR has the unique ability to measure the wave period and direction on top of the SWH and is particularly well suited for long waves, sometimes also referred to as swell."
Figure 50: Sentinel-3A wave data assimilation in the CMEMS global wave forecast model has a strong impact in the north-west of the Pacific Ocean related to the typhoon season and in the Gulf of Mexico after Hurricane Harvey. Analysis increment (in meters) of SWH after 1-day of assimilation of Sentinel-3A wave data in the CMEMS Global Wave Model MFWAM (starting date on 29 August, 2017 at 06:00 UTC to 30 August, 2017 at 0:00 UTC), image credit: ESA, the image contains Copernicus Sentinel data (2017)/ processed by Météo France/CMEMS
• December 22, 2017: EUMETSAT has released a series of videos that provide training on how to access, download and manipulate Sentinel-3 marine data from its Copernicus Online Data Access (CODA) platform. - A set of three Copernicus Sentinel-3 marine user handbooks has also been published. They will enable end users to become familiar with the main features of products based on data coming from instruments onboard Sentinel-3. 80)
• December 21, 2017: Monitoring large, remote bodies of water is logistically challenging, time consuming and expensive. Responding quickly to events that pose a risk to human health has been almost impossible, given the size of some lakes and seas. An innovative satellite data service is now able to change things around. 81)
- Based on satellite remote-sensing data, CyanoLakes RealTime is an online monitoring and mapping service, designed by leading specialist scientists at CyanoLakes (Pty) Ltd. It significantly improves water and health authorities' ability to monitor, respond to and manage cyanobacteria, algal blooms and water weeds in both fresh and salt waters.
- Cyanobacteria blooms pose a serious health threat to humans and animals and are increasingly common due to pollution and a warming climate. Eutrophication can devastate natural ecosystems and increases the cost of water treatment.
- In October 2014, Mark Matthews won the Copernicus Masters Ideas Challenge for applications using satellites. He developed an algorithm able to distinguish between cyanobacteria and algae, which was recognized as a breakthrough in research and innovation, and also solved many of the challenges associated with using satellite data for routine monitoring applications.
- With his new algorithm, Dr. Matthews envisaged an online information service providing daily warnings on the health risks from cyanobacteria blooms. This would allow water and health authorities an unprecedented ability to monitor in near-real time for cyanobacteria and algal blooms, ultimately protecting the general public from this kind of pollution.
Figure 51: Products from Sentinel-3 and MERIS: The products from the maximum peak height algorithm include cyanobacteria, floating cyanobacteria also known as scum, and floating aquatic vegetation (image credit: the image contains MERIS imagery modified by CyanoLakes Pty Ltd.)
- In 2015, after being awarded with a research grant by the Water Research Commission, CyanoLakes (Pty) Ltd began working on a prototype for South Africa.
- The South African Department of Water and Sanitation became the first user of the prototype, using the information to fill gaps in their monitoring database and for reporting.
Figure 52: Cyanobacteria risk level map: The cyanobacteria risk level map of the CyanoLakes RealTime prototype service for 102 water bodies in South Africa (image credit: CyanoLakes (Pty) Ltd.)
- In January 2017, following the public release of data from the Copernicus Sentinel-3 satellite, the prototype started to be used in near realtime operations, enabling a variety of solutions for many fields.
- These applications included filling information gaps in data-poor regions for water scientists and engineers, improving the safety of water sport events, providing aquaculture operators with warnings of harmful algal blooms to reduce economic losses, and wide-scale monitoring and mapping of cyanobacteria blooms and eutrophication for water and health authorities.
Figure 53: Near real-time monitoring of chlorophyll-a: The CyanoLakes RealTime detailed viewer showing chlorophyll-a concentrations for the Vaal Dam, South Africa, on 08 October 2017 (image credit: the image contains modified Copernicus Sentinel data (2017), processed by CyanoLakes Pty Ltd.)
- The OLCI (Ocean and Land Color Instrument) on Sentinel-3A is currently the only sensor in space with the necessary spectral bands, radiometric sensitivity, spatial resolution and coverage for near realtime services related to the detection of cyanobacteria.
- Using the prototype service, the Department of Water and Sanitation was able to monitor the massive outbreak of the invasive water hyacinth at Hartbeespoort dam, which occurred during 2016–17.
- Dr Matthews said, "Sentinel-3 is the backbone of the CyanoLakes RealTime service, given its unique instrument characteristics. Without it, we could not provide our service to the market. We are excited about the launch of Sentinel-3B in 2018 because it will allow us to provide an even better service, with daily updates to clients anywhere around the globe."
Figure 54: Near real-time monitoring of water hyacinth: The CyanoLakes RealTime detailed viewer is showing water hyacinth (magenta) at Hartbeespoort Dam, South Africa, on 11 October 2017 (image credit: the image contains modified Copernicus Sentinel data (2017), processed by CyanoLakes Pty Ltd.)
• November 3, 2017: From the fourth most populous city to the rugged Outback, the Sentinel-3A satellite gives us a wide-ranging view over Australia’s southwestern corner. This perspective from space clearly illustrates human’s influence on our environment: the agricultural landscape that dominates in the lower-left is suddenly interrupted by the more densely vegetated national parks and forests.
- The city of Perth is located on the coast along the left edge of the image (Figure 55). About 150 km north of Perth sits ESA’s tracking station at New Norcia, where a 35 m diameter radio dish communicates with deep-space missions such as Rosetta and Mars Express.
- Moving further inland, grasslands give way to the deserts of Australia’s vast and remote interior – known as the Outback – with a landscape dominated by red soil and sparse vegetation. Several large salt lakes are visible across the image in white, including the appropriately named Lake Disappointment by explorer Frank Hann in search of fresh water (top of image).
- Clouds over the ocean obstruct our view of the southern coast, but the lack of cloud cover over the interior desert pronounces the dry climate, which is a consequence of global wind patterns.
- Sentinel-3 offers a ‘bigger picture’ for Europe’s Copernicus program by systematically monitoring Earth’s oceans, land, ice and atmosphere to understand large-scale global dynamics.
- While the satellite mission carries a suite of cutting-edge instruments, this image, also featured on the Earth from Space video program, was captured on 9 April 2017 by the satellite’s OLCI (Ocean and Land Color Instrument), which helps to monitor ocean ecosystems, supports crop management and agriculture, and provides estimates of atmospheric aerosol and clouds.
Figure 55: Sentinel-3A image of western Australia, acquired on 9 April 2017 with OLCI (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by ESA, CC BY-SA 3.0 IGO)
• October 12, 2017: The Copernicus Sentinel-3A satellite captured this image on 11 October 2017 (Figure 56), when Hurricane Ophelia was about 1300 km southwest of the Azores islands and some 2000 km off the African coast. 82)
- Originally classified as a tropical storm, it has been upgraded to a hurricane. The US National Hurricane Center said that Ophelia could become even stronger in the next days.
Figure 56: The image was acquired at 12:45 GMT on 11 Oct. 2017 by the satellite’s OLCI instrument (image credit: the image contains modified Copernicus Sentinel data (2017), processed by ESA, CC BY-SA 3.0 IGO)
• On 28 August, 2017, EUMETSAT’s near-real-time dissemination service went to the next level when EUMETSAT’s CODA (Copernicus Online Data Access) service, became operationally available to users via the new single-sign on option. 83)
- It ensures that CODA users can access both Copernicus and data from EUMETSAT’s Earth observation portal with one username and password.
- In combination with EUMETCast - a flexible multicasting service delivering the unified data streams from Copernicus and EUMETSAT’s own missions, as well as EUMETview - an interactive visualization service especially for satellite imagery, EUMETSAT’s data services provide solutions for a variety of different needs:
a) Sourcing Copernicus Data with CODA: EUMETSAT’s CODA service is a rolling archive featuring a month’s worth of Sentinel-3 data through an uncomplicated web interface as well as a scripting service, which allows users to automate bulk data downloads (within certain parameters).
After an extensive pilot phase, the CODA service is now fully available to users. CODA is particularly relevant for the ocean and remote sensing scientists, but its benefits reach beyond the scientific community. Developers in the public and private sector, be it for products or information services, can use CODA to develop innovative applications.
Hayley Evers King (Plymouth Marine Laboratory) summarizes her experience: “CODA is ideal for our daily business. It allows us to investigate specific areas and locate data for a particular region anywhere on the globe. This is, for example, useful when spotting algae blooms. The handling is particularly easy and follows a streamlined, user-friendly process. CODA allows us to select data without needing much experience. This is immensely helpful.”
b) Data dissemination via EUMETCast: The vast majority of marine data from the Copernicus-3A satellite, operated by EUMETSAT on behalf of the European Union, are now available on EUMETCast. With this milestone, EUMETSAT’s flexible multicasting service now delivers unified data streams to Copernicus users integrating observations from Copernicus and its own missions. This new marine data stream, involving products from Sentinel-3A, Jason-3, Metop and Meteosat creates a broad range of opportunities for the downstream development of applications, services and – ultimately – added value in Europe.
For Hayley Evers King (Plymouth Marine Laboratory) EUMETCast is important because “... it allows us to routinely and quickly access large amounts of data. We use it together with CODA and EUMETview; having these various sources of data access will increase the number of users for Copernicus data.”
• August 25, 2017: The Copernicus Sentinel-3A satellite saw the temperature at the top of Hurricane Harvey on 25 August 2017 at 04:06 GMT as the storm approached the US state of Texas. The brightness temperature of the clouds at the top of the storm, some 12–15 km above the ocean, range from about –80°C near the eye of the storm to about 20°C at the edges. 84)
- Hurricanes are one of the forces of nature that can be tracked only by satellites, providing up-to-date imagery so that authorities know when to take precautionary measures. Satellites deliver information on a storm’s extent, wind speed and path, and on key features such as cloud thickness, temperature, and water and ice content.
Figure 57: SLSTR (Sea and Land Surface Temperature Radiometer) image of Hurricane Harvey, acquired on 25 Aug. 2017 at 04:06 GMT, approaching the coast of Texas (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by ESA, CC BY-SA 3.0 IGO)
• August 11, 2017: Southern Europe is in the grip of a relentless heatwave, fuelling wildfires and water shortages. Information from the Copernicus Sentinel-3A satellite has been used to map the sweltering heat across the region. 85)
- The map of Figure 58 shows that on 7 August 2017, temperatures of the land surface rose above 40°C – not an usual occurrence over the last weeks. Much of Italy, including Rome, Naples, Florence, Sardinia and Sicily has been suffering these highs. With numerous towns and cities on the ministry of health’s maximum heat alert, the Italians have aptly dubbed the heatwave ‘Lucifer’. Extreme temperatures have also been recorded in Spain and Portugal, the Balkans and Greece.
- As well as wildfires and water shortages, the heat has also led to some tourist attractions being closed, ill health and even some fatalities, and the drought is also threatening crops.
- The map uses data from the satellite’s SLSTR (Sea and Land Surface Temperature Radiometer), which measures energy radiating from Earth’s surface in nine spectral bands – the map therefore represents temperature of the land surface, not air temperature which is normally used in forecasts. The white areas in the image are where cloud obscured readings of land temperature.
Figure 58: Southern Europe is in the grip of a heatwave, fuelling wildfires and water shortages. Information from the Copernicus Sentinel-3A satellite has been used to map the sweltering heat across the region (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by ESA, CC BY-SA 3.0 IGO)
• July 6, 2017: With the Copernicus Sentinel-3A satellite fully fledged and its data freely available, the task of monitoring and understanding our changing planet has been made that much easier. Seeing the effect spring has on our plant life is just one of its many uses. — Launched in February 2016 and carrying a suite of instruments, Sentinel-3 is the most complex of all the Sentinel missions. 86)
- As the workhorse mission for Europe’s environmental monitoring Copernicus program, it measures Earth’s oceans, land, ice and atmosphere systematically so that large-scale global changes can be monitored and understood. While Sentinel-3 offers this ‘big picture’, it can also be used to monitor smaller-scale environmental issues such as urban heat islands.
- Sentinel-3 is well on the way to being at the heart of operational oceanography, but it also provides unique and timely information about changing land cover and vegetation health.
- For instance, the animation of Figure 59 uses information from the satellite’s ocean and land color instrument to measure changing amounts of chlorophyll in plants. Here we clearly see the progress of spring greening in the northern hemisphere, for example.
Figure 59: The Copernicus Sentinel-3A’s ocean and land color instrument can ‘see’ chlorophyll in vegetation. The animation shows how chlorophyll, which is essential in photosynthesis, around the world changed between 1 April and 27 May 2017. While tropical rainforests can be seen to maintain a high degree of chlorophyll, the animation clearly shows the progress of spring greening in the northern hemisphere. This is particularly evident in the eastern part of the USA. It also captures the progress of agricultural planting for summer crops across China where planting normally takes place between March and May. Here various stages of growth are captured. The chlorophyll index ranges from 1 to 6.5 (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by University of Southampton–J. Dash/Brockman Consult (S3-MPC))
- Since its initial commissioning, when the satellite and instruments were meticulously fine-tuned, Sentinel-3A has been in a ‘ramp up’ phase. - This means that over the last year, while the satellite was being prepared for its life as a fully operational mission, only ‘direct instrument’ data were available. Another step in the processing chain is needed to translate them into more tangible information for users worldwide.
