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.
Note: As of January 2020, the previously single large Sentinel-3 file has been split into two files, to make the file handling manageable for all parties concerned, in particular for the user community.
• This article covers the Sentinel-3 mission and its imagery in the period 2020 and 2019
Status of the Sentinel-3 mission
• January 17, 2020: The Copernicus Sentinel-3 mission takes us over the Japanese archipelago – a string of islands that extends about 3000 km into the western Pacific Ocean. 38)
- While the archipelago is made up of over 6000 islands, this image focuses on Japan's four main islands (Figure 11).
- Honshu’s land mass comprises approximately four-fifths of Japan’s total area. Honshu’s main urban areas of Tokyo, Nagoya, and Osaka are clearly visible in the image. The large grey area in the east of the island, near the coast, is Tokyo, while the smaller areas depicted in grey are the areas around Nagoya and Osaka.
- Honshu is also home to the country’s largest mountain, Mount Fuji. A volcano that has been dormant since it erupted in 1707, Mount Fuji is around 100 km southwest of Tokyo and its snow covered summit can be seen as a small white dot.
- The Sea of Japan, also referred to as the East Sea, (visible to the west of the archipelago) separates the country from the east coast of Asia. The turquoise waters surrounding the island of Hokkaido can be seen at the top of the image, while the waters in the right of the image have a silvery hue because of sunglint – an optical effect caused by the mirror-like reflection of sunlight from the water surface back to the satellite sensor.
- Sentinel-3 is a two-satellite mission to supply the coverage and data delivery needed for Europe’s Copernicus environmental monitoring program. Each satellite’s instrument package includes an optical sensor to monitor changes in the color of Earth’s surfaces. It can be used, for example, to monitor ocean biology and water quality.
Figure 11: While the archipelago is made up of over 6000 islands, this image focuses on Japan's four main islands. Running from north to south, Hokkaido is visible in the top right corner, Honshu is the long island stretching in a northeast–southwest arc, Shikoku can be seen just beneath the lower part of Honshu, and Kyushu is at the bottom. This image, which was captured on 24 May 2019, is also featured on the Earth from Space video program (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)
• January 9, 2020: Ferocious bushfires have been sweeping across Australia since September, fuelled by record-breaking temperatures, drought and wind. The country has always experienced fires, but this season has been horrific. A staggering 10 million hectares of land have been burned, at least 24 people have been killed and it has been reported that almost half a billion animals have perished. 39)
- Photographs and film footage have without doubt left the world shocked, but the view from space shows the scale of what Australians are having to deal with.
- New South Wales has been worst hit. The Copernicus Sentinel-3 image of Figure 13 shows smoke pouring from numerous fires in the state on 3 January.
Figure 12: While this image was captured by the mission’s OLCI (Ocean and Land Color Instrument), which offers camera-like images, the mission’s SLSTR (Sea and Land Surface Temperature Radiometer) instrument can record fire hotspots. This instrument works like a thermometer in the sky, measuring thermal infrared radiation to take the temperature of Earth’s land surfaces. The instrument’s two dedicated fire channels are used to compile the World Fire Atlas. The animation here shows how the number of fires increased between October 2019 and January 2020. The measurements were taken by the Copernicus Sentinel-3A satellite at night only, and since the spatial resolution is limited to 1 km, the animation, as shocking as it is, actually underestimates the number of fires (image credit: ESA, the image contains modified Copernicus Sentinel data (2019-2020), processed by ESA)
Figure 13: This Copernicus Sentinel-3 image shows smoke pouring from numerous fires in New South Wales on 3 January 2020 (image credit: ESA, the image contains modified Copernicus Sentinel data (2020), processed by ESA, CC BY-SA 3.0 IGO)
• November 13, 2019: The Copernicus Sentinel-3 mission captured the multiple bushfires burning across Australia’s east coast. Around 150 fires are still burning in New South Wales and Queensland, with hot and dry conditions accompanied with strong winds, said to be spreading the fires. 40)
- Hundreds of homes have been damaged or destroyed, and many residents evacuated. Flame retardant was dropped in some of Sydney’s suburbs as bushfires approached the city center. Firefighters continue to keep the blazes under control.
- The Copernicus Emergency Mapping Service was activated to help respond to the fires. The service uses satellite observations to help civil protection authorities and, in cases of disaster, the international humanitarian community, respond to emergencies.
- Quantifying and monitoring fires is fundamental for the ongoing study of climate, as they have a significant impact on global atmospheric emissions. Data from the Copernicus Sentinel-3 World Fire Atlas shows that there were almost five times as many wildfires in August 2019 compared to August 2018.