- This milestone has now been passed so that the best quality data possible are now freely available from the satellite’s ocean and land color instrument and from the sea and land surface temperature sensor, which measures energy radiating from Earth’s surface.
- This level of data from its other instrument – a radar altimeter, which measures the height of the sea surface, rivers, lakes and land – have been available since last December.
- ESA’s Sentinel-3 mission manager, Susanne Mecklenburg, explained, “Sentinel-3 is an extremely complex mission, and I’m very proud to say that it’s delivering on its promise.
Figure 60: Feeling the heat: Sentinel-3’s Sea and Land Surface Temperature Radiometer includes dedicated channels for measuring fires. This will help to map carbon emissions from burnt biomass and to assess damage and estimate recovery of burnt areas. Information to help manage forest fires will be available using Sentinel-3 measurements combined with meteorological forecasting data. In addition, forests can be monitored systematically to assess risk and develop efficient plans to prevent forest fires (image credit: ESA/ATG medialab)
- “We have been working closely with our colleagues at Eumetsat to make sure it is ready to deliver top-quality data. This is important because while Eumetsat operates the satellite, both organizations manage the mission together.
- “ESA is responsible for the land data products and Eumetsat for the marine products – all of which are made available for the Copernicus services and other users. Measurements made by the satellite’s color instrument over land now offer users key information to monitor the health of our vegetation, which is essential for agricultural practices, and to help plan resources. This also complements other missions such as the Copernicus Sentinel-2 and Proba-V. Together, they will be a powerful tool to map our changing lands.”
- Sentinel-3 shows how Earth’s surface temperature changes, which is also important for weather forecasting and for monitoring climate change. Over land, measurements can be used for urban planning, for example.
- Later in the year, data products will also be available for monitoring fires.
Figure 61: Sentinel-3A senses Earth’s heat: Information from Sentinel-3A’s radiometer, which measures radiation emitted from Earth’s surface, reveal how the temperature of Earth’s land changes between July and November 2016. Measurements are in kelvin (image credit: ESA, the image contains modified Copernicus Sentinel data (2016), processed by UK National Center for Earth Observation/University of Leicester)
• June 23, 2017: Sentinel-3 gives us a nearly cloud-free view of France and the surrounding countries (Figure 62). Much of the landscape is covered with agricultural features. In fact, farmers manage nearly half of Europe’s land area. While agriculture brings benefits for economy and food security, it puts the environment under pressure. Satellites can help to map and monitor land use, and the information they provide can be used to improve agricultural practices. 87)
- On the right side of the image we can see the snow-covered Alps, while the Pyrenees mountains are visible near the bottom.
- To the west of the Alps a green area of mountains and plateaus is visible, called the Massif Central. The region has more than 400 volcanoes, considered by scientists to be extinct.
- On the right side of the image, the light brown area flanked by dark areas is the Rhine River forming part of France’s border with Germany. The dark area to the east is the Black Forest, while the dark area to the west are the Vosges Mountains.
- Just above the center of the image, we can see Paris – the site of ESA’s headquarters as well as the Paris Air & Space Show taking place this week.
Figure 62: This image of France was captured by the Copernicus Sentinel-3A satellite’s OLCI (Ocean and Land Color Instrument) on 7 April 2017. OLCI monitors ocean ecosystems, supports crop management and agriculture, and provides estimates of atmospheric aerosol and clouds – all of which bring significant benefits through more informed decision-making (image credit: ESA, this image contains modified Copernicus Sentinel data (2017), processed by ESA, CC BY-SA 3.0 IGO)
• April 27, 2017: The Copernicus Sentinel-3A satellite brings us over the Bering Sea, north of the Alaska Peninsula, on 26 March. Seasonal sea ice dominates the upper part of the image. Ice plays an important role in the sea’s ecosystem. Growing algae attach to the bottom of the ice; when the ice melts in the spring, it leaves behind a layer of nutrient-rich freshwater on which the algae thrive. Organisms higher up the food chain then eat the algae. 88)
- In the top-right corner of Figure 63, we can see part of Alaska’s mainland blanketed with snow, as well as Nunivak Island appearing like a massive piece of floating ice. At the center of the image are the islands of Saint Paul and Saint George – part of the Pribilof Islands. An estimated two million seabirds nest on these islands annually.
- The swirling clouds on the right side of the image are the result of a meteorological phenomenon known as a von Kármán vortex street. As wind-driven clouds pass over the Unimak Island on the right edge of the image, they flow around the high volcanoes to form the large spinning eddies that can clearly be seen in the image.
• March 10, 2017: A Sentinel-3 image in the Russian Far East and of the Kamchatka Peninsula is provided , located between the Pacific Ocean to the east and the Sea of Okhotsk to the west, where clouds blend with the ice and snow beneath from the bird’s-eye view (Figure 64). One of the fascinating features is the pattern of floating sea ice, appearing in light blue. Along the left, one can see cracks in the ice covering the water. In the middle/right, small pieces of fragmented ice, driven by wind and currents, create the swirls of blue along the coast of the Kamchatka Peninsula. 89)
- Kamchatka, a 1250 km long peninsula with an area of 270, 000 km2, has a landscape covered with volcanoes due to its location along the highly active Pacific ‘Ring of Fire’. There are about 160 volcanoes on the peninsula, 29 of which are still active. The central mountain range running down the spine of the peninsula, is visible in the image, while the eastern range is mostly covered by clouds. Between them lies the central valley, appearing somewhat brown from the lack of snow cover.
- It is no surprise that the area is often referred to as the ‘land of fire and ice’. Owing to minimal development, the peninsula is known for its abundance of large brown bears. Other common animals include foxes, wolves, reindeer and wolverines.
Figure 64: The OLCI (Ocean and Land Color Instrument) on Sentinel-3A image of the Kamchatka Peninsula (on the right side) was acquired on Feb. 15, 2017 (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by ESA)
• December 13, 2016: Launched in February 2016 with a suite of cutting-edge instruments, Sentinel-3A is arguably the most comprehensive of all the Copernicus Sentinel missions. Since then, the satellite has been thoroughly tested and fine-tuned. This led to the release of its first Earth color data in October and first radiometer data last month. Now, the public also have access to data from its radar altimeter. 90)
- Sentinel-3A’s topography package will bring a step change in satellite altimetry, measuring the height of the sea surface, waves and surface wind speed over the oceans. It also provides accurate topography measurements over sea ice, ice sheets, rivers, lakes and land. Over the oceans, the radar altimeter contributes information for forecasting, which is essential for safe maritime operations, for example. Monitoring sea-level change and diminishing Arctic ice is also important for monitoring the effects brought about by climate change.
- As the image of Antarctica shows (Figure 65), the radar altimeter is also important for measuring changes in the height of land ice. The data may seem relatively sparse at the moment, but this is because they only show a few days’ readings. Accurately measuring changes in the height of the huge ice sheets that blanket Antarctica and Greenland is important for climate research and understanding sea-level change.
- ESA’s CryoSat mission currently measures changes in ice height and paved the way for Sentinel-3’s radar altimeter. Importantly, Sentinel-3’s radar altimeter is the first to provide 100% coverage over all of Earth’s surfaces in ‘synthetic aperture radar’ mode. For accuracy, Sentinel-3’s topography package also includes a microwave radiometer that is used to correct measurements from the radar altimeter affected by water vapor in the atmosphere.
- While changes in ice height may be relatively slow, the radar altimeter will also be used to measure changes that can be more abrupt, such as the height of water in lakes and rivers.
- The Sentinel-3 mission is managed jointly by ESA and EUMETSAT. The day-to-day operations of the Sentinel-3A satellite are carried out by EUMETSAT. ESA, as the developer of the mission, continues to monitor its health and performance. ESA is responsible for the land data products and EUMETSAT for the marine products – all of which are made available for application through Copernicus services.
Figure 65: The SRAL (SAR Radar Altimeter) of the Sentinel-3A spacecraft measured the height of the Antarctic ice sheet (image credit: ESA, the image contains modified Copernicus Sentinel data (2015), processed by UCL–MSSL)
• November 17, 2016: Following the release of Sentinel-3A’s first Earth color data, the public now have access to data from the satellite’s radiometer, which measures energy radiating from Earth’s surface in nine spectral bands. Arguably the most comprehensive of all the Copernicus Sentinel missions, Sentinel-3A carries a suite of state-of-the-art instruments to systematically measure Earth’s oceans, land, ice and atmosphere. 91)
- Since it was launched in February 2016, the satellite has been thoroughly tested and fine-tuned. This led to the release of first-level data from its OLCI (Ocean and Land Color Instrument) in October and now data from the SLSTR (Sea and Land Surface Temperature Radiometer) are also available.
- Information from the radiometer will be used to create global maps of SST (Sea Surface Temperature) for ocean and weather forecasting. Over land, the instrument will be used, for example, to detect heat stress, which is useful for improving agricultural practices and monitoring urban heat islands. As cities continue to expand, understanding how heat islands develop is important for planners and developers.
- Importantly, the radiometer has dedicated channels for measuring fires. This will help to assess damage and estimate recovery of burned areas.
- Further processing is needed to turn this kind of data (Figure 66) into actual ocean- and land-surface temperature maps. These next-stage data will start to be released in early 2017. Nevertheless, differences between the land, coasts and sea can be seen clearly in this brightness temperature image. — Data from the satellite’s radar altimeter will be made available in December.
- While the day-to-day operations of the Sentinel-3A satellite are carried out by EUMETSAT, the mission is managed jointly by ESA and EUMETSAT. ESA is responsible for the land data products and EUMETSAT for the marine products – all of which are made available for application through Copernicus services.
Figure 66: The image, which stretches from northern France and Belgium to Italy and western Greece, is an example of first-level data from the radiometer. It shows ‘brightness temperature’, which corresponds to radiation emitted from the surface (image credit: ESA, the image contains modified Copernicus Sentinel data (2015), processed by ESA)
• October 20, 2016: Today, the Copernicus Sentinel-3A satellite has taken another step towards being fully ‘operational’ as the first data from its OLCI (Ocean and Land Color Instrument) are made available to monitor the health of our planet. Following its launch in February, the satellite and instruments have been thoroughly tested and fine-tuned – leading to this important milestone. - Carrying a suite of instruments, Sentinel-3A is arguably the most complex of all the Copernicus Sentinels. 92) 93)
- It has been designed to measure Earth’s oceans, land, ice and atmosphere to monitor large-scale global dynamics and to provide critical near-realtime information for numerous ocean, land and weather applications.
- The Sentinel-3 validation team, a group of expert users, has been receiving sample products since May. Their feedback is essential to both ESA and EUMETSAT to ensure the data are of the highest quality, as is needed for the myriad of operational applications that the mission will serve.
- At the ‘end of commissioning’ review in July, it was noted that a couple of points had to be addressed before the first data were officially released to the public.
- Susanne Mecklenburg, ESA’s Sentinel-3 mission manager, said, “It is imperative that these first-level data are the best quality possible so we are being extremely careful. It is now very gratifying to see data from the satellite’s Ocean and Land Color Instrument being released to users worldwide. “Data from the other two instruments – the SLSTR (Sea and Land Surface Temperature Radiometer) and SRAL (SAR Radar Altimeter) – will be made available in November and December, respectively.”
- Offering new eyes on Earth, the OLCI (Ocean and Land Color Instrument) will monitor the global oceans, and inland waters, including phytoplankton, water quality, harmful algal blooms, sediment transport in coastal areas, El Niño and La Niña events, and climate change. It will also support observations of vegetation and crop conditions, as well as provide estimates of atmospheric aerosol and clouds – all of which bring significant benefits to society through more informed decision-making.
- While the operations of the Sentinel-3A satellite are carried out by EUMETSAT, the mission is managed jointly by ESA and EUMETSAT. ESA is responsible for the land data products and EUMETSAT for the marine products – all of which are made available for application through Copernicus services.
- Hilary Wilson, EUMETSAT's Sentinel-3 project manager, said, “The release of Sentinel-3A’s first operational data is the culmination of a lot of hard work by ESA, EUMETSAT and the expert user teams. It represents an important milestone for the Copernicus Marine Environment Monitoring Service and also for the wider marine monitoring community. Routine operations of the satellite have been proceeding smoothly since EUMETSAT took over this responsibility in July and we are now focusing on bringing the remaining marine products to this community.”
Figure 67: These two images, taken on 13 June 2016 and 22 August by Sentinel-3A’s OLCI (Ocean and Land Color Instrument), show differences in ice cover in northeast Greenland. Differences in sea ice off the coast are clear to see. (image credit: ESA, the image contains modified Copernicus Sentinel data (2016), processed by ESA). 94)
• October 2016: Commissioning phase results of the Sentinel-2A Optical Payloads. 95)
Table 6: Optical CalVal activities of the Sentinel-3A spacecraft launch on Feb. 16, 2016 from Plesetsk/Russia Cosmodrome
With the successful launch of Sentinel-3A, a new era for the Copernicus Services has started offering data over oceans and lands with unprecedented coverage. Together with Sentinel-3B, its twin satellite scheduled for launch in 2017, and later on with the launch/replacement of the Sentinel-3C and D units, a 20-year period of continuous observations is guaranteed. Among the five instruments embarked, the OLCI and SLSTR optical payload ensure the continuity of the ENVISAT mission with very much improved performance. During the calibration validation (CalVal) phase functional, performance, product verification and validation were performed confirming the overall excellent performance of the optical payload.
Figure 68: This image of Europe was taken by Sentinel-3A’s OLCI on 16 October 2016. The framed part of the image shows, for example, how its 1270 km-wide swath captures an area stretching from Spain to Italy. The ‘zoom in’, depicted in Figure 69, shows the French landscape in detail and changes in water color off the south coast. The OLCI features 21 distinct bands in the 400–1020 µm spectral region tuned to specific ocean color, vegetation and atmospheric correction measurement requirements. As well as its wide swath, it has a spatial resolution of 300 m for all measurements, overlapping the satellite’s SLSTR (Sea and Land Surface Temperature Radiometer) swath (image credit: ESA, the image contains modified Copernicus Sentinel data (2016), processed by ESA)
Figure 69: Zoom in image from Figure 68. The islands of Corsica and Sardinia can be seen in the west with coast of Tuscany and the island of Elba to the northeast. The waters along the east coast of Corsica and along the Italian coast are colored by discharge from the land following recent heavy rainfall (image credit: ESA, the image contains modified Copernicus Sentinel data (2016) processed by EUMETSAT) 96)
• September 30, 2016: The SLSTR (Sea and Land Surface Temperature Radiometer) visible channels onboard Sentinel-3A were turned on from 2nd March 2016 and the infrared channels on the 23rd March 2016. The first level 1 (L1b) data was released to expert and validation users on the 14th June 2016, with the level 2 (L2) data similarly released on the 21st June 2016. A successful commissioning review was held on the 12th July 2016, following this EUMETSAT resumed operations of the Sentinel-3A satellite. EUMETSAT processes Sentinel-3 marine data and products at its Sentinel-3 Marine Center, for real time delivery to end-users. 97)
- SST (Sea Surface Temperature) from SLSTR provides increased global coverage than AATSR due to an increased swath width (up to 1400 km) for both nadir only and dual (740 km) view scans. An example of the daily SST global coverage from Sentinel-3A for one day is shown in Figure 70.
• Sept. 20, 2016: Wildfires break out in the boreal forests of eastern Russia most summers, but 2016 has been particularly bad, with numerous blazes since July. The image of Figure 71, which was taken by the Copernicus Sentinel-3A satellite on 14 September, shows smoke billowing from a string of fires northwest of Lake Baikal in Siberia. These huge smoke plumes stretch over 2000 km. It is thought that drier conditions associated with warmer weather – this June being the hottest on record – have contributed to the unusually large number of fires. 98)
Figure 71: As numerous wildfires continue to burn in Siberia, the Copernicus Sentinel-3A satellite has captured images of huge smoke plumes, acquired on Sept. 14, 2016, stretching 2000 km and winds blowing the smoke to the west (image credit: ESA, the image contains modified Copernicus Sentinel data (2016), processed by ESA)
• July 13, 2016: Getting the bigger picture on the health of our planet drew another step closer today as Europe’s Sentinel-3A satellite was handed over to EUMETSAT for operations. -Since it was launched in February, the satellite and its instruments have been meticulously fine-tuned to make sure that everything is fit and ready for the task in hand: to systematically map Earth’s surface for a myriad of services related to both the oceans and land. 99) 100)
- ESA’s Bruno Berruti has been responsible for taking Sentinel-3A from the drawing board and into orbit ready for service. He said, “As the last phase of the ‘project’, the handover signals the end to an intense five months during which we ensured the satellite and instruments are all working well so that they can start delivering routine data.”
- “After only five months of commissioning, we have already released samples for most types of data products. The coming months will see a gradual ramp-up of our processing and data dissemination activities to make sure that the user community is served in the best possible way. The intention is to release first operationally qualified data products to all users in September.”
- So while Sentinel-3A is well on the road to start delivering data that is expected to make unique contribution to the paradigm shift in monitoring our planet, ESA remains busy preparing its identical twin for launch in 2017.
Figure 72: The animation shows the difference in the day and night temperatures of the land surface. These daytime measurements were taken by Sentinel-3A’s radiometer on 20 June 2016 and the night time readings were taken on 21 June (image credit: ESA, the image contains modified Copernicus Sentinel data (2016))
Figure 73: Baltic swirls - captured by Sentinel-3A on 23 June 2016, this image shows an algae bloom in the Baltic Sea. The image was captured with its OLCI (Ocean and Land Color Instrument), which provides biogeochemical measurements to monitor, for example, concentrations of algae, suspended matter and chlorophyll in seawater. The colored tracks in the image are temperature measurements from a zeppelin, which was used as part of HZG (Helmholtz-Zentrum Geesthacht) Clockwork Ocean project. Satellite imagery was used to help locate these eddies (image credit: ESA, the image contains modified Copernicus Sentinel data (2016)/HZG)
• April 6, 2016: Despite only being in orbit a matter of weeks, Sentinel-3A has already delivered some impressive first images. With the thermal-infrared channels now turned on, the satellite completes its set of firsts with a view of ocean features off the coast of Namibia. 101)
- The first image from the Sentinel-3A SLSTR (Sea and Land Surface Temperature Radiometer) thermal-infrared channels depicts thermal signatures over a part of western Namibia and the South Atlantic Ocean. This image shows the ‘brightness temperature’, which corresponds to radiation emitted from the surface. Further processing is needed to turn this into an actual temperature map. The Namibian land surface is shown in red–orange colors, corresponding to a temperature range 301–319 K. The blue colors over the ocean correspond to a temperature range of 285–295 K. The black areas correspond to clouds, which are opaque to thermal-infrared radiation and so prevent a view of the ocean or land surface.
- Cold water is seen along the Namibian coast upwelling from deeper waters. The Benguela current flows north along the west coast of South Africa driven by southeasterly winds creating coastal upwelling. Many eddies and meanders are generated in this complex system and these small-scale features are captured beautifully by SLSTR. Understanding changes in the pattern of these waters is important for fisheries, for example.
Figure 74: Thermal signature of the Namibian coastline observed by SLSTR on Sentinel-3A (image credit: ESA, the image contains modified Copernicus Sentinel data (2016))
• April 1, 2016: The new Sentinel-3A satellite recently began providing data from orbit. This very early image recorded on 3 March 2016, takes us over the River Nile and Delta and the surrounding desert areas of northeast Africa and parts of the Middle East. Very distinct is Egypt, a country connecting northeast Africa with the Middle East, home to millennia-old monuments still sitting along the lush Nile valley. 102)
- In the center of the image, the capital city Cairo with the Nile snaking northwards is clearly visible, along with the Red Sea just further east. Also evident are the islands of Cyprus further north in the Mediterranean Sea and parts of Crete on the very left. Portions of southern Turkey are also visible including some islands of the Aegean Sea.
Figure 75: This SLSTR (Sea and Land Surface Temperature Radiometer) image of Sentinel-3A was acquired on March 3, 2016 showing the River Nile and the extensive Nile Delta (image credit: ESA, the image contains modified Copernicus Sentinel data , processed by ESA)
Legend to Figure 75: The false color image of SLSTR measures the energy radiating from Earth’s surface in nine spectral bands, including visible and infrared bands.
• March 8, 2016: The three instruments on the Sentinel-3A satellite are now offering a tantalizing glimpse of what’s in store for Europe’s Copernicus environmental monitoring effort. The latest images, which feature Europe and Antarctica, come from the sensor that records Earth’s radiant energy. Launched just three weeks ago, Sentinel-3A carries a suite of cutting-edge instruments to provide systematic measurements of Earth’s oceans, land, ice and atmosphere. This information will feed into numerous Copernicus services to monitor and manage our environment. 103)
- The SLSTR (Sea and Land Surface Temperature Radiometer) measures the energy radiating from Earth’s surface in nine spectral bands, including visible and infrared. In addition to providing the temperature of the land and sea surface, dedicated channels will search for fires. This will help to map carbon emissions from burnt biomass and to assess damage and estimate recovery of burned areas. The first images come from the visible channels because the thermal-infrared channels have yet to be activated (Figure 76).
- Another of the images from its visible channels of SLSTR shows a long crack running through the ice shelf to the east of the Antarctic Peninsula (Figure 77).
- An additional false-color image, captured on 2 March, features a large part of Europe, demonstrating the instrument’s 1400 km-wide swath. It also shows vegetated areas in red as well as storm Jake over the UK (Figure 78).
Figure 76: This is one of the first images from Sentinel-3A’s SLSTR (Sea and Land Surface Temperature Radiometer), acquired with the instrument’s visible channels on 3 March 2016 at 11:23 GMT (image credit: Copernicus data (2016))
Legend to Figure 76: This false-color image features the Spanish Canary Islands, the Portuguese island of Madeira and the northwest coast of Africa. The vegetated islands appear red in contrast to the Western Sahara, which has little vegetation. The snow-capped peak of Mount Teide on the island of Tenerife is clearly visible. Both SLSTR and Sentinel-3’s OLCI (Ocean and Land Color Instrument) will be used to monitor plant health. As the SLSTR scans Earth’s surface, it senses visible light and infrared light (heat) in a number of different spectral channels. The thermal infrared channels will soon be working when the instrument has finished outgassing the water vapor. This is necessary because the infrared channels must be cooled to operate properly. The SLSTR will measure global sea- and land-surface temperatures every day to an accuracy of better than 0.3ºC.
Figure 77: Another early image of SLSTR shows a long crack running through the ice shelf to the east of the center part of the Antarctic Peninsula. The crack is about 2 km wide, but widens to 4 km or more in some places. There are also finer cracks and structures visible in the ice shelf. Structure in the cloud, cloud shadows and details of the land emerging from the ice can also be seen. The image was acquired on 3 March 2016 at 11:53 GMT with the instrument’s visible channel (image credit: Copernicus data (2016)
Figure 78: Acquired with the SLSTR instrument's visible channels on 2 March 2016 at 10:04 GMT, this false-color image features a large part of Europe showing vegetated areas in red. Moreover, the image demonstrates the instrument’s 1400 km-wide swath. The image also clearly shows storm Jake over the UK (image credit: Copernicus data (2016))
• March 4, 2016: Just after the SRAL (SAR Radar Altimeter) instrument on Sentinel-3A, it traced the height of the sea surface over a stretch of the North Atlantic, some of the most dynamic ocean waters in the world. Showing features relating to the Gulf Stream, the track compares very well with the background map of sea-surface height. The map (Figure 79), produced by the CMEMS (Copernicus Marine Environment Monitoring Service), comprises near-realtime data for one day from the CryoSat-2, Jason-2 and SARAL/AltiKa satellites. 104)
- The altimeter is designed to deliver accurate measurements of sea-surface height, significant wave height and surface-wind speeds over the world’s oceans for Copernicus ocean forecasting systems and for monitoring sea-level change.
- Pierre-Yves Le Traon from Mercator Ocean said, “These first results are very promising and illustrate the great potential Sentinel-3 has for the CMEMS. Sea-surface height data from the satellite’s altimeter will, for example, significantly improve our capability to analyze and forecast ocean currents. This is essential for the applications we serve such as marine safety, ship routing and predicting the fate of marine pollution events.”
- The altimeter has heritage from the CryoSat-2 and Jason-2 missions. This first image is in low-resolution mode but it will provide measurements at a resolution of approximately 300 m in the along-track direction after processing. SRAL will be the first satellite altimeter to provide 100% coverage over all of Earth’s surfaces in ‘synthetic aperture radar mode’, directly resulting from experience with CryoSat-2.
Figure 79: This is the first track measured by Sentinel-3A’s SRAL (SAR Radar Altimeter) immediately after it was switched on. The track, which captures features in the Gulf Stream current, compares well to the background data that comprises near-realtime data from the CryoSat-2, Jason-2 and Altika satellites (image credit: Copernicus data (2016)/CMEMS)
• March 2, 2016: Featuring Spain, Portugal and North Africa, this is one of the first images from the Sentinel-3A satellite. The image was taken by the satellite’s OLCI (Ocean and Land Color Instrument) on 1 March 2016 and clearly shows the Strait of Gibraltar between the Atlantic and Mediterranean. Swirls of sediment and algae in the seawater can be seen along the southwest coast of Spain and along the coast of Morocco. The instrument picks out Morocco’s dry desert and snow-covered peaks of the Atlas Mountains and greener vegetated northern areas of Spain. 105)
Figure 80: OLCI image of Sentinel-3 acquired on March 1, 2016 showing the Strait of Gibraltar, Spain Portugal and North Africa (image credit: ESA)
• Feb. 25, 2016: Each year, about a quarter of the carbon dioxide we release into the atmosphere ends up in the ocean, but how it happens is still not fully understood. The Sentinel-3A satellite is poised to play an important role in shedding new light on this exchange. 106)
- Initially, the fact that the oceans are absorbing a significant amount of the carbon dioxide we pump into the atmosphere by burning biomass and fossil fuels would appear to be a good thing. However, as more carbon dioxide dissolves into the oceans, it leads to ocean acidification, making it difficult for some marine life to survive.
- Monitoring and understanding the carbon cycle is important because carbon is the fundamental building block of all living organisms. Also, the process of carbon moving between the oceans, atmosphere, land and ecosystems helps to control our climate.