Figure 14: In this image, captured on 12 November 2019 at 23:15 UTC (13 November 09:15 local time), the fires burning near the coast are visible. Plumes of smoke can be seen drifting east over the Tasman Sea. Hazardous air quality owing to the smoke haze has reached the cities of Sydney and Brisbane and is affecting residents, the Australian Environmental Department has warned (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)
• October 25, 2019: Wildfires have been making headlines again this month, with multiple fires burning in Lebanon and California, but these are just some of the many fires 2019 has seen. Fires in the Amazon sparked a global outcry this summer, but fires have also been blazing in the Arctic, France, Greece, Indonesia as well as many other areas in the world. 41)
Figure 15: Fires around the world. Global fires detected in August 2018 compared to August 2019. The Sentinel-3 World Fire Atlas recorded 79,000 wildfires in August 2019, compared to just over 16,000 fires during the same period in 2018 (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA on ONDA Copernicus DIAS)
- Data from the Sentinel-3 World Fire Atlas shows that there were almost five times as many wildfires in August 2019 compared to August 2018, but a detailed analysis reveals precisely where these fires have been occurring – most of which were in Asia.
- The Copernicus Sentinel-3 mission recorded 79,000 fires in August this year, compared to just over 16,000 fires detected during the same period last year. These figures were achieved by using data from the Sentinel-3 World Fire Atlas Prototype, which is also able to provide a breakdown of these fires per continent.
Figure 16: The trend of wildfires detected in 2019 are shown in red, while fires detected in 2018 can be seen in green. The Sentinel-3 World Fire Atlas shows 70,000 fires in August 2019, compared to just over 16,632 fires in August 2018 (image credit: ESA)
- The data reveals 49% of fires were detected in Asia, around 28% were detected in South America, 16% in Africa, and the remaining were recorded in North America, Europe and Oceania.
- Working like thermometers in the sky, the sensors on satellites measure thermal infrared radiation to take the temperature of Earth's land surfaces. This information is used to detect and monitor the heat emitted by the fires.
- Using its two dedicated fire channels, the Sentinel-3 World Fire Atlas uses a simplified operational version derived from Wooster et al. 2012 in order to identify all active fires at night.
- Data gathered are used to plot the number of fires occurring monthly. The number of input images from Copernicus Sentinel-3A satellite were around the same from one year to the other.
Figure 17: Out of the 79,000 wildfires detected in 2019, this pie chart shows the breakdown of the fires by continent. Around half of the fires were detected in Asia, 28% in South America, 16% in Africa and the remaining in Europe, Oceania and North America (image credit: ESA)
- Even if the atlas cannot pick up all fires due to satellite overpass constraints and cloud coverage, it is statistically representative from one month to the other and from one year to the other.
- ESA’s Olivier Arino comments, “We have never seen an increase of wildfires of this kind since the ATSR World Fire Atlas was created in 1995.”
- Quantifying and monitoring fires is important for the ongoing study of climate because they have a significant impact on global atmospheric emissions, with biomass burning contributing to the global budgets of greenhouse gases, like carbon dioxide.
- One of the biggest problems during and after fires is obtaining an overall view of the fires evolution and potential damage. With fires seen from space, Earth observation is also being used to detect and monitor the active spots over affected areas.
• October 18, 2019: The Korean Peninsula in East Asia can be seen in this image captured by the Copernicus Sentinel-3 mission. The peninsula is over 900 km long and is located between the Sea of Japan, also known as the East Sea, to the east and the Yellow Sea to the west. 42)
- The peninsula is divided into two countries – the Democratic People's Republic of Korea (North Korea) and the Republic of Korea (South Korea).
- North Korea is divided into nine provinces, with Pyongyang as the capital. Pyongyang, which can be seen in light grey in the upper left of the image, lies on the banks of the Taedong River and on a flat plain about 50 km inland from the Korea Bay.
- The capital of South Korea is Seoul, which is in the northwest of the country, slightly inland and around 50 km south of the North Korean border.
- As the image shows, the Korean peninsula is mostly mountainous and rocky, making less than 20% of the land suitable for farming.
- The Yellow Sea owes its name to the silt-laden waters from the Chinese rivers that empty into it. It is also one of the largest shallow areas of continental shelf in the world with an average depth of around 50 m.
- The waters off the coast of Korea are considered among the best in the world for fishing. The warm and cold currents attract a wide variety of species and the numerous islands, inlets and reefs provide excellent fishing grounds.
Figure 18: This image, which was captured on 21 May 2019 on Sentinel-3, is also featured on the Earth from Space video program (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)
• October 10, 2019: This enormous typhoon Hagibis, which is being compared to a Category 5 hurricane, can be seen in this image captured by the Copernicus Sentinel-3 mission on 10 October at 01:00 GMT (10:00 Japan Standard Time). The eye of the storm has a diameter of approximately 60 km. 43)
Figure 19: Typhoon Hagibis is headed towards Japan’s main island of Honshu, where it is expected to make landfall over the weekend. Japan is bracing for potential damage from strong winds and torrential rain (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)
• September 3, 2019: This Copernicus Sentinel-3 image features Hurricane Dorian as it pummels the Bahamas on 2 September 2019 at 15:16 GMT (11:16 EDT). This mighty storm has been parked over the northwest Bahamas for more than 24 hours unleashing a siege of devastation. Storm surges, wind and rain have claimed at least five lives and destroyed homes and infrastructure. 44)
- Dorian is reported to be one of the most powerful Atlantic hurricanes on record. Residents in Florida, US, are also starting to feel the effects of Dorian, though its path is difficult to predict as it creeps slowly over the Bahamas. However, the US National Hurricane Center expect life-threatening storm surges along Florida’s east coast and along the coasts of Georgia and South Carolina. As the US authorities respond to the devastation, Europe’s Copernicus Emergency Mapping Service has been activated to provide flood maps based on satellite data.