- Over the last four years an international team of scientists and engineers have been using satellites along with measurements from ships and pioneering cloud computing techniques to study how carbon dioxide is transferred from the atmosphere into the oceans. Their work reveals that the seas around Europe absorb an astonishing 24 million tonnes of carbon each year. This is equivalent in weight to two million double decker buses or 72 000 Boeing 747s. 107)
- The team are making their data and cloud computing tools, the ‘FluxEngine’, available to the international scientific community so that other groups can analyse the data for themselves.
• Feb. 25, 2016: Working around the clock, mission teams have brought Sentinel-3A through the critical LEOP (Launch and Early Orbit Phase) in just 49 hours, much earlier than planned and a record for such a complex satellite. LEOP was completed on Feb. 18. All operations were executed on time, and the satellite and ground systems performed perfectly during the whole period. 108)
- The speedy completion of LEOP means that the five-month commissioning phase has already started, and Sentinel-3 project and operations teams will start validating the correct functioning of spacecraft and its payload. -In parallel, teams at ESA/ ESRIN in Frascati are working on configuring and validating the complex data delivery channels and bringing these into full service.
- In the afternoon of 23 February, Sentinel-3A tested its first delivery of science data, downlinking test data from the few instruments already switched on, including the SLSTR (Sea and Land Surface Temperature Radiometer). The 10-minute downlink was conducted using the satellite’s X-band radio transmitter, confirming its ability to deliver high-rate data via the mission’s designated ground station, at Svalbard on the Norwegian archipelago of Spitsbergen.
• After a first burn starting about five minutes after liftoff and a second about 70 minutes later, Rockot’s upper stage delivered Sentinel-3A into its planned orbit, 815 km above Earth. The satellite separated 79 minutes into the flight (Ref. 31).
- The first signal from Sentinel-3A was received after 92 minutes by the Kiruna station in Sweden. The telemetry links and attitude control were then established by controllers at ESA/ESOC in Darmstadt, Germany, allowing them to monitor the health of the satellite.
- The mission is the third of six families of dedicated missions that make up the core of Europe’s Copernicus environmental monitoring network. Copernicus relies on the Sentinels and contributing missions to provide data for monitoring the environment and supporting civil security activities. Sentinel-3 carries a series of cutting-edge sensors to do just that.
Figure 81: Operations image of the week: The men and women now flying Sentinel-3A comprise a ‘team of teams’ who specialize in areas such as mission operations, flight dynamics and ground stations (image credit: ESA) 109)
Legend to Figure 81: This ‘team of teams’ involves some 50 engineers and scientists at ESOC, including spacecraft engineers, specialists working on tracking stations and the sophisticated ‘ground segment’ – the hardware and software used to control the satellite and distribute its data – and experts working in flight dynamics, software and networks, as well as simulation and training teams.
Representatives from ESA’s Sentinel project team, as well as several operations engineers integrated within the Flight Control Team and shared with EUMETSAT, are also working to ensure the success of this crucial mission.
Copernicus: Sentinel-3 Continued
Sensor complement (optical payload, topographic payload)
In the context of GMES (Global Monitoring for Environment and Security), the objectives of the Sentinel-3 mission, driven by ESA and the user community, encompass the commitment to consistent, long-term collection of remotely sensed data of uniform quality in the areas of sea / land topography and ocean color. Measurements over oceans will be provided jointly with other operational missions, such as the Jason series, to contribute to the realization of a permanent Global Ocean Observing System (GOOS). Regarding ice, it is foreseen to monitor land ice (also denoted as ice sheet) including ice margins and sea ice. At last, measurements over rivers and lakes will help in the water level monitoring of spots of interest throughout the world.
Sentinel-3 will support primarily services related to the marine environment, such as maritime safety services that need ocean surface-wave information, ocean-current forecasting services that need surface-temperature information, and sea-water quality and pollution monitoring services that require advanced ocean color products from both the open ocean and coastal areas. Sentinel-3 will also serve numerous land, atmospheric and cryospheric application areas such as land-use change monitoring, forest cover mapping and fire detection. 110)
Figure 82: Sentinel-3 spacecraft with payload layout (image credit: ESA)
Figure 83: FOVs (Field of Views) of the Sentinel-3 instruments (image credit: ESA)
Optical payload (OLCI, SLSTR)
The primary mission objective of the optical payload is to ensure the continuation of the successful Envisat observations of MERIS for ocean color and land cover and AATSR for sea surface temperature. In addition, due to the overlapping field of view from both optical sensors, new applications will emerge from the combined exploitation of all spectral channels.
OLCI (Ocean and Land Color Instrument):
OLCI is a medium resolution pushbroom imaging spectrometer of MERIS heritage, flown on Envisat, but with a slightly modified observation geometry: the FOV (Field of View) is tilted towards the west (~ 12º away from the sun), minimizing the sun-glint effect over the ocean and offering a wider effective swath (~ 1300 km, overall FOV of 68.6º). The sampling distance is 1.2 km over the open ocean and 0.3 km for coastal zone and land observations. The instrument mass is ~ 150 kg, a size of 1.24 m x 0.83 m x 1.32 m, the power demand is 124 W; it has been designed and developed at Thales Alenia Space España.
The FOV of OLCI is divided between five cameras on a common structure with the calibration assembly. Each camera has an optical grating to provide the minimum baseline of 16 spectral bands required by the mission together with the potential for optional bands for improved atmospheric corrections.
The OLCI bands are optimized to measure ocean color over open ocean and coastal zones. A new channel at 1.02 µm has been included to improve atmospheric and aerosol correction capabilities. Two additional channels in the O2A absorption line (764.4 and 767.5 nm, in addition to the existing channel at 761.25 nm) are included for improved cloud top pressure (height) with an additional channel at 940 nm in the H2O absorption region, to improve water vapor retrieval. A channel at 673 nm has been added for improved chlorophyll fluorescence measurement.
Table 7: OLCI band specifications, in cyan MERIS heritage, in yellow additional bands 114)
Figure 84: The OLCI instrument supports a re-definition of its spectral bands through a programmable acquisition design to support a high-degree of flexibility during the mission (image credit: ESA)
Figure 85: Schematic view of the OLCI instrument configuration (image credit: ESA)
Each camera is constituted of a Scrambling Window Element to comply with the polarization requirement, a COS (Camera Optical Sub-assembly) for the spectral splitting of the different wavelengths, a FPA (Focal Plane Assembly) with a CCD for the signal detection and a VAM (Video Acquisition Module) for the monitoring of the analog signal. The optical sub-assembly of each camera includes its own grating and provides the 21 spectral bands required by the mission in the range 0.4-1.0 µm. 115)
The control of the instrument assembly is realized by a CEU (Common Electronic Unit), which assumes the function of instrument control, power distribution and digital processing.
A calibration assembly, including a rotation wheel with five different functions for normal viewing, dark current, spectral and radiometric calibrations insures the calibration of the instrument. The calibration wheel has 5 positions: 116)
• The Earth Observation aperture
• The Shutter, blocking incoming light it allows dark offset acquisition (and gives the calibration zero)
• The nominal radiometric diffuser, from which calibration gains are derived
• The reference radiometric diffuser, dedicated to the monitoring of the nominal diffuser ageing
• The spectral diffuser allowing spectral calibration at 3 wavelengths.
Figure 86: Schematic view of the OLCI observation geometry with the 5 camera assembly (image credit: ESA)
Compared to ENVISAT, the following improvements have been implemented:
• Along-track SAR capability for coastal zones, inland water and sea-ice altimetry
• Off-nadir tilted field of view for OLCI cameras to minimize sun-glint contamination (i.e. loss of ocean color data)
• New spectral channels in OLCI and SLSTR allowing improved retrieval of geophysical products and detection of active fire
• Synergy products (e.g. vegetation) based on combination of OLCI and SLSTR data (OLCI swath fully covered by SLSTR swath).
In each camera unit, Earth light enters a calibration assembly which includes filters as well as spectral calibration sources which are PTFE (Polytetrafluorethylen, also known as Teflon) sun diffusers to track spectral calibration, gain calibration and instrument aging effects. Next, light passes through the scrambling window assembly featuring a scrambling window and an inverse filter.
The camera optics subassembly houses all optics needed for the separation of spectral bands and focusing the light onto the detector. After passing through the ground imager optics, the light is dispersed by a diffraction grating and passed onto the CCD detector system building the centerpiece of the focal plane assembly.
The spectrometer generates a dispersed image of the entrance slit onto a two-dimensional detector where one dimension is the spatial extension of the slit and the other is the spectral dispersion of the slit image created by the grating.
Figure 87: OLCI optical design (image credit: ESA)
Figure 88: Photo of the OLCI engineering model (image credit: ESA, TAS)
Table 8: Technical characteristics of the Sentinel-3 OLCI instrument
SLSTR (Sea and Land Surface Temperature Radiometer):
SLSTR is an upgraded and advanced version of the AATSR instrument on Envisat, offering a wider swath which completely overlaps the OLCI swath, as required to produce accurate vegetation products. The SLSTR is designed for ocean and land-surface temperature observations. Unlike AATSR, SLSTR has a double-scanning mechanism, yielding a much wider swath stretching almost from horizon to horizon. The OLCI and SLSTR swaths are overlapping broadly, yielding extra information. SLSTR has a wide nadir view and a narrow oblique view. 117) 118) 119)
Selex Galileo of Finmeccanica signed a contract with Thales Alenia Space, to supply the SLSTR instrument. Overall, the SLSTR team involves some 20 European companies or institutions, referred to as “SLSTR consortium” (among them RAL (Rutherford Appleton Laboratory), Jena-Optronik, TAS-F, ABSL, ESA/ESTEC), for the development of this rather complex payload.
The instrument design follows the dual view concept of the ATSR series with some notable improvements. An increased swath width in both nadir and oblique views (1400 and 740 km) provides measurements at global coverage of Sea and Land Surface Temperature (SST/LST) with daily revisit times, which is useful for climate and meteorology (1 km spatial resolution).
Improved day-time cloud screening and other atmospheric products will be possible from the increased spatial resolution (0.5 km) of the VIS and SWIR channels and additional SWIR channels at 1.375 µm and at 2.25 µm. Two additional channels using dedicated detector and electronics elements are also included for high temperature events monitoring (1 km spatial resolution).
The two Earth viewing swaths are generated using two telescopes and scan mirrors that are optically combined by means of a switching mirror at the entrance of a common FPA (Focal Plane Assembly). The eleven spectral channels (3 VIS, 3 SWIR, 2 MWIR, 3 TIR) are split within the FPA using a series of dichroics. The SWIR, MWIR and TIR optics/detectors are cooled down to 80 K with an active cryocooler, while the VIS detectors work at a stabilized uncooled temperature.
The SST and LST data generated from SLSTR observations are intended to ensure continuity of the data sets started in 1991 by the ATSR series. The plan is for the Sentinel 3 Mission to operate with two satellites operating concurrently for 20 years when fully implemented, although initially a single SLSTR will provide equivalent or a better performance (Table 9) when compared to its predecessors in the following ways:
- Increase of the dual view swath width from 500 to 744 km centered on the subsatellite track (Figure 89) gives a mean global coverage revisit time at the equator of 1.9 days (1 spacecraft) or 0.9 days (2 spacecraft)
- Enlarged single view swath width of 1400 km provides a mean global coverage revisit time at the equator of 1 day (1 spacecraft) or half a day (2 spacecraft)
- A nadir on-ground resolution of 0.5 km at nadir (instead of 1 km) for all VIS (S1-S3) and SWIR (S4-S6) channels. Radiance measurements from these channels are used for both land & clouds daytime observations
- Two added channels in the SWIR band to allow improved cloud and aerosol detection to give more accurate SST/LST retrievals
- Two dedicated channels for fire and high temperature events monitoring at 1 km resolution
- A mission design lifetime of 7.5 years which is higher compared to the earlier instruments.
High SNR (600) for the VIS channels at Earth albedo signals (30%) and low NEDT (< 30 mK) for the TIR channels is achieved via the use of a very efficient detector technology (TIA Si for VIS at 260 K, PV-CTIA for SWIR, and PC for TIR at 80 K). The polarization sensitivity is less than 5% EOL (2% BOL) thanks to the use of high reflecting/transparent optical coatings within the channel passbands.
Legend to Table 10: λ = center wavelength, Δλ = spectral width, Albedo Ref (Top of Atmosphere Reflectance), T (Top Of Atmosphere Brightness Temperature), SNR (Signal-To-Noise Ratio), NEΔT (Noise Equivalent Temperature Difference).
In the SLSTR viewing geometry, two SLSTR dedicated telescopes and scan mirrors generate the Earth view swaths (Figure 89) as portions of two conical CCW (Counter-Clock-Wise) scans; in between these observation views, SLSTR acquires calibration data with the BBs (Black Bodies) and the VISCAL (Visible Calibration unit).
Table 11: Technical characteristics of the Sentinel-3 SLSTR instrument
The SST/LST measurement accuracy is obtained by means of a high accurate calibration of the three infrared channels (S7-S8-S9), which are used for the correction of the water vapor atmospheric absorption (split window during day and triple window during night), and the observation of the same on-ground pixel by means of two atmospheric path views for the correction of aerosols effects (Figure 90).
Two different synchronized conical scanners are used to optimize the IR radiometric accuracy, allowing constant optical area beam and incidence angle (23.5º) for all scan points (both scene and BBs), low polarization effects and frequent views of BBs (every scan) with the same Earth observation geometry.
The conical scan, inherited from AATSR (Advanced Along Track Scanning Radiometer), is of fundamental importance for TIR (Thermal Infrared) radiometric accuracy as each scanner uses a constant optical area at a fixed angle. Optimizing incidence angles and reflectivities provides low polarization. Both scans are performed by means of a mirror inclined at θ=23.5º with respect to the rotation axis with an half cone angle of β=47º: the inclined view rotation axis is pointed to nadir direction while the near-nadir view rotation axis is backward inclined of γ=41º (Figure 90).
All on-ground pixels are viewed with an OZA (Observation Zenith Angle) less than 55º, so limiting the radiance variations with sea emissivity changes due to salinity, temperature and wind speed and permitting emissivity modelling only as a function of OZA.
A path length ratio between the two views of 1.54 is achieved for a good optimization of the SST retrieval algorithm. Two detector pixels (IR channels) simultaneously view 2 km along-track in nadir view, to ensure adequate signal integration and also reducing the scan speed with respect to previous AATSR thereby allowing heritage qualifications for the scanners’ bearing operational lifetime requirement of 7.5 years.
The SLSTR instrument comprises two physical units, that are integrated separately to the platform:
- SLOSU (SLSTR Optical Scanning Unit), simply referred to as OSU, is mainly composed by OME (Opto-Mechanical Enclosure) together with thermal radiators, FEE (Front End Electronics), the cooling system, and DA (Detection Assembly)
- SLCPE (SLSTR Control and Processor Electronics), simply called CPE, controls all subsystems and manages the data interface with the satellite.
For each view, IR and VIS radiant energy is reflected by a scan mirror mounted on a scan mechanism towards a paraboloid mirror. The energy is then focused and reflected into a common FPA, with a flip mirror mechanism switching from one view to the other. A fast switching flip-mirror alternates the optical beams from the two scanners so that they superimpose at prime focus and acquire signals from both Earth views (nadir and oblique) and the on-board calibrators (BBs and VISCAL). To increase lifetime, each scanner has a period of 300 ms, a factor of two slower than its predecessor AATSR.
This technology is housed in the OME, which mounts the two telescopes, the flip mirrors, the calibrator units (BBs and VCU), and the SUE (Scan Unit Electronics). The OME feeds the optical beams into the FPA (Focal Plane Assembly) which spectrally separates eleven channels (3 VIS, 3 SWIR, 2 MIR, 3 TIR) with dichroics. There are nine DUs (Detector Units), each one is equipped with a precision filter to define its spectral response. The IR channels’ optics/detectors are cooled to 80 K by an active cryocooler that has vibration compensation. The separated housed visible channels need to be run at a stable ambient temperature.
Figure 93: Left: SLSTR, consisting of two baffles, a scan mirror, an off-axis parabolic mirror and a folding plane mirror for each telescope (near nadir and backward view), Right: A plane flip mirror is used to switch the beam between the two telescopes, focalizing at the entrance diaphragm of the FPA (Focal Plane Assembly), within which is separated into 9 spectral channels with cooled (265 K for VIS and 89 K for SWIR-TIR channels) dichroics, mirrors, filters, focalizing optics and detectors (image credit: ESA)
OSU (Optical Scanning Unit) technologies:
1) General design considerations for the OME (Opto-Mechanical Enclosure): Two conical scanners provide the two swaths by using rotating mirrors inclined at 23.5º with respect to its rotation axis. The oblique swath (740 km) is obtained with the scanner rotation axis pointed versus nadir, while for the near nadir one (1400 km), the scanner rotation axis is inclined backward at 41º. In this way, the same on ground swath of 740 km can be observed with two observation zenith angles (< 55º) with a minimum atmospheric path length ratio of 1.54 (Figure 94).
The OME layout for the scanners has been carefully optimized. Placing the two blackbodies (one hot at ~300 K, and another cold at ~265 K) in front of all other optical elements at the intersections of the two scanned cones (Figure 96 and Figure 97) affords both scanners frequent, continuous and consistent IR in-flight calibration.
The radiation from the earth or calibration targets is reflected via the two scanners and focused into a common focal plane. This plane at the telescope prime focus is an intermediate focus in the system since the FPA re-images it on to the detectors. The two reflective telescopes are realized with single off-axis paraboloid mirrors and use folding mirrors to feed a small, fast and precise FMD (Flip Mirror Device) as shown in Figure 95).
The FMD steps a highly reflective 13 mm diameter mirror about an attrition-free rotation axis, driven by a limited angle torque motor and a limited angular range optical encoder. Optical design optimization has permitted the tilt angle to be limited to < ±9.5º, so the SLSTR achieves the required flex-pivot lifetime. The FMD has a steady state angular stability accuracy of 10-15 arcsec.
Figure 96: Bended boundary line between green (OB) and red (NA) scan cone showing potential calibration source positions. Black bodies have to be positioned on this line to be seen by both scanners (image credit: SLSTR consortium)
SLSTR has a VISCAL (Visible Calibration unit) which is illuminated by the sun for ~1 minute each orbit. Besides the IR blackbodies already mentioned, the OME relays photons from the VISCAL via both scanners. The optical beams have been geometrically accommodated, but viewing them also has to be integrated in the timing duty cycle of the scanners. An optimized FMD switching time of 34 ms has been chosen, combining a minimized stepping angle with this minimizes drive thermal dissipation.
What the 34 ms does, means that calibration sources will be observed via both views only every two scans or 600 ms rather than every scan. A timing diagram for is shown in Figure 98, where the upper (magenta) and lower (blue) lines represent the oblique and nadir view scanner observations, while the middle brown line represents the position of the flip mirror and its transitions between the two views.
2) Switching device FMD (Flip Mirror Device): The FMD (Figure 99) has been implemented by using components with limited development cycles:
- A conventional glass based flat mirror with 13 mm free aperture
- Attrition free axis bearing implemented by means of flexural pivots with infinite life time for max excursion angle of 15º and a powerless 0º position
- A limited angular range high resolution (19 bit) and accurate (< 15 arcsec) optical encoder
- An efficient LAT motor,
- A transition time between both operation positions separated by 19 of 34 ms
- 4 x switching operations per 600 ms
- More than 2 billion operations life-time.
The final components selection includes a LAT-Motor actuator with an inertia of 1.46 x 10-6 kgm2 providing a torque of 36 mNm.
3) Scanning devices: To meet the improved GSD and mission performance requirements of the GMES program, the SLSTR scanning mechanism accuracy has been specified to be of the order of a quarter of 1 arcmin.
SLSTR has two scanners, referred to as SMU (SLSTR Scanning Mechanism), and one unit for their synchronization the SUE (Scan Unit Electronics). This synchronization is performed relative to an instrument clock for each 1 km pixel. An optical encoder with 21 bit resolution, a repeatability of 2 arcsec and a maximum absolute position error of about 6 arcsec had been selected.
Figure 100: Illustration of the SMU device (image credit: SLSTR consortium)
The primary performance requirement for the SMUs is to function in a system that defines the angle of mirrors via which SLSTR optically scans, positioning the pixels to the required accuracy. For vector components across the rotational axis, this limits run-out and lubrication track roughness. Along the scan swathes or around the SMU axis, accurate rotation angles are required at the time when each pixel is acquired.
In the SLSTR design, the SMUs drive the control position. This represents a considerably improved implementation over the AASTR design, in which the SMUs were driven at a constant angular velocity, and the swath pixel data was sampled and selected just on a time delay from a synchronization pulse. - For every SLSTR 'pix10sync signal', the position is being measured, implemented in a 100 Hz cut-off frequency closed loop circuit and transmitted to the ground for geolocation referencing. These measures offer a much more robust system in the event of some bearing torque degradation. Small scan mirror angle errors due to electronic noise or mechanical torques, either in the bearings or in external sources, can be corrected in part by means of the real-time positional control loop, the actual angles can then be used for later ground processing.
The maximum control loop pointing error is 5 arcsec. In case this tolerance will be exceeded, a flag will be set and transmitted with the telemetry data package of each scan.
The selected implementation for the SMU control system is an FPGA (Floating Point Gate Array) with an oscillator frequency of 40 MHz. A cascaded control scheme (Figure 101) is used with three levels:
- Current control loop (inner control loop) including the motor current commutation
- Speed control loop
- Position control loop (outer control loop).
These loops work with different sampling frequencies. The current control works with 27.5 kHz, while the outer loop is fixed at the frequency generated from the PIX10SYNC impulse each 82 µs. The maximum PWM resolution is limited to 10 bit.
The Scanning mirror will be manufactured from beryllium alloy to provide a low overall mass and inertia as well as sufficient stiffness and thermal stability. It is mounted to the shaft via an angular adapter and a centering shim. Using a mechanism shaft of very high accuracy, the angular adapter can provide the required value and accuracy of the scan cone angle. The centering shim allows the dismounting of the mirror to the adapter without alignment loss.
The scan mirror I/F has a stress relieved mechanical design to prevent the mirror surface from disturbances arising from mounting forces (Figure 102).
FPA (Focal plane Assembly) technologies:
1) General design considerations for the FPA: The FPA has an enclosure composed of a base-plate, a cylinder and an aluminum dome (Figure 103). The base-plate has an input field-limiting aperture, dichroics for spectral channels separation and lens optics to focus the beam onto each detector units. Note that the two scans are combined time multiplexed before they reach the FPA aperture.
The IR (S7, S8, S9) and SWIR (S4, S5, S6) channels are implemented on either side of the cryo-optical bench, cooled to 80 K (Figure 104), while the VIS (S1, S2, S3) channels are disposed outside in a separate enclosure at about 260 K. All S4-S9 optics, the detectors and the baseplate are cooled down to 80 K by a cryocooler with good established space heritage, life-time and reduced vibrations.
The VIS beam is sent to a VIS box (containing all VIS dichroics, optics and detectors) which is controlled to a temperature of 260 K and is situated in the upper part of the FPA.
2) Detection module: The detection module is formed by 9 detectors optimized for low photon flux and high temporal response. Three custom photovoltaic (PV) silicon detectors cover the VIS and NIR bands (0.4 µm to 1.0 µm), four HgCdTe PV detectors are used on the SWIR/MWIR band (1 µm to 4.4 µm) and two photoconductive (PC) HgCdTe detectors provide coverage for wavelengths beyond 10 µm.
The Si detectors are mated to companion TIA (Trans-Impedance Amplifier) arranged in a particular configuration to achieve high bandwidth with very high feedback impedance. The HgCdTe PV detectors are wire-bonded to readout integrated circuits (ROIC) that provide integration and multiplexing. The CAIA (Capacitive Trans-Impedance Amplifier) readout cells provide customized integrating capacitance for each band with high efficiency at low photon fluxes and high bandwidth. The HgCdTe PC detectors are designed for high responsivity and detectivity and are technologically critical, requiring a material cut-off wavelength close to 14 µm at 90 K (S9). They are mounted inside the DA (Detector Assembly) to operate, respectively, at about 260-270 K (Si) and 80 K (HgCdTe), and are connected to the FEE (Front End Electronics) through a combination of flexible (inside the DA) and standard (outside the DA) cables for a total length of 1.5 m.
To cover the visible bands, the PV Si detectors are realized with two different epitaxial layers and three anti-reflection coatings optimized in the region of interest in order to achieve a global quantum efficiency of 80%.
Each device is composed by four 185 x 205 µm elements, separated by a narrow guard ring to reduce dark current and improve the MTF characteristics. A TIA feedback resistance, in the order of 240 MΩ, is needed to meet the severe performance requirement causing non-trivial difficulties in achieving electrical bandwidth higher than 15 kHz.
The SWIR/MWIR detectors operate with low photo-generated currents at nominal dwell time of 40 µs. With these levels of photon fluxes, the classical direct injection (DI) structure does not operate, because of its electrical bandwidth and injection efficiency. These detectors are based on CTIA technology to enhance the electrical bandwidth of the ROIC input stage while its capacitance is designed to comply with the expected radiance for each band to minimize the readout noise that is the main contribution to the noise figure at low signal levels. The S4-S6 detectors are formed by two columns of four 100 x 100 µm elements to have off-chip oversampling of the observed scene, while the S7 detector is formed by single column of 200 x 200 µm elements.
Additional elements for the F1 fire detector are mounted inside S7; these are arranged in a column of four pixels with an active area of 25 x 100 µm to reduce the photo-generated current. These elements are in fact bonded to the ROIC in a BDI ( Buffered Direct Injection) technology of the input stage, to comply with 500 K target temperature. The 1 km response is obtained by integrating the reduced FOV signal for the dwell time, because the fire detector elements are mounted perpendicular to the along-scan direction. The BDI technique was chosen to implement the large capacitance needed to store the signal generated by high fluxes at high temporal variations but it is also able to handle the calibration signal from on-board 300 K BB at long observation periods with sufficient SNR.
The global quantum efficiency of the SWIR/MWIR devices is 80%, including the narrow band filter, with saturation charges and readout noises, respectively, of 0.6 M to 4 M and 150 to 350 electrons, depending on band, together with linearity of 1%. The global quantum efficiency of the MWIR fire elements is 70%, including the narrow band filter, with saturation charge and readout noise, respectively, of 20 M and 550 electrons, together with linearity of 1%. The photodiodes, the ROICs and the temperature sensors are mounted on the fan-out ceramics with thick film conductors as shown in Figure 105.