Figure 20: Copernicus Sentinel-3 image of Hurricane Dorian over the Bahamas on 2 September 2019 (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA)
• August 27, 2019: Thousands of fires have broken out in the Amazon rainforest. Satellite data show that there are almost four times as many fires this year compared to the same period last year. Apart from Brazil, parts of Peru, Bolivia, Paraguay and Argentina have also been affected. 45)
- While forest fires normally occur in Brazil’s dry season, which runs from July to October, the unprecedented increase is reported to come from both legal and illegal deforestation which allows land to be used for agricultural purposes, rising global temperatures are also thought to be making the region more susceptible to fire.
- The Amazon basin is the world’s largest tropical rainforest, spanning four countries and is home to millions of plants and animals. It produces around 20% of the world’s oxygen – hence the region being called ‘the lungs of the world’ – and is crucial for helping to regulate global warming as the forests absorb millions of tonnes of carbon emissions every year.
- Using Copernicus Sentinel-3 data, as part of the Sentinel-3 World Fires Atlas, almost 4000 fires were detected from 1 August to 24 August 2019, while last year there were far fewer during the same period, just 1110 fires.
- “By processing 249 images for August 2018 and 275 images for August 2019, we are able to see the incredible number of fires burning in the Amazon. This was achieved by the World Fire Atlas night time algorithm, in order to avoid any possible false alarms with the daytime algorithm,” says ESA’s Olivier Arino.
- Plumes of smoke have spread across the Amazon region. Strong winds have blown smoke to São Paulo – more than 2500 km away— causing a black out in the city. According to the Copernicus Atmosphere Monitoring System (CAMS), smoke has travelled as far as the Atlantic coast.
- CAMS also reports that the fires have released 228 megatons of carbon dioxide into the atmosphere, as well as copious amounts of carbon monoxide. The fires also threaten the lives of many indigenous people.
- The Copernicus Emergency Mapping Service was activated to help respond to the fire. The service uses satellite observations to help civil protection authorities and, in cases of disaster, the international humanitarian community to help to respond to emergencies.
Figure 21: Number of wildfires in the Amazon. Using Copernicus Sentinel-3 data, as part of the Sentinel-3 World Fires Atlas, 3951 fires were detected at night from 1 August to 24 August 2019, compared to 1110 fires detected in 2018 during the same period (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA on ONDA Copernicus DIAS)
Figure 22: Wildfires in Brazil from Copernicus Sentinel-3. An unprecedented amount of fires have broken out in Brazil’s Amazon rainforest. In this image, captured on 21 August 2019, the fires and plumes of smoke can clearly be seen (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)
- The severity of the fires has reached the highest political levels. Deemed an international crisis, the G7 nations who met in France yesterday, have offered a 20 million euro emergency funding to assist Brazil and its neighboring countries to put out the fires, according to French President Emmanuel Macron.
- Josef Aschbacher, ESA’s Director of Earth Observation Programs, said, “As we continue to face the ongoing climate crisis, satellites are essential in monitoring wildfires in remote areas, especially for a key component of the Earth system such as the Amazon.”
Figure 23: Wildfires on the border between Bolivia, Paraguay and Brazil from Copernicus Sentinel-2. This false-color animation captured by the Copernicus Sentinel-2 mission shows the fires breaking out on the border between Bolivia, Paraguay and Brazil. The animation contains three separate images from 8, 18 and 23 August 2019. On the 23 August, the smoke from the fire is visible in blue, while clouds can be seen in white. The orange areas show the burned land (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA; CC BY-SA 3.0 IGO)
• 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. 46)
Figure 24: 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. 47)
Figure 25: 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. 48)
- 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 26: 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. 49)
- 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 27: 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. 50)
Figure 28: 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 29: 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 30: 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. 51)
Figure 31: 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 32: 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. 53) 54)
- 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 33: 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. 55)
- The Copernicus Sentinel-3A satellite takes us over the high, snow-studded Alps under clear skies (Figure 34). 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 34: 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)
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. 56)
Figure 35: Sentinel-3 spacecraft with payload layout (image credit: ESA)
Figure 36: 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 4: OLCI band specifications, in cyan MERIS heritage, in yellow additional bands 60)
Figure 37: 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 38: 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. 61)
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: 62)
• 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 39: 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 40: OLCI optical design (image credit: ESA)
Figure 41: Photo of the OLCI engineering model (image credit: ESA, TAS)
Table 5: Technical characteristics of the Sentinel-3 OLCI instrument