The TIR detectors use the bulk photoconductor concept with particular layout to increase their responsivity. Although the increasing of the bias current would appear to be a straightforward way to improve the detector sensitivity, it involves also the increasing of the noise. Indeed, low current operation provides optimum SNR and an improved response spatial uniformity.
A responsivity as large as 150 KV/W is required to minimize the noise coming from the downstream electronics and in particular to the voltage noise of the OpAmp used for the high gain preamplifier. The resistor values of the PC elements are low and hence, the time constant is of few µs, due to the recombination time of the photo-generated carriers. The detectors contain two pixels, and each pixel is made with pairs of elements, one active and the other blind. The advantage of this arrangement is that the two parts are balanced when there is no radiation falling on the active part. This measure reduces the offset considerably as well as the thermal and ageing effects, since the signals from the active and blind part are differentially amplified. The global quantum efficiency of the LWIR (Long Wave Infrared) S9 elements is 70%, including the narrow band filter, with a detectivity of about 2.5 x 1011 Jones at 80 K, a linearity of 1%, and an element matching of 2%. Note: the 'Jones' is a unit of specific detectivity. - All detectors are in compliance with the SLSTR radiation requirements.
The SLSTR FPA requires special care in the routing, shielding and grounding of very low noise PC signals in the presence of high level PV signals. The FPA signals are received by the FEE (Front-End Electronics) in radiation-tolerant and redundant design with free fault-propagation circuits interfacing the detectors. The FEE operation is controlled by FPGA allowing some flexibility in the acquisition characteristics via ground commands. The main demands of the detection system are:
- Same temporal-spatial response for the VIS/SWIR, TIR and fire channels
- Extremely low thermal variation of the gain and offset
- Very low noise.
3) FEE (Front End Electronics): The FEE is subdivided into a common and a cold-redundant part as depicted in Figure 106.
The former (in red) includes the functions strictly associated with DU like bias and p/s voltages, differential receivers of the analog signals and temperature sensor switching. It is formed by nine separated and independent sections having dedicated protections to avoid failure propagation.
The latter comprises the functions associated to the video processing of the analog signals, including amplification, filtering, offset correction, sampling and/or integration up to the A/D conversion, the generation and distribution of the timing signals, the HK (Housekeeping) signal acquisition, the power supply filtering, the LVDS interface for the synchronization signals, and the SpaceWire interface for command/telemetry. To manage the 20 analog chains, the FEE architecture uses 10 radiation hardened ADC RHF1401, working in parallel and interfacing to the FPGA RTAX2000 though 10 private buses.
Three types of analog processing are employed for the VIS, SWIR/MWIR, and the TIR signals. The implementation of the F2 channel is of some interest; it is derived from the S8 chain after the preamplification and offset correction as shown in Figure 107.
Each PC signal is generated by a bridge configuration, using the active and blind elements whose bias currents can be adjusted by resistors, to reduce the dc offset when blanked. It is amplified by using two stages with an intermediate offset correction feature for a total gain of about 300. The PC preamplifier uses the RH1028 with excellent noise performance having a voltage noise less than the noise of a 50 Ω resistor. However, careful attention has to be paid to limit the effect of the bias noise.
Multiple acquisitions with the decimation technique are used to reduce the ADC (Analog Digital Converter) noise to about 100 µV rms (~ 1 LSB of 14 bit ADC) with each data coded in a 16 bit string. In this way the ADC noise contribution to the overall noise is further reduced allowing fixed amplification, because each channel noise is dominated by the corresponding detector noise.
SLSTR budget: The SLSTR instrument has a nominal mass of 140 kg (160 kg max) and the nominal power consumption of 155 W.
Topography payload: (SRAL, MWR, GNSS receiver, DORIS, LRR)
The objective of the topography mission is to provide measurements over the open Ocean, coastal zones, ice sheets, rivers and lakes. Measurements over open oceans will contribute jointly with other operational missions to the realization of a permanent Global Ocean Observing System (GOOS). The main parameters measured over the open sea are SSH (Sea Surface Height) and SWH (Significant Wave Height) allowing to retrieve sea surface wind speed.
The science goals of the topography mission can be summarized as: 120)
• to continue and extend the current set of altimetry measurements at least at the level of quality of the Envisat RA (Radar Altimeter)
• to provide along-track SAR processing to improve acquisitions for coastal zones, in-land water and sea-ice topography
• to provide open loop tracking through an onboard stored DEM (Digital Elevation Model) to improve acquisitions over inhomogeneous or rough topography.
All altimetry products will be delivered as NRT (Near-Real- Time) within 3 hours after acquisition with an orbit estimate from the GNSS receivers. STC (Standard Time Critical) and NTC (Non Time Critical) products will improve orbit estimates with complementary information from DORIS and the laser reflector.
SRAL (SAR Radar Altimeter)
SRAL is a redundant dual-frequency (C-band + Ku-band) nadir-looking altimeter instrument, and the core instrument of the topographic payload. The overall objectives are to provide altimetric data (basic measurements of surface heights, sea wave heights and sea wind speed) relative to a precise reference frame. SRAL has a strong heritage of the instrument techniques implemented for the Poseidon-3 altimeter on Jason-2 (launch June 20, 2008), SIRAL (SAR Interferometer Radar Altimeter) on CryoSat-2 (launch April 8, 2010), and AltiKa (Altimeter in Ka-band) on the SARAL mission of ISRO and CNES (launch 2012). The SRAL instrument is being developed at TAS (Thales Alenia Space) of Toulouse, France. 121) 122) 123) 124)
The SRAL radar uses a linearly frequency-modulated pulse (chirp) and the pulse compression is carried out on-board by means of the deramp technique. The main frequency used for surface height measurements is the Ku-band (13.575 GHz, bandwidth=350 MHz), whereas the C-band frequency (5.41 GHz, bandwidth=320 MHz) is used for the ionospheric corrections. The frequency plan is compliant with the ITU (International Telecommunication Union) regulations. A 50 ms pulse duration for both frequencies has been sized as a trade-off result between a high BT product and the timing constraints of the burst pattern of the SAR mode.
Figure 108: Comparison between Jason (black) and Sentinel-3 (purple) ground tracks for a complete cycle (image credit; ESA)
Figure 109: The measurement principle of the topography payload (image credit: ESA)
Figure 110: A diagram of the corrections applicable to the altimeter range measurement and the contributions to the height of the instantaneous sea surface above a reference earth ellipsoid (image credit: Gary M. Mineart)
The main characteristics of the SRAL (SENTINEL-3 Ku/C Radar Altimeter) are: 125)
• radar measurement modes: LRM and SAR
• tracking modes: closed and open-loop
• pulse repetition frequency: 1.9 kHz (LRM), 17.8 kHz (SAR)
• total range error: 3 cm
• launch mass: 60 kg.
Figure 111: Photo of the SRAL instrument assembly (image credit: ESA)
The SRAL altimeter instrument is made of one nadir looking antenna subsystem which is externally mounted on the satellite +Zs panel and central electronic chains composed each of a DPU (Digital Processing Unit) and a RFU (Radio Frequency Unit). The central electronic chains are mounted inside the satellite on the -YS panel and are treated according to a cold redundancy scheme.
Figure 112: SRAL accommodation on the Sentinel -3 spacecraft (image credit: ESA)
SRAL modes of operation:
The SRAL instrument includes measurement modes, calibration modes and support modes. The measurement modes are composed of two radar modes associated to two tracking modes. The two radar modes are the following:
• LRM (Low Resolution Mode). It refers to the conventional altimeter pulse-limited resolution mode (so far, the LRM mode is being used on all altimetry missions). It consists of regular emission/reception sequences at a fixed PRF (Pulse Repetition Frequency) of around 1920 Hz leading to an ambiguity rank of 10.
• SARM (SAR Mode): This is a high along-track resolution mode composed of bursts of Ku-band pulses.
These modes are associated to two tracking modes which consist of the following:
- Closed-loop mode: refers autonomous positioning of the range window (ensures autonomous tracking of the range and gain by means of tracking loop devices implemented in the instrument).
- Open-loop mode: refers to the positioning of the range window based on a-priori knowledge of the terrain height from existing high-resolution global digital elevation models.
The open-loop is intended to be used instead of the more conventional closed-loop tracking over some surfaces, to improve the acquisitions over inhomogeneous or rough topography. While in open-loop, the setting of the tracking window of the altimeter is driven by predetermined commands, stored on board, combined with real-time navigation information available from the GNSS receiver. The main advantage is that the measurements are continuous, avoiding the data gaps typical of closed-loop tracking, which has problems in tracking the rapid topographic changes at coastal margins and in mountainous regions.
Table 12: Summary of SRAL support modes
Figure 113: Comparison of a conventional pulse-limited radar altimeter’s (a) illumination geometry (side view) and footprint (plan view) and (b) impulse response, with a delay/Doppler altimeter’s (c) illumination geometry and footprint and (d) impulse response (image credit: JHU/APL) 126)
Figure 114: Instrument architecture of SRAL (image credit: TAS, ESA)
The SRAL instrument generates either C-band or Ku-band pulses in order to simplify the hardware design. However, the periodic emission of elementary patterns (1 C-band pulse is surrounded by 6 Ku-band pulses, denoted by 3Ku/1C/3Ku) ensures a sufficient correction of the ionosphere bias (Figure 115).
After de-ramping and digital processing, the echo received from each pulse is sampled on 128 points corresponding to a 60 m range window. The C- and Ku-band echoes are submitted each to a FFT (Fast Fourier Transform) to return to the time domain after deramp. Then, C- and Ku-band echoes are accumulated separately over a 50 ms cycle corresponding to an accumulation of 84 Ku-band pulses and 14 C-band pulses over that cycle.
SARM (SAR Mode): The implementation of a nadir SAR mode provides an enhanced along-track (azimuth) resolution (~ 300 m) w.r.t. the LRM mode. This feature allows to acquire height measurements over along-track sliced areas sampled at the 300 m resolution. It is of prime interest to discriminate finely sea/ice transitions, sea/land transitions in a coastal area or inland water areas.
The SAR mode consists of periodical emissions/receptions of bursts composed of 64 Ku-band pulses surrounded by 2 C-band pulses (Figure 116) again for ionosphere delay correction. The 64 Ku-band pulses are generated coherently within a burst to carry out azimuth resolution enhancement on a burst basis by means of Doppler filtering. The burst emission / reception cycle is completed before the next burst cycle. The burst cycle duration is about 12.5 ms in such a way that a 4-burst cycle is equal to the LRM cycle of 50 ms. The PRF within a burst is around 18 kHz.
Figure 117: Final shape of resolution cells in SAR mode (image credit: ESA)
Calibration mode: Two specific calibration modes have been designed to refresh the calibration parameters required for ground processing and to monitor the good health of the instrument in flight configuration. The CAL-1 mode allows to calibrate the internal impulse responses (range and azimuth impulse responses in C- and Ku-band) whereas the CAL-2 mode allows to calibrate the gain profile of the range window by averaging thermal noise measured at each C- and Ku-band antenna port.
Table 13: Some SRAL instrument parameters
The “dual-like” features of the SRAL instrument (dual frequency, dual radar mode, dual tracking mode) make it possible to acquire very accurate topography data over all types of surfaces covered by the Sentinel-3 mission. And the “dual” central electronic chain ensures a high degree of reliability.
SRAL antenna: The antenna is made up of a 1.20 m parabolic reflector with a C/Ku dual frequency feed horn placed in a centered configuration at a focal length of about 430 mm. The feed is supported by 3 struts separated by a 120º angle: Two of them are doubled to improve the sidelobe ratio performance and the third one supports the Ku-band waveguide. It must be pointed out that the position of the strut ends on the reflector to match the reflector brackets position in order to improve the mechanical robustness of the antenna.
The antenna provides a minimum gain of 41.5 dBi in Ku-Band and 31.6 dBi in C-band at bore sight in the signal bandwidths. The side-lobe level is lower than –18 dB in Ku-band in order to minimize the Range Ambiguity Ratio.
The SRAL antenna is manufactured by MDA (MacDonald Dettwiler and Associates Ltd.), Richmond, BC, Canada. The center-fed linearly-polarized antenna has a mass of < 7 kg. 127)
Figure 118: Photo of the SRAL EM antenna (image credit: TAS, ESA)
RFU (Radio Frequency Unit): The RFU equipment (Figure 119) is made up of slices which are stacked together except the C- and Ku-band duplexers that will be fixed independently on the satellite panel. The RFU up-converts chirp signals from 50 MHz to C- and Ku-band and provides an output power of 38 dBm in Ku-band and 43 dBm in C-band. The up -conversion stage also includes an expansion of the chirp bandwidth by a factor of 16. Received echoes in C- and Ku-band are deramped down to 100 MHz. The deramp output produces a useful signal bandwidth of 2.86 MHz which is then processed by the DPU.
DPU (Data Processing Unit): The DPU is a rack of 6 boards plus an interconnection board. Its main functions are:
• Generation of a chirp signal centered at 50 MHz at PRF (Pulse Repetition Frequency) rate
• Processing of deramped echoes including digitization, I/Q demodulation, FFT and echo accumulation
• Transmission of science data on SpaceWire link
• Echo processing (range and tracking) for the closed loop mode operation
• Storage and management of on-board Digital Elevation Model for Open-Loop tracking
• Management of 1553 TM/TC interface with the platform
Figure 120: Illustration of the DPU device (image credit: TAS, ESA)
MWR (Microwave Radiometer):
MWR is a nadir looking sounder, operating at 23.8 and 36.5 GHz (K/Ka-band) covering a bandwidth of 200 MHz in each channel. The objective is to provide water vapor and cloud water contents in the field of view of the altimeter, necessary to compensate for the propagation delay induced by these atmospheric components and affecting the radar measurements. Such corrections are only possible over the ocean, where the background noise is stable and can be quantified either by the 3rd (optional) radiometer channel, or derived from the altimeter measurements of the backscattered power. Alternatively, over ice and land surfaces where MWR data cannot be used, wet troposphere corrections will be derived based on global meteorological data and dedicated models.
The MWR instrument is being developed by Airbus DS-CASA Espacio of Madrid, Spain (formerly EADS CASA Espacio) under contract with Thales Alenia Space France (TAS-F) and ESA. MWR measures the thermal radiation emitted by Earth (brightness temperature). The received signal is proportional to the abundance of the atmospheric component emitted at the observed frequency and the sea-surface reflectivity. This information reveals the delay added to the altimeter pulses by moisture in the troposphere. 128) 129) 130) 131)
The MWR instrument is comprised of the following part elements: antenna assembly, REU (Radiometer Electronics Units), the main structure and the thermal control hardware. Both K-band and Ka-band channels are fully redundant, except for the antenna assembly, with cold redundancy without cross-strapping.
Figure 121: MWR block diagram (image credit: ESA)
Table 14: Main characteristics of the MWR instrument
Conceptually, the MWR is a balanced Dicke radiometer for brightness temperatures below the Dicke load temperature. The balancing is achieved by means of a noise injection circuit. For brightness temperatures higher that the Dicke load temperature a conventional Dicke mode is used. The radiometer employs a single offset reflector of 60 cm in diameter and two separate feeds for the two channels. Calibration is achieved through a dedicated horn antenna pointing at the cold sky.
Figure 122: Block diagram of the detailed functional architecture of the MWR instrument (image credit: ESA)
The REU (Radiometer Electronics Unit) consists of the RFFE (Radio Frequency Front End) and the RPM (Radiometer Processing Module). The RFFE is located as close as possible to the measurement feeds to optimize the length of the waveguides and thus the radiometric performance. It contains the amplifiers, switches (calibration and redundancy) and other performance determining elements. The RPM contains the thermal control, the RFFE control, the noise injection loop, the power supplies (also for the RFFE) and provides the electronic interface to the platform. The REU includes a mode to blank the receiver inputs when the radar altimeter emits its pulses to avoid potential disturbances. This mode is accessible by ground commands.
Antenna assembly: The antenna assembly consists of the main nadir-looking reflector that has a diameter of 60 cm, the two measurement feeds and a calibration feed-horn, also known as sky horn. The main antenna is a single offset reflector fed by one horn for each channel. The calibration antenna is based on a wide band single corrugated horn and a frequency diplexer. Both antennae receive a single linear polarization and their signals are routed towards the receivers by means of separate waveguides.
The antenna assembly receives the noise temperature emitted by the objects within the antenna field of view. Discrimination between the different measurement frequencies is done by using different feed horns, each covering a separate frequency band. A separate sky measurement is provided by means of a dedicated sky horn. In this way, the satellite can continue the regular nadir measurements without the need of any maneuvers to turn over the satellite to look at the cold sky. The different frequencies received in the sky horn, are separated by a wave guide diplexer. The signals received by the feeds are guided towards the receiver electronics by means of waveguides. The physical temperatures of the different sections of the antenna assembly are measured and sent to the RPM (Radiometer Processing Module) of the REU.
The RFFE (Radio Frequency Front End) is manufactured by Thales Alenia Space Italia (TAS-I) and is implemented in a very compact design of 200 mm x 290 mm x 120 mm, a total mass of 5 kg, and 10 W power consumption.. Each RFFE channel holds a MSA (Microwave Switching Assembly), a redundant receiver (Rx) and a NS (Noise Source). The RPM is comprised of the FPGA-based radiometer control and acquisition and the DC/DC converters. It provides the electrical interface of the MWR to the Sentinel-3 SMU.
Figure 123: Illustration of the RFFE instrument (left), outline of the RPM (right) image credit: TAS-I)
The MSA includes the Quad-switch, made of 4 ferrite switches, which is in charge of the selection of the nominal/redundant and observation/calibration paths. A redundant driver provides the required pulsed current to the switches so that one of the following two states is selected:
1) Nominal → Observation / Redundant → Calibration, or
2) Nominal → Calibration / Redundant → Observation.
The RF part of the Quad-switch is the only non-redundant section of the RFFE, but its reliability is estimated to be better than 0.99924 (11.56 FIT). The MSA is completed with a waveguide coupler for the noise injection and the Dicke switch with a 25 dB matched load. Measurements data from the EQM model shows a remarkable performance in terms of return loss (> 25 dB), isolation (> 40 dB) and insertion loss (< 1 dB) for the MSA, with switching times lower than 2.5 µs and low power consumption.
The NS contains a single noise diode and two cascaded PIN switches, providing high isolation (> 80 dB) in the off state. Additional attenuators are included inside the NS to tune the ENR (Excess Noise Ratio) at the output of the Dicke switch to be around 0 dB. Fine tuning can be also achieved by adjusting the current to the noise diode. The Rx is a superheterodyne receiver; its first element is the LNA, which provides low noise figure and permits good instrument sensitivity even with a SSB (Single Side-Band) architecture. The RF signal is down-converted to an intermediate frequency of 2.5 GHz, which is common to both frequency modules. The local oscillator is based on an X-band DRO and frequency multiplier, while the bandwidth selection is achieved by means of a microstrip band-pass filter centered at the IF frequency. A distributed Rx gain of some 65 dB allows the input noise power to be detected through linear behavior of all Rx components. The square law detector diode is followed by a 4th order Butterworth filter with a cut-off frequency of 50 kHz. The LPF is optimized taking into account the sampling frequency of the ADC (100 kHz) and the offset produced in the retrieval of Ta when the filter’s group delay is increased.
Figure 124: Conceptual view of the MWR instrument and accommodation on the Sentinel-3 spacecraft (image credit: ESA)
Instrument operation: The MWR is a digitally controlled NIR (Noise Injection Radiometer) instrument. It measures the brightness temperature of the observed scene (the surface of the Earth through the atmosphere, as the instrument is pointing to nadir in its nominal attitude). The noise captured by the RF receiver at both operating frequencies (23.8 and 36.5 GHz) is compared to a very stable Dicke load. This load produces a noise power equivalent to its physical temperature, which is precisely known, and to the receiver bandwidth. The NIR principle reduces the effect of receiver gain and offset instabilities and has successfully been implemented in the frame of the SMOS mission. Every Dicke period is divided into three phases (Figure 125):
- The first half of the Dicke cycle contains two phases: First, a precisely known noise contribution is added to the scene measurement. Second, only the noise power from the scene is received.
- During the second half of the Dicke cycle the noise of the Dicke load is measured.
The working principle is based on the balance between the total amount of power received in the first and second half of the Dicke cycle. This is achieved by varying the duty cycle of the noise injection during the first half of the Dicke cycle. - Then, the injected pulse length is integrated along several Dicke cycles. With this approach, the performance of the instrument is independent of variations of the receiver gain, as all its contributions are cancelled in the balancing process. As a result, the brightness temperature captured by the antenna (Ta) can be derived from known parameters:
Ta = Tref - η Tn (1)
Tref being the physical temperature of the Dicke load, Tn the temperature of the noise injected into the system, and η the length of the noise injection (normalized to half a Dicke cycle; equation (1) means that noise has been injected during the whole first half of the Dicke cycle). Equation (1) can only be used in case Ta is below Tref. If this condition is not met (for very hot scenes, or for a cold receiver), the instrument loses its balanced condition and works in a DNB (Dicke Non-Balanced) mode, in which η goes to 0 and the antenna temperature is retrieved from voltage difference (Ve) between the power integration of the first and the second half of the Dicke cycle:
Ve = G x (Ta-Tref) ⇒ Ta = Ve/G + Tref (2)
where G is the gain of the receiver that is also periodically calibrated. Equations (1) and (2) are the so-called NIR and DNB equations. Both η and Ve are integrated over 12 Dicke cycles, and then included in the science TM packets.
GNSS (Global Navigation Satellite System) receiver
A dual-frequency instrument based on GPS constellation - and optionally on Galileo. The objective is to provide data for precise orbit determination (POD), established after ground processing. In addition, the GNSS receiver will provide real-time navigation bulletins periodically, as required by the open loop tracking mode of the altimeter, with an accuracy of about 3 m rms. This information is used to control SRAL's open-loop tracking and for Sentinel-3 navigation. Ground processing yields the altitude to an accuracy of < 8 cm within 3 hours for operational applications, and 2 cm after some days of refinement.
The 11 kg GNSS receiver can track up to 12 satellites at the same time. The signals transmitted by the navigation satellites are also disturbed by the ionosphere. The effect is corrected by comparing two signals at different frequencies within 1160-1590 MHz.
Figure 126: Photo of the GNSS receiver (image credit: ESA)
LRR (Laser Retroreflector):
LRR is a passive device, composed of a set of corner cubes (mass of 1 kg). The LRR is mounted on the Earth panel of the spacecraft. Its purpose is to enable the accurate localization of the satellite from the ground, through laser ranging techniques. A network of laser ground stations (SLR) will be used for this purpose and their measurements will contribute to refining and validating the POD solutions derived from GNSS data.
Figure 127: Photo of the LRR instrument (image credit: ESA)
Sentinel-3 (S3) Ground Segment:
The main objective of the Sentinel-3 mission is the provision of ocean observation data in routine, long term and continuous fashion with a consistent quality and a very high level of availability. In addition, the mission will be designed to generate land optical observation products, ice topography, vegetation and land hydrology products. The different elements composing the Sentinel-3 system and the related interfaces for the provision of the operational marine and land services are shown Figure 128. 132)
In the frame of the Cooperation Agreement on the GMES space component, ESA and EUMETSAT have established a specific implementing arrangement concerning the cooperation on Sentinel-3. On this basis, following completion of spacecraft in-orbit commissioning, EUMETSAT will be responsible for the operation of the mission covering the products for the marine user community and the monitoring and control of the spacecraft in routine phase.
The GMES Sentinel-3 ground segment is in charge of the overall commanding and monitoring of the spacecraft constellation (2 S/C) as well as the acquisition, processing and dissemination of their observational data. The two primary components of the ground segment are the FOS (Flight Operation System) and the PDGS (Payload Data Ground Segment).
The different elements composing the Sentinel-3 system and the related interfaces for the provision of the operational marine and land services include (Figure 128) :
• The Sentinel-3 satellite(s), which produce and downlink the observation data
• The Sentinel-3 Ground Segment, which acquires the observation data and produce the operational products
• Interfaces to:
- The S3-Mission Management charged with the overall responsibility for the routine mission of the Sentinel-3 system
- DAS (Data Access Ground Segment) as the gateway between the users and the Sentinel-3 Ground Segment. The DAS forwards users’ requests to the PDGS and delivers the related data products generated by the PDGS. The DAS is capable to manage users’s communities, providing access to different classes of Users, with different priorities, specific rules, access privilege and security limitations
- SSALTO (Segment Sol multimissions d'ALTimétrie, d'Orbitographie et de localisation précise) operated under responsibility of CNES. SSALTO is a multi-mission ground segment which interfaces with the FOS to provide command information for DORIS and it will interface PDGS to receive DORIS mission raw data and to provide auxiliary data for PDGS POD (Precise Orbit Determination)
- Expert Teams as a privileged set of users which support the Satellite Commissioning phase and for this reason interface with the PDGS.
FOS (Flight Operation System):
The FOS main responsibility encompasses the spacecraft monitoring and control, including execution of all platform activities and the commanding of the payload schedules.
• The Ground Station and Communications Network performing telemetry, telecommand and tracking operations within the S-band frequency. The S-band ground station used throughout all mission phases will be the ESA Kiruna terminal (complemented by the Svalbard and Troll as backup stations).
• FOCC (Flight Operations Control Center), including:
- The Mission Control System, supporting hardware and software Telecommand coding and transfer, HKTM (Housekeeping Telemetry) data archiving and processing tasks essential for controlling the mission, as well as all FOCC external interfaces
- The Mission Planning System (part of the Mission Control System), supporting command request handling and the planning and scheduling of spacecraft/payload operations
- The Spacecraft Simulator, supporting procedure validation, operator training and the simulation campaign before each major phase of the missions
- The Flight Dynamics System, supporting all activities related to attitude and orbit determination and prediction, preparation of slew and orbit maneuvers, spacecraft dynamics evaluation and navigation.
• A General Purpose Communication Network, providing the services for exchanging data with any other external system during all mission phases.
Two instances of the FOS exist in the Sentinel-3 system. Up to end of commissioning of the space segment (i.e. launch+5 months), the FOS instance based at ESA/ESOC is responsible for flight operations, following hand-over the responsibility for flight operations will be with the FOS instance based at EUMETSAT.
To optimize the use of resources, reduce the development risk, and to meet the challenging schedule, it has been agreed to adopt a specific cooperation approach in relation to the FOS implementation, IVV (Integration, Verification and Validation) and operations. The concept relies on:
- The use of the same facilities for the FOS main components, including Mission Control System, Spacecraft Simulator, Flight Dynamics specific modules
- The development under a Joint Team of common Operations Preparation (OPSPREP) products, including Flight Operations Procedures and Spacecraft Operations Database
- The provision by ESA/ESOC of specific services to EUMETSAT, including the TT&C ground stations, the back-up Control Center, the debris collision prediction, and operations support during all mission phases.
During all mission phases, ESA/ESOC will provide a set of specific services in support of the EUMETSAT FOS. This includes the use of the S-band ground station passes needed for satellite control. This service will rely on the ESTRACK Kiruna site as primary station, with Svalbard and Troll acting as back-up in support of contingency or special operations. The baseline is that two S-band passes per day are provided to EUMETSAT, which is compatible with the Sentinels operations concept. The S-band Services will cover a mechanism for provision of additional S-band passes upon EUMETSAT request.
Figure 129: Mission context showing the data distribution for an operational ocean forecasting system (image credit: ESA)
PDGS (Payload Data Ground Segment):
The access to the Sentinel data will be provided through a dedicated Ground Segment infrastructure where the PDGS is one building block. For Sentinel 3, this is currently implemented by ESA in conjunction with EUMETSAT. TelespazioVega Deutschland is leading the Core PDGS implementation, with ACS, Werum, and Telespazio Italy as partners. 133)
The PDGS is primarily in charge of receiving and processing the Sentinel-3 instrument payload data, including HKTM data; ensuring that satellite tasking is performed according to the overall GMES user requirements and satellite capabilities; guaranteeing that suitable Sentinel-3 products meeting the expected quality and timeliness constraints are available to the GMES Users; and of all necessary support activities.
For the Sentinel 3 mission, the following product delivery timeliness are foreseen:
• NRT (Near Real Time) products made available to the users in less than 3 hours after sensing.
• STC (Short Time Critical) products made available to the users in less than 48 hours after sensing.
• NTC (Non-Time Critical) products made available less than 1 month after sensing.
As per mission requirements and on-board instruments, the processing is broadly grouped into Land data processing and Marine data processing.
Distributed Centers: The Sentinel-3 PDGS functions are distributed across a number of centers and locations, as illustrated in Figure 130.
• A primary CGS (Core Ground Station) at Svalbard, providing X–band service, and where the acquisition and ingestion function is deployed, together with the NRT processing chains for the Land products.
• A backup CGS that provides a backup X-band service covering for planned unavailability of the prime CGS X-band service.
• A Land PAC (Processing and Archiving Center) for the NTC production of the OLCI products.
• A Land PAC for the STC/NTC production of the SRAL and MWR products.
• A MPC (Mission Performance Center) hosting the Mission Performance Monitoring function for Land products
• A Marine Center, hosting the NRT and STC/NTC production for all Marine products, as well as their own Mission Performance Monitoring function.
Specific platforms are deployed at the centers to support different activities: operational platform for the routine operations, reference platform for validation, reprocessing platform to support reprocessing campaigns, and development platforms to support maintenance and evolution of the system.
Automation: The Sentinel-3 PDGS operational concept is driven by the requirement of largely automated operation with no routine manual interaction required for nominal processing. All PDGS elements are autonomously running in nominal operations during time periods where an operator is not available. This is achieved by:
• a data driven approach implemented across the whole PDGS, whereby software components are activated on the availability of the data they require as input
• machine-to-machine interfaces between components for propagating configuration and workflow changes.
For the routine operation (i.e. systematic processing, circulation, archiving, etc.), immediate operator intervention is only required in case of critical failures, that are identified by the monitoring component. A notification-based approach allows the system to execute the nominal workflows based on the availability or change of input data without explicit user intervention.
Nominal operations are completely automatic and can be managed without operator intervention.
NRT (Near Real Time) system: One of the key requirements driving the design of the Sentinel-3 PDGS is the capability to acquire, process, and make the data available to the users in Near Real Time, i.e. within 3 hours from sensing.
In the Sentinel-3 mission all the data downlinked by the satellite is processed in near real time, producing a complete set of Level 0, Level 1 and Level 2 products. Level 1 and Level 2 products are directly accessible by the end users, while the access to the Level 0 products is limited to special users.
The deployment of components across different ground stations adds a constraint to the architecture, as:
• Raw data is acquired in the CGS Center
• Land NRT products are processed in the CGS Center
• Marine NRT products are processed in the Marine PDGS Center, starting from the same raw data used in the Land case.
In order to cope with the NRT timeliness requirements and the processing deployment constraints, the design of the acquisition and ingestion functionality has been built focussing on two key concepts:
• data "granules" pipelined processing
• NRT data circulation.
According to the mission requirements, the data flowing from the satellites to the acquisition ground stations is not processed as a single monolithic entity after the completion of the passes. The chosen approach has been to divide the input data stream in size-optimized granules while the acquisition is on- going.
Marine Center: The Marine Center implements the processing, archiving, and dissemination for all marine products and all levels of processing, for NRT as well as NTC/STC timeliness. It relies on the following functions:
The Processing function applies all the necessary data processing levels, starting from the reconstructed payload science data (Instrument Source Packets) to produce Level 0 to the algorithms and formatting techniques to generate higher level products. The processing function is capable of producing the desired products in a systematic way. It is essentially composed of a processing management layer, controlling the data processors required for each level of processing and each timeliness.
The Long Term and Short Term Archives ensure the long-term and short-term storage of the payload data products and of the auxiliary data according to the longterm/short-term storage policy. These functions include all operations to be put in place to store and to circulate the data within the PDGS and to ensure their integrity according to the applicable requirements. They also include the inventory (at the appropriate level) of the stored data to enable their retrieval by the PDGS for internal purposes and by the PDGS users.
Figure 131: Schematic view of the Marine Center functions (image credit: ESA, EUMETSAT)
EUMETCast is the EUMETSAT generic multi-mission dissemination system based on the standard DVB-S (Digital Video Broadcast-Satellite) multicast technology. EUMETCast is the main dissemination mechanism for distribution of data to EUMETSAT’s end users (Ref. 133).
Sen3Exp (Sentinel-3 Experiment) campaign:
ESA conducted the Sen3Exp airborne campaign in June and July 2009. The campaign started in Barrax, La Mancha, Spain. An aircraft operated by the Spanish National Institute for Aerospace Technology (INTA), equipped with three hyperspectral imaging spectrometers, made two flights over the area. Meanwhile, satellite data were acquired by Envisat’s MERIS and AATSR instrument and by the CHRIS (Compact High Resolution Imaging Spectrometer) instrument aboard ESA’s PROBA-1 satellite. At the same time, ground teams, under the direction of Prof. Jose Moreno from the University of Valencia, made atmospheric radiometric and biophysical measurements. 134)
Legend to Figure 132: Hyperspectral imaging spectrometers were installed on board INTA’s CASA-212-200 aircraft in support of the Sen3Exp airborne campaign. The AHS (Airborne Hyperspectral System) occupies the left-hand port; the CASI-1500i (Compact Airborne Spectrographic Imager) on the left and the SASI-600 (Shortwave Infrared Airborne Spectrographic Imager) occupy the right-hand port.
The campaign then moved to Pisa in Italy, from where a pine forest at San Rossore could be reached. At San Rossore, Prof. Federico Magnani from the University of Bologna oversaw the week-long ground measurement program. The dataset was again complemented with MERIS, AATSR and CHRIS satellite data.
In July, activities focused on the marine environment where measurements were taken at two oceanic sites: the Boussole monitoring buoy in the Ligurian Sea and the Aqua Alta Oceanographic Tower (AAOT) in the Adriatic Sea, close to Venice. Both sites have played an important role in supporting ocean color algorithm development and product validation for many years.
Figure 133: Photo of the AAOT (Aqua Alta Oceanographic Tower) in the northern Adriatic Sea (image credit: ESA)
Boussole typifies the global ocean, where the measured signal is determined solely by the absorption of phytoplankton. AAOT is in an area where there is both open ocean water and also water that is optically complex because phytoplankton, suspended sediments and colored dissolved organic matter also affect the measured signal. Such water can be found in all coastal regions and represents a challenge to analyze and interpret the data from spaceborne measurements.
A unique, comprehensive and valuable dataset has been created that will significantly support the development of the Sentinel-3 mission.
Permanent calibration station for altimeters in Crete with microwave transponder
The Technical University of Crete (TUC) is installing a new permanent microwave transponder ground infrastructure on the Island of Crete, Greece, to serve as an alternative and independent technique for the calibration of, mainly, European altimetric missions. The facility was initially planned as a calibration site for the Sentinel-3 in the south west of Crete, Greece, using the developed transponder. However, this ground infrastructure, along with other permanent facilities in Crete, may also be used for the calibration of other Ku-band altimetric missions such Jason-2, Cryosat-2, etc. 135)
The idea for incorporating land based transponders was initially introduced in 2000. 136) A microwave transponder is an electronic equipment which receives the pulsed radar signal, transmitted by the altimeter of the over-passing satellite and actively amplifies and retransmits the signal towards the spacecraft, where it is recorded. The time delay of the signal is measured, from which the absolute range between the transponder and the satellite can be deduced. The main advantage of this technique, compared to the conventional sea-surface calibration, stands for the fact that no ocean dynamics errors are involved in satellite altimeter’s calibration.
However, in the past, only few transponders have been built and implemented for this reason. The ESA premises in Svalbard, Norway host a transponder developed by RAL, UK in 1987 that has been used mainly for the Cryosat-2 calibration. The Gavdos Island Cal/Val facility in Greece hosted the Austrian Academy of Sciences transponder and that transponder has been effectively used for the calibration of Envisat and Jason-2 missions. There has been another transponder placed in Rome, Italy which was used for the Envisat sigma-0 calibration.
In 2011, the Geodesy and Geomatics Engineering Laboratory at the Technical University of Crete in Greece developed a new Ku-band microwave transponder. The TUC transponder is mobile, allowing calibration at different locations but also modular for operating in other frequencies, provided that some parts are modified. It is capable of recording the incoming and outgoing signals, while it can be controlled and operated remotely. The transponder frequency has been selected to be compatible with past, current and future European as well as international altimetry missions that operate in this microwave range (i.e., Jason series, Cryosat-2, Sentinel-3). Additionally, it is equipped with a GPS (Global Navigation Satellite System) receiver and appropriate meteorological sensors to provide precise time-tagging, as well as the atmospheric delay corrections during transponder calibration. This is of importance for the accurate determination of the altimetric range because the atmosphere affects the altimetric measurements. Furthermore, this prototype transponder is the only microwave transponder that incorporates circularly polarized antennas. The latter, allows performing calibration experiments on different satellite missions at the same location, approaching from different directions, providing that the satellite ground track is in a range of 3-5 km away from the transponder location.
Table 15: TUC’s transponder radio frequency characteristics
The TUC transponder has been characterized for 4 months (March-July 2012) at the CPTR (Compact Payload Test Range) facilities in ESA/ESTEC, the Netherlands.
The transponder has already been used for the calibration of several Cryosat-2 passes (10-May, 8-June and 3-August 2013) over the SLR2 (Satellite Laser Ranging 2) site (35° 32.084' N, 24° 04.061' E) in North West Crete, Greece, and a clear response has been captured on the satellite’s data (Figure 134).
A TUC transponder site has been selected on Crete Island which represents a triple cross-over point between Sentinel-3A, -3B and Jason-2&3 (and also Jason-CS, as it will most likely fly over the same Jason-series tracks). This criterion was used to finally define and freeze the ground tracks for Sentinel-3 mission.
The CDN2 (35° 20.729' N, 23° 46.577'E) site is exactly under Jason, 100 m east of Sentinel-3A and 300 m west of the Sentinel-3B ground tracks. The CNES team will verify the satellite signal observed using Jason-2 around the CDN2 candidate.
Figure 135: A triple cross-over point for Sentinel-3A (red), -3B (purple) and Jason series (yellow) exist at the CDN2 site in western Crete (image credit: TUC)
The instruments at the CDN2 site will be protected using either weather-proof boxes or a container with appropriate covers to avoid/reduce any satellite echoes by their metallic parts. Figure 136 illustrates an indicative spatial distribution of the necessary and ancillary instrumentation to be constructed at the CDN2 Sentinel-3 altimeter calibration site. Besides the instrumentation and infrastructure, the preparatory steps taken for the establishment of the CDN2 site involve also the development of appropriate software for data archival and transmission and for the determination of the transponder’s precise positioning.
The Sentinel-3 altimeter calibration site is expected to be fully operational in early 2014, that is about one year prior to the Sentinel-3A launch. During this period, calibration campaigns for the Jason-2 and Cryosat-2 altimetry mission will be performed to test the transponder’s operational capabilities in real-field conditions. These campaigns will aim at: a) delivering altimeter calibration values for these satellites, b) getting familiarized with the remote operation procedures to be followed, and 3) identifying potential upgrades necessary for improving the transponder’s performance.
The transponder is to be upgraded, improved, and characterized before its final deployment and support for Sentinel-3A commissioning phase in 2015.
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108) ”Nonstop LEOP full stop,” ESA, Feb. 25, 2016, URL: http://www.esa.int/Our_Activities/Operations/Nonstop_LEOP_full_stop
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).
The Sentinel series:
Provides data continuity for:
Validation provided by: