Copernicus: Sentinel-2 — The Optical Imaging Mission for Land Services
Sentinel-2 is a multispectral operational imaging mission within the GMES (Global Monitoring for Environment and Security) program, jointly implemented by the EC (European Commission) and ESA (European Space Agency) for global land observation (data on vegetation, soil and water cover for land, inland waterways and coastal areas, and also provide atmospheric absorption and distortion data corrections) at high resolution with high revisit capability to provide enhanced continuity of data so far provided by SPOT-5 and Landsat-7. 1) 2) 3) 4) 5) 6) 7) 8)
Table 1: Copernicus is the new name of the former GMES program 9)
The overall GMES user requirements of the EU member states call for optical observation services in the areas of Global Climate Change (Kyoto Protocol and ensuing regulations), sustainable development, European environmental policies (e.g. spatial planning for Soil Thematic Strategy, Natura 2000 and Ramsar Convention, Water Framework Directive), European civil protection, common agricultural policy, development and humanitarian aid, and EU Common Foreign & Security Policy.
To meet the user needs, the Sentinel-2 satellite data will support the operational generation of the following high level products like:
• Generic land cover, land use and change detection maps (e.g. CORINE land cover maps update, soil sealing maps, forest area maps)
• Maps of geophysical variables (e.g. leaf area index, leaf chlorophyll content, leaf water content).
The mission is dedicated to the full and systematic coverage of land surface (including major islands) globally with the objective to provide cloud-free products typically every 15 to 30 days over Europe and Africa. To achieve this objective and to provide high mission availability, a constellation of two operational satellites is required, allowing to reach a 5-day geometric revisit time. The revisit time with only one operational satellite as it will be the case at the beginning of the deployment of the system is 10 days. - In comparison, Landsat-7 provides a 16-day geometric revisit time, while SPOT provides a 26-day revisit, and neither of them provides systematic coverage of the overall land surface.
The following list summarizes the top-level system design specifications derived from the user requirements:
• Sentinel-2 will provide continuity of data for services initiated within the GSE (GMES Service Element) projects. It will establish a key European source of data for the GMES Land Fast Track Monitoring Services and will also contribute to the GMES Risk Fast Track Services.
• The frequent revisit and high mission availability goals call for 2 satellites in orbit at a time, each with a 290 km wide swath using a single imaging instrument
• Continuous land + islands carpet mapping imaging within the latitude range of -56º to +83º (the selected orbit excludes imagery from Antarctica)
• 10 m, 20 m, and 60 m spatial resolution (in the VNIR to SWIR spectral range) to identify spatial details consistent with 1 ha MMU (Minimum Mapping Unit)
• An accurate geolocation (< 20 m) of the data is required (without GCPs) and shall be produced automatically to meet the timeliness requirements. The geolocation accuracy of Level 1 b imagery data w.r.t. WGS-84 (World Geodetic System - 1984) reference Earth ellipsoid of better than 20 m at 2σ confidence level without need of any ground control points is required.
• Very good radiometric image quality (combination of onboard absolute and on ground vicarious calibration).
• The mission lifetime is specified as 7.25 years and propellant is to be sized for 12 years, including provision for de-orbiting maneuvers at end-of-life.
• 2 weeks of satellite autonomy and maximum decoupling between flight operations and mission exploitation
Table 2: Sentinel-2 fast track service compliance to land user requirements
To provide operational services over a long period (at least 15 years following the launch of the first satellites), it is foreseen to develop a series of four satellites, with nominally two satellites in operation in orbit and a third one stored on ground as back-up.
In partnership: The Sentinel-2 mission has been made possible thanks to the close collaboration between ESA, the European Commission, industry, service providers and data users. Demonstrating Europe’s technological excellence, its development has involved around 60 companies, led by Airbus Defence and Space in Germany for the satellites and Airbus Defence and Space in France for the multispectral instruments. 10)
The mission has been supported in kind by the French space agency CNES to provide expertise in image processing and calibration, and by the German Aerospace Center DLR that provides the optical communication payload, developed by Tesat Spacecom GmbH.
This piece of technology allows the Sentinel-2 satellites to transmit data via laser to satellites in geostationary orbit carrying the European Data Relay System (EDRS). This new space data highway allows large volumes of data to be relayed very quickly so that information can be even more readily available for users.
Seven years in the making, this novel mission has been built to operate for more than 20 years. Ensuring that it will meet users’ exacting requirements has been a challenging task. Developing Sentinel-2 has involved a number of technical challenges, from early specification in 2007 to qualification and acceptance in 2015.
The satellite requires excellent pointing accuracy and stability and, therefore, high-end orbit and attitude control sensors and actuators. The multispectral imager is the most advanced of its kind, integrating two large visible near-infrared and shortwave infrared focal planes, each equipped with 12 detectors and integrating 450,000 pixels.
Pixels that may fail in the course of the mission can be replaced by redundant pixels. Two kinds of detectors integrate high-quality filters to isolate the spectral bands perfectly. The instrument’s optomechanical stability must be extremely high, which has meant the use of silicon carbide ceramic for its three mirrors and focal plane, and for the telescope structure itself.
The geometric performance requires strong uniformity across the focal planes to avoid image distortion. The radiometric performance excluded any compromise regarding stray light, dictating a tight geometry and arrangement of all the optical and mechanical elements. The instrument is equipped with a calibration and shutter mechanism that integrates a large spectralon sunlight diffuser.
Each satellite has a high level of autonomy, so that they can operate without any intervention from the ground for periods of up to 15 days. This requires sophisticated autonomous failure analysis, detection and correction on board.
The ‘carpet mapping’ imaging plan requires acquisition, storage and transmission of 1.6 TB per orbit. This massive data blast results from the combination of the 290 km swath with 13 spectral channels at a spatial resolution as high as 10 m.
In addition, the optical communication payload using the EDRS data link is a new technology that will improve the amount and speed of data delivery to the users. This was very recently demonstrated by Sentinel-1A, which also carries an optical communication payload.
Land in focus: Ensuring that land is used sustainably, while meeting the food and wood demands of a growing global population – a projected eight billion by 2020 – is one of today’s biggest challenges. The Copernicus land service provides information to help respond to global issues such as this as well as focusing on local matters, or ‘hotspots’, that are prone to specific challenges.
However, this service relies on very fast revisit times, timely and accurate satellite data in order to make meaningful information available to users – hence, the role of Sentinel-2. Through the service, users will have access to information about the health of our vegetation so that informed decisions can be made – whether about addressing climate change or how much water and fertilizer are needed for a maximum harvest.
Sentinel-2 is able to distinguish between different crop types and will deliver timely data on numerous plant indices, such as leaf area index, leaf chlorophyll content and leaf water content – all of which are essential to accurately monitor plant growth. This kind of information is essential for precision farming: helping farmers decide how best to nurture their crops and predict their yield.
While this has obvious economic benefits, this kind of information is also important for developing countries where food security is an issue. The mission’s fast geometric revisit of just five days, when both satellites are operational, and only 10 days with Sentinel-2A alone, along with the mission’s range of spectral bands means that changes in plant health and growth status can be easily monitored.
Sentinel-2 will also provide information about changes in land cover so that areas that have been damaged or destroyed by fire, for example, or affected by deforestation, can be monitored. Urban growth also can be tracked.
The Copernicus services are managed by the European Commission. The five ‘pan-European’ themes covering 39 countries are addressed by the land service, including sealed soil (imperviousness), tree cover density, forest type, and grasslands. There is currently insufficient cloud-free satellite data in high resolution with all the necessary spectral bands that cover Europe fast enough to monitor vegetation when it is growing rapidly in the summer. Sentinel-2 will fill this gap.
This multi-talented mission will also provide information on pollution in lakes and coastal waters at high spatial resolution and with frequent coverage. Frequent coverage is also key to monitoring floods, volcanic eruptions and landslides. This means that Sentinel-2 can contribute to disaster mapping and support humanitarian aid work.
Leading edge: The span of 13 spectral bands, from the visible and the near-infrared to the shortwave infrared at different spatial resolutions ranging from 10 to 60 m on the ground, takes global land monitoring to an unprecedented level.
The four bands at 10 m resolution ensure continuity with missions such as SPOT-5 or Landsat-8 and address user requirements, in particular, for basic land-cover classification. The six bands at 20 m resolution satisfy requirements for enhanced land-cover classification and for the retrieval of geophysical parameters. Bands at 60 m are dedicated mainly to atmospheric corrections and cirrus-cloud screening.
In addition, Sentinel-2 is the first civil optical Earth observation mission of its kind to include three bands in the ‘red edge’, which provide key information on the vegetation state.
Thanks to its high temporal and spatial resolution and to its three red edge bands, Sentinel-2 will be able to see very early changes in plant health. This is particularly useful for the end users and policy makers to detect early signs of food shortages in developing countries (Ref. 10).
Table 3: Facts and figures
In April 2008, ESA awarded the prime contract to Airbus Defence and Space (former EADS-Astrium GmbH) of Friedrichshafen, Germany to provide the first Sentinel-2A Earth observation satellite. In the Sentinel-2 mission program, Astrium is responsible for the satellite’s system design and platform, as well as for satellite integration and testing. Astrium Toulouse will supply the MSI (MultiSpectral Instrument), and Astrium Spain is in charge of the satellite’s structure pre-integrated with its thermal equipment and harness. The industrial core team also comprises Jena Optronik (Germany), Boostec (France), Sener and GMV (Spain). 11) 12) 13) 14)
Sentinel-2 uses the AstroBus-L of EADS Astrium, a standard modular ECSS (European Cooperation for Space Standards) compatible satellite platform compatible with an in-orbit lifetime of up to 10 years, with consumables sizeable according to the mission needs. The platform design is one-failure tolerant and the standard equipment selection is based on minimum Class 2 EEE parts, with compatibility to Class 1 in most cases. The AstroBus-L platform is designed for direct injection into LEO (Low Earth Orbit). Depending on the selection of standard design options, AstroBus-L can operate in a variety of LEOs at different heights and with different inclinations. An essential feature of AstroBus-L is the robust standard FDIR (Failure Detection, Isolation and Recovery) concept, which is hierarchically structured and can easily be adapted to specific mission needs.
Figure 1: Artist's rendition of the Sentinel-2 spacecraft (image credit: ESA, Airbus DS)
The satellite is controlled in 3-axes via high-rate multi-head star trackers, mounted on the camera structure for better pointing accuracy and stability, and gyroscopes and a GNSS receiver assembly. The AOCS (Attitude and Orbit Control Subsystem) comprises the following elements: 18)
• A dual frequency GPS receiver (L1/L2 code) for position and time information
• A STR (Star Tracker) assembly for precise attitude information (use of 3 STRs)
• A RMU (Rate Measurement Unit) for rate damping and yaw acquisition purposes
• A redundant precision IMU (Inertial Measurement Unit) for high-accuracy attitude determination
• Magnetometers (MAG) for Earth magnetic field information
• CESS (Coarse Earth Sun Sensors) for coarse Earth and Sun direction determination
• 4 RW (Reaction Wheels) and 3 MTQ (Magnetic Torquers)
• RCS (Reaction Control System) a monopropellant propulsion system for orbit maintenance with 1 N thrusters
The different tasks of the AOCS are defined the following modes:
• Initial Acquisition and Save Mode (rate damping, Earth acquisition, yaw acquisition, steady-state)
• Normal Mode (nominal and extended observation)
• Orbit Control Mode (in- and out-of-plane ΔV maneuvers).
Figure 2: Overview of the AOCS architecture (image credit: EADS Astrium)
The geolocation accuracy requirements of < 20 m for the imagery translate into the following AOCS performance requirements as stated in Table 4.
For Sentinel-2 it was decided to mount both the IMU and the star trackers on the thermally controlled sensor plate on the MSI. So the impact of time-variant IMU/STR misalignment on the attitude performance can be decreased to an absolute minimum. Furthermore, the consideration of the time-correlated star tracker noises by covariance tuning was decided.
Figure 3: Sentinel-2 spacecraft architecture (image credit: Astrium GmbH)
Figure 4: Block diagram of the Sentinel-2 spacecraft (image credit: EADS Astrium)
The EPS (Electric Power Subsystem) consists of:
• Solar Array (one deployable and rotatable single wing with three panels). Total array area of 7.1 m2. Use of 2016 triple junction GaAs solar cells with integrated diode. Total power of 2300 W (BOL) and 1730 W (EOL). The mass is < 40 kg.
• SADM (Solar Array Drive Mechanism) for array articulation. Use of a two phase stepper motor with µ-stepping to minimize parasitic distortions during MSI operation, motor step angle 1.5º. Mass of < 3.2 kg.
• PCDU (Power Control and Distribution Unit). PCDU with one unregulated 28 V ±4 V main power bus. Mass of < 21.6 kg; the in-orbit life is 12.25 years.
• Li-ion batteries with 8 cells in series. Total capacity of 102 Ah @ EOL. Mass = 51 kg.
Figure 5: Block diagram of the electrical power subsystem (image credit: EADS Astrium)
The OBC is based on the ERC32 PM (Processor Module) architecture. The PLDHS (Payload Data Handling System) provides source data compression from 1.3 Gbit/s to 450 Mbit/s [state-of-the-art lossy compression (wavelet transform)].
The spacecraft mass is ~ 1200 kg, including 275 kg for the MSI instrument, 35 kg for the IR payload (optional) and 80 kg propellant (hydrazine). The S/C power is 1250 W max, including 170 W for the MSI and < 100 W for the IR payload. The spacecraft is designed for a design life of 7.25 years with propellant for 12 years of operations, including deorbiting at EOL (End of Life).
Table 5: Overview of some spacecraft parameters
Figure 6: Schematic view of the deployed Sentinel-2 spacecraft (image credit: EADS Astrium)
Figure 7: The Sentinel-2 spacecraft in launch configuration (image credit: ESA)
Payload data are being stored in NAND flash memory technology SSR (Solid State Recorder) based on integrated CoReCi (Compression Recording and Ciphering) units of Airbus DS, available at various capacities. The CoReCi is an integrated image compressor, mass memory and data ciphering unit designed to process, store and format multi-spectral video instrument data for the satellite downlink. The mass memory utilizes high performance commercial Flash components, ESA qualified and up-screened for their use in space equipment. This new Flash technology allows mass and surface area used in the memory to be reduced by a factor of nearly 20 when compared with the former SD-RAM (Synchronous Dynamic Random Access Memory) based equipment. The first CoReCi unit has been successfully operating on SPOT-6 since September 2012. Sentinel-2A is carrying a CoReCi unit. 19) 20)
The MRCPB (Multi-Résolution par Codage de Plans Binaires) compression algorithm used is a wavelet transform with bit plane coding (similiar to JPEG 2000). This complex algorithm is implemented in a dedicated ASIC, with speeds of up to 25 Mpixel/s. Alternatively this unit can be supplied with a CCSDS compression algorithm using a new ASIC developed with ESA support. The ciphering is based on the AES algorithm with 128 bit keys. The modularity of the design allows the memory capacity and data rate to be adapted by adjusting the number of compressor and memory boards used.
• February 27, 2017: The ninth Vega light-lift launcher is now complete at the Spaceport, with its Sentinel-2B Earth observation satellite installed atop the four-stage vehicle in preparation for a March 6 mission from French Guiana. 21)
• January 12, 2017: Sentinel-2B arrived at Europe’s spaceport in Kourou, French Guiana on 6 January 2017 to be prepared for launch. After being moved to the cleanroom and left for a couple of days to acclimatise, cranes were used to open the container and unveil the satellite. Over the next seven weeks the satellite will be tested and prepared for liftoff on a Vega rocket. 22)
• November 15, 2016: Sentinel-2B has successfully finished its test program at ESA/ESTEC in Noordwijk, The Netherlands. The second Sentinel-2 Airbus built satellite will now be readied for shipment to the Kourou spaceport in French Guiana begin January 2017. It is scheduled for an early March 2017 lift-off on Vega. 23)
- Offering "color vision" for the Copernicus program, Sentinel-2B like its twin satellite Sentinel-2A will deliver optical images from the visible to short-wave infrared range of the electromagnetic spectrum. From an altitude of 786 km, the 1.1 ton satellite will deliver images in 13 spectral bands with a resolution of 10, 20 or 60 m and a uniquely large swath width of 290 km.
• June 15, 2016: Airbus DS completed the manufacture of the Sentinel-2B optical satellite; the spacecraft is ready for environmental testing at ESA/ESTEC. The Sentinel-2 mission, designed and built by a consortium of around 60 companies led by Airbus Defence and Space, is based on a constellation of two identical satellites flying in the same orbit, 180° apart for optimal coverage and data delivery. Together they image all Earth’s land surfaces, large islands, inland and coastal waters every five days at the equator. Sentinel-2A was launched on 23 June 2015, its twin, Sentinel-2B, will follow early next year. 24)
- The Sentinel-1 and -2 satellites are equipped with the Tesat-Spacecom’s LCT (Laser Communication Terminal). The SpaceDataHighway is being implemented within a Public-Private Partnership between ESA and Airbus Defence and Space.
Figure 8: Sentinel-2B being loaded at Airbus Defence and Space’s satellite integration center in Friedrichshafen, Germany (image credit: Airbus DS, A. Ruttloff)
• April 27, 2015: The Sentinel-2A satellite on Arianespace’s next Vega mission is being readied for pre-launch checkout at the Spaceport, which will enable this European Earth observation platform to be orbited in June from French Guiana. — During activity in the Spaceport’s S5 payload processing facility, Sentinel-2A was removed from the shipping container that protected this 1,140 kg class spacecraft during its airlift from Europe to the South American launch site. With Sentinel-2A now connected to its ground support equipment and successfully switched on, the satellite will undergo verifications and final preparations for a scheduled June 11 liftoff. 25)
Figure 9: Sentinel-2A is positioned in the Spaceport’s S5 payload processing facility for preparation ahead of its scheduled June launch on Vega (image credit: Arianespace)
• April 23, 2015: The Sentinel-2A satellite has arrived safe and sound in French Guiana for launch in June. The huge Antonov cargo aircraft that carried the Sentinel-2A from Germany, touched down at Cayenne airport in the early morning of 21 April. 26)
• April 8, 2015: The Sentinel-2A satellite is now being carefully packed away in a special container that will keep it safe during its journey to the launch site in French Guiana. The satellite will have one final test, a ‘leak test’, in the container to ensure the propulsion system is tight. Bound for Europe’s Spaceport in French Guiana, Sentinel-2A will leave Munich aboard an Antonov cargo plane on 20 April. Once unloaded and unpacked, it will spend the following weeks being prepared for liftoff on a Vega rocket. 27)
• February 24, 2015: Sentinel-2A is fully integrated at IABG’s facilities in Ottobrunn, Germany before being packed up and shipped to French Guiana for a scheduled launch in June 2015. 28)
Figure 10: Photo of the Sentinel-2A spacecraft in the thermal vacuum chamber testing at IAGB's facilities (image credit: ESA, IABG, 2015)
• In August 2014, Airbus Defence and Space delivered the Sentinel-2A environmental monitoring satellite for testing . In the coming months, the Sentinel-2A satellite will undergo a series of environmental tests at IABG, Ottobrunn, Germany, to determine its suitability for use in space. 29) 30)
Figure 11: Sentinel-2A solar array deployment test at IABG (Airbus Defence & Space), image credit: ESA 31)
- Sentinel-2A is scheduled to launch in June 2015; Sentinel-2B, which is identical in design, is set to follow in March 2017. Together, these two satellites will be able to capture images of our planet’s entire land surface in just five days in a systematic manner.
Figure 12: Photo of the Sentinel-2A spacecraft at the satellite integration center in Friedrichshafen, Germany (image credit: Airbus DS, A. Ruttloff)
RF communications: The payload data handling is based on a 2.4 Tbit solid state mass memory and the payload data downlink is performed at a data rate of 560 Mbit/s in X-band with 8 PSK modulation and an isoflux antenna, compliant with the spectrum bandwidth allocated by the ITU (international Telecommunication Union).
Command and control of the spacecraft (TT&C) is performed with omnidirectional S-band antenna coverage using a helix and a patch antenna. The TT&C S-band link provides 64 kbit/s in uplink (with authenticated/encrypted commands) and 2 Mbit/s in downlink..
The requirements call for 4 core X-band ground stations for full mission data recovery by the GMES PDS (Payload Data System).
In parallel to the RF communications, an optical LEO-GEO communications link using the LCT (Laser Communication Terminal) of Tesat-Spacecom (Backnang, Germany) will be provided on the Sentinel-2 spacecraft. The LCT is based on a heritage design (TerraSAR-X) with a transmit power of 2.2 W and a telescope of 135 mm aperture to meet the requirement of the larger link distance. The GEO LCT will be accommodated on AlphaSat of ESA/industry (launch 2012) and later on the EDRS (European Data Relay Satellite) system of ESA. The GEO relay consists of an optical 2.8 Gbit/s (1.8 Gbit/s user data) communication link from the LEO to the GEO satellite and of a 600 Mbit/s Ka-band communication link from the GEO satellite to the ground. 34)
To meet the user requirements of fast data delivery, DLR (German Aerospace Center) is funding the OCP (Optical Communication Payload), i.e. the LCT of Tesat, – a new capability to download large volumes of data from the Sentinel-2 and Sentinel-1 Earth observation satellites - via a data relay satellite directly to the ground. A contract to this effect was signed in October 2010 between ESA and DLR. 35)
Since the Ka-band downlink is the bottleneck for the whole GEO relay system, an optical ground station for a 5.625 Gbit/s LEO-to-ground and a 2.8 Gbit/s GEO-to-ground communication link is under development.
Orbit: Sun-synchronous orbit, altitude = 786 km, inclination = 98.5º, (14+3/10 revolutions/day) with 10:30 hours LTDN (Local Time at Descending Node). This local time has been selected as the best compromise between cloud cover minimization and sun illumination.
The orbit is fully consistent with SPOT and very close to the Landsat local time, allowing seamless combination of Sentinel-2 data with historical data from legacy missions to build long-term temporal series. The two Sentinel-2 satellites will be equally spaced (180º phasing) in the same orbital plane for a 5 day revisit cycle at the equator.
The Sentinel-2 satellites will systematically acquire observations over land and coastal areas from -56° to 84° latitude including islands larger 100 km2, EU islands, all other islands less than 20 km from the coastline, the whole Mediterranean Sea, all inland water bodies and closed seas. Over specific calibration sites, for example DOME-C in Antarctica, additional observations will be made. The two satellites will work on opposite sides of the orbit (Figure 13).
• The first stage separated 1 min 55 seconds after liftoff, followed by the second stage and fairing at 3 min 39 seconds and 3 min 56 seconds, respectively, and the third stage at 6 min 32 seconds.
• After two more ignitions, Vega’s upper stage delivered Sentinel-2B into the targeted Sun-synchronous orbit. The satellite separated from the stage 57 min 57 seconds into the flight.
• Telemetry links and attitude control were then established by controllers at ESOC in Darmstadt, Germany, allowing activation of Sentinel’s systems to begin. The satellite’s solar panel has already been deployed.
• After this first ‘launch and early orbit’ phase, which typically lasts three days, controllers will begin checking and calibrating the instruments to commission the satellite. The mission is expected to begin operations in three to four months.
Sentinel-2B will join its sister satellite Sentinel-2A and the other Sentinels part of the Copernicus program, the most ambitious Earth observation program to date. Sentinel-2A and -2B will be supplying ‘color vision’ for Copernicus and together they can cover all land surfaces once every five days thus optimizing global coverage and the data delivery for numerous applications. The data provided by these Sentinel-2 satellites is particularly suited for agricultural purposes, such as managing administration and precision farming.
With two satellites in orbit it will take only five days to produce an image of the entire Earth between the latitudes of 56º south and 84º north, thereby optimizing the global coverage zone and data transmission for numerous applications.
To ensure data continuity two further optical satellites, Sentinel-2C and -2D, are being constructed in the cleanrooms of Airbus and will be ready for launch as of 2020/2021.
Figure 14: Illustration of the Sentinel-2B spacecraft in orbit (image credit: Airbus DS, Ref. 38)
Figure 15: This technical view of the Sentinel-2 satellite shows all the inner components that make up this state-of-the-art high-resolution multispectral mission (video credit: ESA/ATG medialab)
Figure 16: As well as imaging in high resolution and in different wavelengths, the key to assessing change in vegetation is to image the same place frequently. The Sentinel-2 mission is based on a constellation of two satellites orbiting 180° apart, which along with their 290 km-wide swaths, allows Earth’s main land surfaces, large islands, inland and coastal waters to be covered every five days. This is a significant improvement on earlier missions in the probability of gaining a cloud-free look at a particular location, making it easier to monitor changes in plant health and growth (video credit: ESA/ATG medialab)
Note: As of March 2020, the Sentinel-2 file has been split into a total of four files. — As of May 2019, the previously single large Sentinel-2 file has been split into two additional files, to make the file handling manageable for all parties concerned, in particular for the user community.
• This article covers the Sentinel-2 mission and its imagery in the period 2020
Mission status and imagery of 2020
• March 20, 2020: Earth’s land is covered by a range of different types of vegetation, from forest and marsh to crops and bodies of water, as well as the artificial surfaces that are an increasingly common feature of our landscape. 40)
- Mapping land cover is not only essential for monitoring change, but it also underpins numerous practical applications. However, generating these maps entails handling huge amounts of satellite data and some technical expertise. Thanks to the Copernicus Sentinel-2 mission and new cloud-computing resources, fully automated land-cover maps in 10 m resolution are on the horizon.
- Natural processes, climate change and the way we use land to feed, shelter and support a growing population means that Earth’s land cover is in a continual state of change.
- Information on land cover is important at many levels – at local, regional, national and global scales, and over different timescales.
- Up-to-date maps are a basic source of information to track the impact that human activity, natural processes and climate change have on land cover. These maps are critical for making informed policy, development and resource management decisions, and for disciplines such as agriculture, forestry, water management, urban planning, environmental protection and crisis management.
- While the Copernicus Sentinel-2 mission delivers ideal images to map land cover, producing maps means that huge amounts of time-series data have to be processed. To make this possible, the ESA-funded Sentinel-2 for Science Land Cover project explored novel ways of capitalizing on the latest cloud-computing technologies and machine learning to automate mapping. While still in the experimental stage, the results demonstrate that fully-automated mapping is just around the corner. For example, Europe’s land-cover has been mapped showing 13 land cover classifications.
- Orbiting almost 800 km above, the two-satellite Copernicus Sentinel-2 mission is on hand to map land cover in 10 m resolution.
Figure 17: Europe land-cover mapped in 10 m resolution of 2017 (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by CBK PANsí mi)
- Each identical satellite carries a multispectral imager that can distinguish between different classes of cover such as forest, cultivated areas, grassland, water and artificial surfaces like roads and buildings. The mission can also be used to determine plant indices such as the amount of chlorophyll and water in leaves so that changes in plant health and growth can be monitored.
Figure 18: Austria land cover map (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by CBK PANsí mi)
- Through this experiment, different methodologies were explored and tested over different areas of the world, including the full European region.
- The scientists used dedicated software developed by the Space Research Center of the Polish Academy of Sciences, CBK PAN, to process the satellite images and auxiliary data.
- Stanislaw Lewinski, from CBK PAN, said, “Indeed mapping land cover is a real technical undertaking, but thanks to funding from ESA we have developed a classification methodology that is mainly automated to make land-cover mapping easier.
Figure 19: Poland land cover map (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by CBK PANsí mi)
- “Our system is based on Copernicus Sentinel-2 imagery where each image tile has been classified separately using a set of images from different times, and we chose a pixel-based approach to maintain the mission’s 10 m resolution. Importantly, it also involved many tests in selected areas across Europe. The final maps have been produced on a platform called CREODIAS with the algorithms and software that we developed.”
- CREODIAS is a large-scale computing and data storage platform that enables processing and publication of results of large-scale data analysis activities. The result is a map of Europe at 10 m resolution displaying 13 land-cover classes.
- ESA’s Espen Volden noted, “While we are still in the experimental stage and some land-cover classes don’t reach an accuracy that can be exploited directly and there are some other artefacts, the results are very promising. We demonstrate that fully-automated mapping is on the horizon, opening the way to much more frequently updated land-cover information than has been possible so far.”
Figure 20: Italy’s land classified map (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by CBK PANsí mi)
Viewing and download
- The map of Europe at 10 m resolution displaying 13 land-cover classes can be viewed in full resolution on the CREOSIAS EO Browser (select S2GLC and click search), or accessed as a WMS layer for expert use.
- All classified Sentinel-2 tiles can be downloaded from the CREODIAS finder.
Figure 21: Greece land cover map (image credit: ESA, the image contains modified Copernicus Sentinel data (2017), processed by CBK PANsí mi)
• March 20, 2020: The Copernicus Sentinel-2 mission takes us over Kuwait in the Middle East. With a total area of around 17,800 km2, Kuwait is considered one of the smallest countries in the world. At its most distant points, it is around 200 km north to south and 170 km east to west. 41)
- Situated in the northeast of the Arabian Peninsula, Kuwait shares its borders with Iraq to the north and Saudi Arabia to the south. Kuwait is generally low lying, with the highest point being only 300 m above sea level.
Figure 22: The flat, sandy Arabian Desert covers the majority of Kuwait and appears as a vast expanse of light sand-colored terrain in this image, captured on 25 July 2019. During the dry season, between April and September, the heat in the desert can be severe with daytime temperatures reaching 45ºC and, on occasion, over 50ºC. This image 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)
- Kuwait City, visible jutting out into Kuwait Bay, holds most of the country’s population – making Kuwait one of the most urbanized countries in the world.
- The various colors of Kuwait Bay come from a combination of wind and the amount of sunlight reflected off the waters. The Sheikh Jaber Al-Ahmad Al-Sabah Causeway can be seen cutting across the bay. The bridge is 36 km long – making it the fourth largest bridge in the world.
- Al-Jahra lies around 50 km west of Kuwait City and is visible as a small, green oasis on the west side of Kuwait Bay. It is the center of the country’s principal agricultural region – producing primarily fruits and vegetables. The circular shapes to the right of Al-Jahra are an example of the pivot irrigation or center-pivot irrigation method, where equipment rotates around a central pivot and crops are watered with sprinklers.
- Just south of Kuwait City lies the Great Burgan oil field – considered the second largest oil field in the world. The Great Burgan comprises three smaller fields: Burgan, Al-Maqwa and Al-Ahmadi. The oil fields can be identified an extensive network of interlocking roads which connect the individual wellheads.
- Satellites, such as Copernicus Sentinel-2, allow us to capture images such as these from space, but also allows us to monitor changing places on Earth. Flying 800 km above, satellites take the pulse of our planet by systematically imaging and measuring changes taking place, which is particularly important in regions that are otherwise difficult to access.
• March 13, 2020: The Copernicus Sentinel-2 mission takes us over Victoria Falls – one of the world’s greatest natural wonders. Victoria Falls, known locally as Mosi-oa Tunya or ‘the smoke that thunders,’ lies along the course of the Zambezi River, on the border between Zambia to the north and Zimbabwe to the south. The Zambezi River flows for around 3500 km from its source on the Central African Plateau and empties into the Indian Ocean. 42)
Figure 23: In this image, captured on 22 February 2019, the river cuts from left to right in the image before plunging over Victoria Falls – visible as a white line in the image. While it is neither the highest nor the widest waterfall in the world, Victoria Falls has a width of around 1700 m and a height of over 100 m which classifies it as the world’s largest sheet of falling water. This image is also featured on the Earth from Space video program (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA)
- The spray from the falls normally rises to a height of over 400 m and is sometimes visible from up to 40 km away. The water from the Zambezi River then continues and enters a narrow, zigzagging series of gorges, visible in the bottom right of the image.
- Despite recent reports of Victoria Falls drying up, the Zambezi River is subject to large seasonal fluctuations – with water levels rising and dropping dramatically throughout the year. According to the Zambezi River Authority, the lowest recorded water flows recorded were during the 1995—96 season, which had an annual mean flow of around 390 m3/s, compared to the long-term mean annual flow of around 1100 m3/s.
- The town of Victoria Falls, in Zimbabwe, can be seen west of the falls, while the town of Livingstone – named after the famous Scottish explorer – is visible just north of the falls, in Zambia. The Harry Mwanga Nkumbula airport can be seen west of the town.
- The circular shapes in the image are an example of an irrigation method called pivot irrigation or center-pivot irrigation, where equipment rotates around a central pivot and crops are watered with sprinklers.
• March 6, 2020: Thousands of fires broke out in the Amazon last year – sparking an international media frenzy. A detailed analysis, using data from the European Space Agency’s Climate Change Initiative, indicates that while there was a small increase of fires in 2019 compared to 2018, fires in Brazil were similar to the average annual number of fires detected over the past 18 years. 43)
- While forest fires are common in the Amazon region, they vary considerably from year-to-year driven by changes in climate, as well as variations in deforestation and forest degradation.
- Attention on fires last year sparked an international demand for up-to-date information on active fires – particularly in Brazil. However, these numbers were never compared to the number of fires over a longer period of time.
- Detailed in a recent paper published in Remote Sensing, scientists using data from ESA’s Fire CCI project, analyzed burned areas in South America in both 2018 and 2019 – and compared the data to the 2001-18 yearly average. 44)
- According to the report, the total burned area in South America was around 70% more in 2019 compared to the same period of 2018, however only slightly more than the yearly average over the past 17 years.
- These results are particularly interesting for Brazil, which only saw a 1.7% increase of burned area in 2019 compared to the long-term average.
- Bolivia, on the other hand, saw a 51.4% increase of burned areas in 2019, compared to the yearly average.
- Emilio Chuvieco, science leader of the Fire CCI project, comments, “Studies such as these are important to quantify and monitor fire activities in places such as the Amazon. However, they indicate the importance of long-term data series and studies using higher resolution sensors, such as the Copernicus Sentinel-2 multispectral instrument, to detect fires.”
Figure 24: This map shows the increase or decrease of the total burned area in 2019 compared to the 2001-2018 average (image credit: Lizundia-Loiola, J., Pettinari, M.L., & Chuvieco, E. (2020). Temporal Anomalies in Burned Area)
- Earth observing satellites can be used to detect and monitor fires over frequently affected areas. These burned area estimates are from ESA’s Fire Climate Change Initiative project, which produces long-term datasets of burned area information from satellites, as part of the ESA Climate Change Initiative.
- The data is of use for those interested in historical burned patterns, fire management and emissions analysis and climate change research, by providing a consistent burned area time series.
- Josef Aschbacher, ESA’s Director of Earth Observation Programs, says, “These observations show the challenge we are facing - the processes on Earth and in the forests are very dynamic. The unusual increase of fire activity in 2019 demonstrates that satellite data is essential to get a clear and independent picture in order to also understand long-term trends.”
Figure 25: This graph shows the country distribution of burned areas for 2018, 2019 as well as the average for the 2001-2018 period. Brazil has a 1.7% increase of burned area in 2019 compared to the long-term average [image credit: Lizundia-Loiola, J., Pettinari, M. L., & Chuvieco, E. (2020)]
- Tropical forests are home to around half of the world’s biodiversity, and are considered a fundamental part of Earth’s ecosystem. Quantifying fires in forests is important for the ongoing study of climate, as they have a significant impact on atmospheric emissions – with biomass burning contributing to the global budgets of greenhouse gases.
• March 4, 2020: A powerful winter storm, with lake-effect snow, brought blizzard conditions to New York last week and buried the area surrounding the Great Lakes under a blanket of snow. Days of strong winds, with speeds of over 90 km/h, blew lake water ashore, encasing several homes in ice. 45)
- Lake-effect snow is a weather phenomenon that occurs when cold, dry air picks up moisture by passing over relatively warmer lake waters. The air rises and forms clouds, generating what is known as lake-effect snow. This lake-effect snow is common in the Great Lakes area – where cold air, usually from Canada, moves in.
Figure 26: This image, captured by the Copernicus Sentinel-2 mission on 29 February, shows the extent of the snow in the area surrounding Lake St. Clair, Lake Erie and Lake Huron. A layer of ice can be seen over both Lake St. Clair and Lake Erie (image credit: ESA, the image contains modified Copernicus Sentinel data (2020), processed by ESA, CC BY-SA 3.0 IGO)
• February 28, 2020: Andros Island, the largest island of the Bahamas, is featured in this false-color image captured by the Copernicus Sentinel-2 mission. This image was processed in a way that included the near-infrared channel, which highlights the island’s vegetation in bright red. 46)
- Andros is around 160 km from north to south, and 70 km from east to west at its widest point. The island is largely unpopulated and has undeveloped stretches of land. Even though it is considered a single island, Andros is an archipelago made up of hundreds of small islets and cays connected by estuaries and swamplands together with three major islands: North Andros, Mangrove Cay and South Andros.
- The island’s west coast features many bays, channels and inlets. The turquoise colors of the ocean show shallow waters, whereas the dark blue colors are the deep ocean.
- The West Side National Park covers the west part of Andros and includes its pristine coastal wetlands. The 6000 km2 park is the largest protected area in the region,and is a prime habitat for bonefish and an important feeding area for the endangered West Indian flamingo.
Figure 27: This image was acquired with Sentinel-2 on 5 September 2019, just days after the mighty Hurricane Dorian passed over the Bahamas and unleashed a siege of destruction. Dorian is reported to be one of the most powerful Atlantic hurricanes on record – with storm surges, wind and rain that claimed many lives, destroyed homes and left thousands of people homeless. This image 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)
- Compared to acquisitions captured in the days leading up to Hurricane Dorian making landfall, the area in the top-left of the image appears to be more flooded owing to heavy rainfall, and several submerged islands can be seen.
- In response to Hurricane Dorian, the Copernicus Emergency Mapping Service was activated. The service uses observations from several Earth observation satellites, such as Copernicus Sentinel-1 and-2, to provide flood, risk and recovery maps.
• February 14, 2020: For Valentine’s Day, we bring you this Copernicus Sentinel-2 image capturing a beautiful heart-shaped geographical formation in the dramatic landscape of the southern highlands of Bolivia. 47)
- The highlands are part of the Altiplano, meaning High Plateau, a region that stretches almost 1000 km from Peru to Bolivia. The landscape consists of a series of basins lying about 3500 m above sea level and is the most extensive area of high plateau on Earth, outside Tibet.
- This particular area featured here is a transition between the desert in the west and the tropical forest in the east. The heart-shaped formation has been molded by many layers of different geological formations over time. The many streams and rivers visible in this image have also contributed to the shaping of the landscape as we see it today.
- Satellites, such as Copernicus Sentinel-2, allow us to capture beautiful images such as these from space, but also to monitor changing places on Earth. Flying 800 km above, satellites take the pulse of our planet by systematically imaging and measuring changes taking place, which is particularly important in regions that are otherwise difficult to access. This allows for informed decisions to be made to help protect our world for future generations and for all citizens that inhabit our beloved Earth.
- We are sending all our love for Valentine’s Day from the high plateaus of Bolivia – and hope we continue our celebration of love for Earth every day of the year.
Figure 28: This false-color composite image of Sentinel-2 was processed by selecting spectral bands that can be used for classifying geological features – but here the image processing also highlights this lovely heart for today’s image. Sucre, the capital of the Chuquisaca Department, is visible at the top of the image in grey. Designated a UNESCO World Heritage Site, the city lies at an elevation of around 2800 m above sea level. To the left of Sucre, the Maragua crater can be seen – a popular hiking destination. This image, which was captured on 26 January 2020, is also featured on the Earth from Space video program (image credit: ESA, the image contains modified Copernicus Sentinel data (2020), processed by ESA, CC BY-SA 3.0 IGO)
• February 12, 2020: The Pine Island Glacier recently spawned an iceberg over 300 km2 that very quickly shattered into pieces. This almost cloud-free image, captured on 11 February by the Copernicus Sentinel-2 mission, shows the freshly broken bergs in detail (Figure 29). 48)
- A recent animation using 57 radar images captured by the Copernicus Sentinel-1 mission shows just how quickly the emerging cracks from the glacier grew – leading to this historic calving event.
- Thanks to the combination of both optical and radar images from the Copernicus Sentinel satellite missions, growing cracks were spotted in the Pine Island Glacier last year, and since then, scientists have been keeping a close eye on how quick the cracks were growing.
- The Pine Island Glacier, along with its neighbor Thwaites glacier, connect the center of the West Antarctic Ice Sheet with the ocean, and together discharge significant quantities of ice into the ocean.
Figure 29: This image of the Pine Island Glacier, captured on 11 February 2020 by the Copernicus Sentinel-2 mission, shows the freshly broken bergs in detail (image credit: ESA, the image contains modified Copernicus Sentinel data (2020), processed by ESA, CC BY-SA 3.0 IGO)
• January 31, 2020: World Wetlands Day is celebrated internationally each year on 2 February. It marks the anniversary of the signing of the Convention on Wetlands of International Importance, known as the Ramsar Convention, in Ramsar, Iran, on 2 February 1971. 49)
- World Wetlands Day raises global awareness about the vital role of wetlands for our planet, paying particular attention to wetland biodiversity.
- The equatorial lake (Figure 30) covers an area of around 250 km2 and has an average depth of around 2.4 meters. Lake George is fed by a complex system of rivers and streams originating from the Rwenzori mountains – supplying a system of permanent swamps surrounding the lake.
- A dense fringe of wetland grass, visible in bright green, can be seen around the edges of the lake in the center of the image.
- The wetlands provide a natural living space for a number of mammals including elephants, hippopotamus and antelope. They also provide a habitat for over 150 species of birds including several rare species such as the saddle-billed stork.
- Seen from above, the waters of Lake George appear green as a result of the thick concentration of blue-green algae. Metal pollution, mine seepage and agricultural runoff has caused serious pollution to the lake’s waters and are severely impacting the lake’s health.
- Lake George drains through the Kazinga Channel in the image’s center. The wide, 32km long channel connects Lake George with Lake Edward, which lies on the border between Uganda and the Democratic Republic of the Congo.
- The Kazinga Channel flows through the Queen Elizabeth National Park. The almost 2000 km2 park is known for its wildlife including the African buffalo and the Nile crocodile.
- The park is also famous for its volcanic features, including volcanic cones and deep craters which can be seen dotted around the image. Many contain crater lakes, including the Katwe crater lake, whose salt deposits have been mined for centuries.
Figure 30: This Copernicus Sentinel-2 image takes us over Lake George, in western Uganda. In 1988, Lake George was designated as Uganda’s first Ramsar site, given its importance as a center for biological diversity. 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)
• January 24, 2020: This Copernicus Sentinel-2 image features an area in the Santa Cruz Department of Bolivia, where part of the tropical dry forest has been cleared for agricultural use. 50)
- Since the 1980s, the area has been rapidly deforested owing to a large agricultural development effort where people from the Andean high plains (the Altiplano region) have been relocated to the lowlands of Bolivia.
- The relatively flat lowlands and abundant rainfall make this region suitable for farming. In fact, the local climate allows farmers to benefit from two growing seasons. The region has been transformed from dense forest into a patterned expanse of agricultural land. This deforestation method, common in this part of Bolivia, is characterized by the radial patterns that can be seen clearly in the image.
- Each patterned field is approximately 6.25 km2 and each side is around 2.5 km long (Figure 31).
- Small settlements can be seen in the center of each individual field in the image, which typically contain a church, a school and a soccer field. These communities are joined by a road network depicted by the straight lines that bisect the radial fields and connect the adjacent areas.
- Meandering streams and rivers can be seen flowing through the fields. The long, thin strips of land in the top right of the image are most likely cultivated soybean fields.
- Rainforests worldwide are being destroyed at an alarming rate. This is of great concern as they play an important role in global climate, and are home to a wide variety of plants and animals.
- Because of their unique perspective from space, Earth observation satellites are instrumental in providing comprehensive information on the full extent and rate of deforestation, which is particularly useful for monitoring remote areas.
Figure 31: This composite image was created by combing three separate ‘Normalized Difference Vegetation Index’ images from the Copernicus Sentinel-2 mission. The first image, from 8 April 2019, is visible in red; the second from 22 June 2019, can be seen in green; and the third from 5 September 2019 can be seen in blue. The Normalized Difference Vegetation Index is widely used in remote sensing as it gives scientists an accurate measure of healthy and status of plant growth. This image 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 23, 2020: The Philippines’ Taal volcano erupted on 12 January 2020 – spewing an ash plume approximately 15 km high and forcing large-scale evacuations in the nearby area. 51)
- The optical image of Figure 32 has also been processed using the mission’s short-wave infrared band to show the ongoing activity in the crater, visible in bright red. Ash blown by strong winds can be seen in Agoncillo, visible southwest of the Taal volcano. Ash has also been recorded in other areas of the Batangas province, as well as Manila and Quezon.
- According to The Philippine Institute of Volcanology and Seismology bulletin published today, sulphur dioxide emissions were measured at an average of around 140 tons. The Taal volcano still remains on alert level four, meaning an explosive eruption is possible in the coming hours or days. The highest alert level is five which indicates an eruption is taking place.
- According to the National Disaster Risk Reduction and Management Council, over 50,000 people have been affected so far. In response to the eruption, the Copernicus Emergency Mapping Service was activated. The service uses satellite observations to help civil protection authorities and, in cases of disaster, the international humanitarian community, respond to emergencies.
Figure 32: This almost cloud-free image was captured today 23 January at 02:20 GMT (10:20 local time) by the Copernicus Sentinel-2 mission, and shows the island, in the center of the image, completely covered in a thick layer of ash (image credit: ESA, the image contains modified Copernicus Sentinel data (2020), processed by ESA, CC BY-SA 3.0 IGO)
• January 15, 2020: Heavy rainfall has triggered flooding in southern Iran, particularly in the Sistan and Baluchestan, Hormozgan and Kerman provinces. The downpour has led to blocked roads and destroyed bridges, crops and houses – displacing thousands of people. 52)
- The flooding has also affected Zahedan, as well as Konarak, Saravan, Nik Shahr, Delgan, Bazman, Chabahar, Zarābād and Khash.
- In response to the flood, the Copernicus Emergency Mapping Service was activated. The service uses satellite observations to help civil protection authorities and, in cases of disaster, the international humanitarian community, respond to emergencies.
Figure 33: This image, captured by the Copernicus Sentinel-2 mission, shows the extent of the flooding in the Sistan and Baluchestan province on 13 January 2020. Flooded areas are visible in brown, while the flooded villages are highlighted by dotted circles. Sediment and mud, caused by the heavy rains, can be seen gushing from the Bahu Kalat River, Iran, and Dasht River, Pakistan, into Gwadar Bay (image credit: ESA, the image contains modified Copernicus Sentinel data (2020), processed by ESA, CC BY-SA 3.0 IGO)
• January 10, 2020: The Copernicus Sentinel-2 mission takes us over the Faroe Islands, located halfway between Iceland and Norway in the North Atlantic Ocean. The Faroe Islands are an archipelago made up of 18 jagged islands and are a self-governing nation under the external sovereignty of the Kingdom of Denmark. 53)
- The archipelago is around 80 km wide and has a total area of approximately 1400 km2. The official language of the Faroe Islands is Faroese, a Nordic language which derives from the language of the Norsemen who settled the islands over 1000 years ago.
- The islands have a population of around 50,000 inhabitants – as well as 70,000 sheep. Around 40% of the population reside in the capital and largest city of the Faroe Islands, Tórshavn, visible on the island of Streymoy, slightly above the center of the image.
- The islands are a popular destination for birdwatchers, particularly on the island of Mykines, the westernmost island of the Faroese Archipelago. The island provides a breeding and feeding habitat for thousands of birds, including the Atlantic Puffins.
- Several inland water bodies can be seen dotted around the islands. Lake Sørvágsvatn, the largest lake of the Faroe Islands, is visible at the bottom of Vágar Island to the right of Mykines. Vágar Airport, the only airport in the Faroe Islands, can be seen left of the lake.
- The official language of the Faroe Islands is Faroese, a Nordic language which derives from the language of the Norsemen who settled the islands over 1000 years ago.
- The islands are particularly known for their dramatic landscape, grass-roofed houses and treeless moorlands. The Faroe Islands boast over 1000 km of coastline and because of their elongated shape, one can never be more than five km to the ocean from any point of the islands.
Figure 34: In this image of Sentinel-2, captured on 21 June 2018, several clouds can be seen over the Northern Isles, top right of the image. Low vegetation is visible in bright green. The unique landscape of the Faroe Islands was shaped by volcanic activity approximately 50 to 60 million years ago. The original plateau was later restructured by the glaciers of the ice age and the landscape eroded into an archipelago characterized by steep cliffs, deep valleys and narrow fjords. 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)
• 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. 54)
Figure 35: The Copernicus Sentinel-2 mission has been used to image the fires. The Sentinel-2 satellites each carry just one instrument – a high-resolution multispectral imager with 13 spectral bands. The smoke, flames and burn scars can be seen clearly in the image shown here, which was captured on 31 December 2019. The large brownish areas depict burned vegetation and provide an idea of the size of the area affected by the fires here – the brown ‘strip’ running through the image has a width of approximately 50 km and stretches for at least 100 km along the Australian east coast (image credit: ESA, the image contains modified Copernicus Sentinel data (2019), processed by ESA, CC BY-SA 3.0 IGO)
Sensor complement: (MSI)
MSI (Multispectral Imager):
The instrument is based on the pushbroom observation concept. The telescope features a TMA (Three Mirror Anastigmat) design with a pupil diameter of 150 mm, providing a very good imaging quality all across its wide FOV (Field of View). The equivalent swath width is 290 km. The telescope structure and the mirrors are made of silicon carbide (SiC) which allows to minimize thermoelastic deformations. The VNIR focal plane is based on monolithic CMOS (Complementary Metal Oxide Semiconductor) detectors while the SWIR focal plane is based on a MCT (Mercury Cadmium Telluride) detector hybridized on a CMOS read-out circuit. A dichroic beamsplitter provides the spectral separation of VNIR and SWIR channels. 55) 56) 57) 58) 59) 60) 61)
Airbus DS (former EADS Astrium SAS) of Toulouse is prime for the MSI instrument. The industrial core team also comprises Jena Optronik (Germany), Boostec (Bazet, France), Sener and GMV (Spain), and AMOS, Belgium. The VNIR detectors are built by Airbus DS-ISAE-e2v, while the French company Sofradir received a contract to provide the SWIR detectors for MSI.
Calibration: A combination of partial on-board calibration with a sun diffuser and vicarious calibration with ground targets is foreseen to guarantee a high quality radiometric performance. State-of-the-art lossy compression based on wavelet transform is applied to reduce the data volume. The compression ratio will be fine tuned for each spectral band to ensure that there is no significant impact on image quality.
The observation data are digitized on 12 bit. A shutter mechanism is implemented to prevent the instrument from direct viewing of the sun in orbit and from contamination during launch. The average observation time per orbit is 16.3 minutes, while the peak value is 31 minutes (duty cycle of about 16-31%).
Figure 36: MSI instrument architecture (image credit: ESA)
Table 6: MSI instrument parameters
Spectral bands: MSI features 13 spectral bands spanning from the VNIR (Visible and Near Infrared) to the SWIR (Short-Wave Infrared), featuring 4 spectral bands at 10 m, 6 bands at 20 m and 3 bands at 60 m spatial sampling distance (SSD), as shown in Figure 39.
Table 7: Specification of VNIR and SWIR FPAs 62)
Figure 37: The MSI instrument (left) and the associated VNIR focal plane (right), image credit: Airbus DS-ISAE-e2v
Figure 38: Left: VNIR FPA (image credit: Airbus DS-F, ev2); right: SWIR FPA (image credit: Airbus DS-F, Sofradir)
Figure 39: MSI spatial resolution versus waveleng: Sentinel-2’s span of 13 spectral bands, from the visible and the near-infrared to the shortwave infrared at different spatial resolutions ranging from 10 to 60 m on the ground, takes land monitoring to an unprecedented level(image credit: ESA)
Table 8: MSI spectral band specification
The filter-based pushbroom MSI instrument features a unique mirror silicon carbide off-axis telescope (TMA) with a 150 mm pupil feeding two focal planes spectrally separated by a dichroic filter. The telescope comprises three aspheric mirrors: M2 mirror is a simple conic surface, whereas the other mirrors need more aspherization terms. The spectral filtering onto the different VNIR and SWIR spectral bands is ensured by slit filters mounted on top of the detectors. These filters provide the required spectral isolation.
CMOS and hybrid HgCdTe (MCT) detectors are selected to cover the VNIR and SWIR bands. The MSI instrument includes a sun CSM (Calibration and Shutter Mechanism). The 1.4 Tbit image video stream, once acquired and digitized is compressed inside the instrument.
The instrument carries one external sensor assembly that provides the attitude and pointing reference (star tracker assembly) to ensure a 20 m pointing accuracy on the ground before image correction.
The detectors are built by Airbus Defence and Space-ISAE-e2v: they are made of a CMOS die, using 0.35µm CMOS process, integrated in a ceramic package (Figure 40). The VNIR detector has ten spectral bands, two of them featuring an adjacent physical line allowing TDI operating mode, with digital summation performed at VCU (Video and Compression Unit) level. On-chip analog CDS (Correlated Double Sampling) allows to reach a readout noise of the order of 130 µV rms. For each detector, the ten bands are read through 3 outputs at a sample rate of 4.8MHz. The detector sensitivity has been adjusted for each band through CVF (Charge to Voltage conversion Factor) in view of meeting SNR specifications for a reference flux, while avoiding saturation for maximum flux. A black coating deposition on the non-photosensitive area of the CMOS die is implemented to provide high straylight rejection.
Figure 40: Illustration of the MSI VNIR detector (image credit: Airbus DS, Ref. 59)
Figure 41: MSI electrical architecture (image credit: Astrium SAS, Ref. 58)
The filter assemblies are procured from Jena Optronik (JOP) in Germany. A filter assembly is made of filter stripes (one for each spectral band) mounted in a Titanium frame. The aims of the filter assembly are: i) to separate VNIR spectral domain into the ten bands B1 to B9, ii) to prevent stray light effects. This stray light limitation is very efficient since it is made very close to the focal plane. Each filter stripe, corresponding to each spectral band, is aligned and glued in a mechanical mount. A front face frame mechanically clamps the assembly together.
The FEE (Front End Electronics) are procured from CRISA in Spain. Each FEE unit provides electrical interfaces to 3 detectors (power supply, bias voltages, clock and video signals) plus video signal filtering and amplification.
Video and Compression Unit (VCU) is manufactured by JOP and aims i) at processing the video signals delivered by the FEEs : digitization on 12 bits, numerical processing, compression and image CCSDS packet generation, ii) interfacing with the platform (power supply, MIL-BUS, PPS), iii) providing the nominal thermal control of the MSI.
Figure 42: Internal configuration of MSI (image credit: EADS Astrium)
The mechanical structure of MSI instrument holds the 3 mirrors, the beam splitter device, the 2 focal planes and 3 stellar sensors. It is furthermore mounted on the satellite through 3 bolted bipods. This main structure (Figure 43) has a size of 1.47 m long x 0.93 m wide x 0.62 m high with a mass of only 44 kg.
The optical face of these mirror blanks have been grounded by Boostec before and after CVD coating (i.e. before polishing), with a shape defect of few tens of a µm. M1 and M2 are designed to be bolted directly on the main SiC structure. M3 is mounted on the same structure through glued bipods. 63)
Table 9: MSI mirror characteristics
Mirror manufacturing: The mirror optomechanical design was performed by EADS-Astrium on the basis of the SiC-100 sintered silicon carbide from Boostec who produced the mirror blanks and delivered them to AMOS (Advanced Mechanical and Optical Systems), Liege, Belgium. AMOS is in charge of the deposition of a small layer of CVD-SiC (Chemical Vapor Deposition-Silicon Carbide) on the mirror. The purpose is to generate a non-porous cladding on the mirror surface which allows the polishing process reaching a microroughness state, compatible with the system requirements regarding straylight. 64)
Figure 44: Optical elements and schematic layout of the MSI telescope (image credit: EADS Astrium)
VNIR and SWIR focal plane assemblies: Both focal planes accommodate 12 elementary detectors in two staggered rows to get the required swath. The SWIR focal plane operates at -80ºC whereas the VNIR focal plane operates at 20ºC. Both focal planes are passively cooled. A monolithic SiC structure provides support to the detectors, the filters and their adjustment devices and offers a direct thermal link to the radiator.
Figure 45: Focal plane configuration (image credit: EADS Astrium)
Filters and detectors: Dedicated strip filters,mounted on top of each VNIR or SWIR detector, provide the required spectral templates for each spectral band. The VNIR detector is made of a CMOS die, using the 0.35 µm CMOS technology, integrated in a ceramic package. The detector architecture enables “correlated double.”
The so-called VNIR Filter Assembly contains 10 VNIR bands (from 443 nm to 945 nm) and the so-called SWIR Filter Assembly includes 3 SWIR bands (from 1375 nm to 2190 nm). The sophisticated development of the filter assemblies is caused by the specified spectral performance parameters and the high stray light requirements due to the topology of the spectral bands. 65)
Sampling for the 10 VNIR spectral bands along with TDI (Time Delay IntegrationI) mode for the 10 m bands. Black coating on the die eliminates scattering.
Figure 46: Photo of the VNIR (top) and SWIR spectral filter assemblies (image credit: Jena Optronik)
Figure 47: Photo of a CMOS detector with black coating (image credit: EADS Astrium)
The SWIR detector is made of an HgCdTe photosensitive material hybridized to a silicon readout circuit (ROIC) and integrated into a dedicated hermetic package. The SWIR detector has three spectral bands for which the spectral efficiency has been optimized. The B11 and B12 bands are being operated in (TDI) mode.
Figure 48: Photo of the EM model of the SWIR detector at hybridization stage (image credit: Sofradir)
CSM (Calibration and Shutter Mechanism): In MSI, the two functions of calibration and shutter are gathered in one single mechanism to reduce mass, cost and quantity of mechanisms of the instrument, increasing its reliability at the same time. The CSM is located at the entrance of MSI, a rectangular device of ~ 80 cm x 30 cm, mounted on the frame of the secondary structure. The design and development of the CSM is provided by Sener Ingenieria y Sistemas, S.A., Spain. 66)
Figure 49: Photo of the CSM (Calibration and Shutter Mechanism) mechanical configuration (image credit: Sener)
Requirements and design drivers:
• During launch the CSM has to protect the instrument from sun illumination and contamination by covering the instrument entrance with a rectangular plate (named the door). This is the close position, which has to be maintained under the action of the launch loads.
• Once in orbit, the following functions are required from the CSM:
- To allow Earth observation to the instrument (MSI) the door needs to rotate from the close position 63º inwards the instrument and maintain it stable without power. This is the open position.
- From time to time, in calibration mode of the MSI, the CSM inserts a sun diffuser in front of the primary mirror and the sun diffuser is illuminated by direct solar flux. This mode corresponds to a door position located 55º from the close position outward the instrument. This position must be also stable without any power supply.
Figure 50: Sentinel-2A/MSI sun diffuser. Size: 700 x 250 mm2 ensuring calibration of each pixel into the FOV (image credit: Airbus DS-F)
- In case of emergency, the CSM has to rotate the door to the close position from any initial position to prevent the sun light to heat sensible components of the instrument. Similarly to the previous positions, the close position shall be stable without power supply.
Figure 51: View of the CSM in calibration position (image credit: Sener)
A face to face ball bearing as rotation axis hinge in the opposite side of the actuator is used supported by means of an axially flexible support. Apart from that the pinpuller mounted on a flexible support, holds the door during launch by means of a cylindrical contact with respect to the door bushing. This design is the result of the optimization made in order to reach a stiff and robust but light and hyper-statically low constrained mechanism to make it compatible under possible thermal environments.
The pinpuller provides a reliable launch locking device and allows after pin retraction the mechanism to rotate in both senses.
The MSI instrument design represents state-of-the-art technology on many levels that is being introduced for next generation European land-surface imagers. Obviously, its performance will set new standards for future spaceborne multispectral imagers.
Storage technology introduction:
MMFU (Mass Memory and Formatting Unit):
The introduction of MMFU by EADS Astrium GmbH and IDA (Institut für Datentechnik und Kommunikationsnetze) at TU Braunschweig represents a new spaceborne storage technology based on SLC (Single Level Cell) NAND-Flash memory devices.
Note: NAND (Not And) is a Boolean logic operation that is true if any single input is false. Two-input NAND gates are often used as the sole logic element on gate array chips, because all Boolean operations can be created from NAND gates.
The NAND storage technology is not only an established technology in commercial applications but represents also a real and effective alternative for mass memory systems in space. The main advantages of the NAND-Flash technology are: a) the non-volatile data storage capability and b) the substantially higher storage density.
In the commercial world the NAND technology has become the preferred solution for storing larger quantities of data on devices such as SSDs (Solid State Drives), USB (Universal Serial Bus) Flash memory sticks, digital cameras, mobile phones and MP3-Players. In the space business, this technology has been used in some experiments only, but not in the frame of large scale mass memory systems. This is now going to be changed. 67) 68) 69)
Astrium and IDA have continuously worked for over seven years on the subject “NAND-Flash Technology for Space”. In the frame of an ESA study dubbed SGDR (Safe Guard Data Recorder) this NAND-Flash technology has been introduced and intensively evaluated.
As a result of this extensive testing, the radiation effects of this technology are well known meanwhile and appropriate error handling mechanisms to cope with the observed effects have been developed. For the S2 (Sentinel-2) mission, a complete qualification program has been performed including radiation tests, assembly qualification, construction analysis, electrical characterization, reliability tests like burn-in, destructive physical analysis, stress and life tests.
All these investments led to the final conclusion that the selected SLC NAND-Flash is an adequate technology for high capacity memory systems for space, even for systems with very high data integrity requirements.
Table 10 lists some main requirements and provides in parallel the related figures of two Astrium MMFU implementations. The first implementation is based on SLC NAND-Flash devices and will be launched with the Sentinel 2 satellite. The second option uses SDR-SDRAM devices, which was the initially required baseline technology for this mission.
The related simplified architectural block diagram of the Astrium Sentinel-2 MMFU is shown in Figure 52. The MMFU receives two parallel data streams either from the nominal or redundant VCU (Video Compression Unit). The interfaces are cross-strapped with redundant PDICs (Payload Data Interface Controllers). After reception and adaptation to internal data formats of the received source packets, the data is stored in memory modules. FMM (Flash Memory Module) and respectively SMM for the SDR-SDRAM memory module. For replay, the data is read out from two parallel operated memory modules and routed via two active TFGs (Transfer Frame Generators) providing interfaces for downlink and test. The system is controlled by a Memory System Supervisor, which is based on an ERC32 processor. The required supply voltages are provided by a power converter.
Each function is implemented by nominal and redundant hardware components. The functions and boards are summarized in Table 11:
Storage capacity: Astrium uses for all boards a standard format. Therefore the maximum number of memory and other devices which can be assembled on one board is limited by this form factor. Both types of memory modules are nearly identical in form, fit and function and because they can be mutually replaced; this represents a good basis for comparison.
The selected NAND-Flash device provides a capacity of 32 Gbit plus some spare. It is realized by means of four 8 Gbit dies encapsulated in a standard TSOP1 package. In total, the FMM (Flash Memory Module) includes 76 devices. The devices are arranged in four partitions which can be independently powered. A partition represents also the lowest level for reconfiguration. Each partition contains sixteen devices to store user data and three devices that are used to store parity information. This configuration enables single symbol error correction and double symbol error detection.
The SDRAM based memory module has a similar organization. There are also four partitions and each devices for single symbol error correction. A device is represented by a stack which contains eight SDRAM chips with a capacity of 512 Mbit each. From this follows the user storage capacity per memory module and some other parameters as listed in Table 12.
The number of FMM modules is determined by the total data rate and the operational concept, which requires the operation of two independent data streams. Therefore there are two memory modules operated in parallel. The third one is provided for redundancy.
The number of SMM modules is mainly determined by the required capacity. Also here two modules are operated in parallel and one SMM is included for reliability reasons.
The much higher storage density of the NAND-Flash devices (factor of 8) leads to a massive reduction in the number of required memory modules. For a mass memory system this becomes especially evident, if there is a requirement for a large user capacity as in case of the Sentinel-2 MMFU. Further positive aspects evolve with reduction of the number of modules. The complete system design from electrical and mechanical point of view is greatly relaxed.
Mass and volume: With reduction of the number of memory modules, it is obvious that directly related physical budgets like mass and volume, decline accordingly. Mass is always a critical issue for space missions which can be reduced by using NAND-Flash technology; but also the complete system design of a satellite, in terms of mass, power, thermal and other aspects, can be positively influenced by applying NAND-Flash based memory systems. In case of the Sentinel-2 MMFU, indeed 14 Kg (about 50%) can be saved.
Power: The power consumption is also reduced by more than 50% (Table 10). This is mainly caused by the number of memory modules operated in parallel. In case of Flash, there are only two active memory modules. In case of the SDRAM technology, 10 memory modules are operated in parallel: up to four modules for data access, two modules for read, two modules for write, and all other modules in data retention mode. Data retention means that the modules store user data and the SDRAM chips have to be refreshed and scrubbed for error detection and correction.
In contrast, a Flash-based memory module can be completely switched off without loss of data in the data retention mode. For a minimum, the partitions can be switched off and the power consumption of the controller part of the module is reduced due to low activity.
It is not obvious, that, in all cases, NAND-Flash consumes less power than SDR-SDRAM based systems. The power consumption depends on several factors like required storage capacity, data rates and operations. Generally it can be said, that as long as the required storage capacity determines the number of memory devices, Flash might be the better choice. If the number of memory devices is determined by the required data rate, SDRAM based systems may have a better performance from a power consumption point of view.
Data rates: Table 13 shows that SDR-SDRAM devices provide a much better performance from data rate point of view. The overall performance of a memory module depends on further characteristics like type of interfaces, memory controller performance, and maximum power consumption and others. Generally an SDRAM based memory module has advantages in terms of access speed.
The lower performance of NAND-Flash is determined by three characteristics. During writing the NAND-Flash devices need to be programmed and this takes a time of about 700 µs per 4 kbyte data (one device page). Additionally the so-called blocks of a NAND-Flash device have to be erased before programming. This consumes another 2 ms per block (64 pages). Last but not least, the selected NAND-Flash devices use an eight bit interface for serial commanding, addressing and data transfer with a maximum operating frequency of 40 MHz.
This lack in performance can be mitigated by mainly two measures. The first straight forward measure is parallel operation of NAND-Flash devices. The second measure is interleaved access to several NAND-Flash devices. Interleaving uses the programming time of a NAND-Flash device to write in parallel the next device. These methods allow increasing the write access performance.
Life time and reliability: NAND-Flash devices provide a limited endurance. This is caused by an inherent wear out mechanism of the Flash memory cells which limits the number of erase and write cycles to about 105 cycles. To mitigate the endurance limitation, most Flash memory systems are equipped with an address management system, which distributes the write accesses rather uniformly over the address space. This technique is called Wear Leveling.
Furthermore the very high device capacity of NAND-Flash devices offers the opportunity to implement a physical address space, which exceeds the required logical user address space by a factor of n. This enhances the wear out limit of the logical addresses by the factor of n too. Hence there are two methods to keep the total count of write accesses to the same physical address below the wear out limit.
Radiation and error rates: In general, sensitivity of electronic devices to space radiation is a major topic and is also shortly discussed here through a comparison of NAND-Flash and SDR-SDRAM devices.
The mass memory system based on NAND-Flash shows clear advantages and fits well to the high storage capacity and moderate data rates of the Sentinel-2 application. The very high storage density of the NAND-Flash devices leads to a reduced number of memory modules with advantages in terms of power consumption, mass and volume. Furthermore this feature improves the reliability and eases the system design from mechanical and electrical points of view.
Figure 53: Photo of the EQM (Engineering Qualification Model), Sentinel-2 MMFU (image credit: Astrium)
Table 14: Parameters of the Sentinel-2 MMFU 70)
For Copernicus operations, ESA has defined the concept and architecture for the Copernicus Core ground segment, consisting of a Flight Operations System (FOS) and a Payload Data Ground Segment (PDGS). Whereas the flight operations and the mission control of Sentinel-1 and -2 is performed by ESOC (ESAs European Space Operations Center in Darmstadt, Germany), the operations of Sentinel-3 and the Sentinel-4/-5 attached payloads to meteorological satellites is performed by EUMETSAT.
The ground segment includes the following elements:
• Flight Operations Segment (FOS): The FOS is responsible for all flight operations of the Sentinel-2 spacecraft including monitoring and control, execution of all platform activities and commanding of the payload schedules. It is based at ESOC, Darmstadt in Germany and comprises the Ground Station and Communications Network, the Flight Operations Control Centre and the General Purpose Communication Network.
• Payload Data Ground Segment (PDGS): The PDGS is responsible for payload and downlink planning, data acquisition, processing, archiving and downstream distribution of the Sentinel-2 satellite data, while contributing to the overall monitoring of the payload and platform in coordination with the FOS.
The Service Segment, geographically decentralized, will utilize the satellite data in combination with other data to deliver customized information services to the final users.
The baseline ground station network will include four core X-band ground stations for payload observation data downlink and one S-band station for Telemetry, Tracking and Control (TT&C). To a limited extent, the system can also accommodate some direct receiving local user ground stations for Near-Real Time applications.
The systematic activities of the PDGS include the coordinated planning of the mission subsystems and all processes cascading from the data acquired from the Sentinel-2 constellation, mainly:
1) The automated and recurrent planning of the satellite observations and transmission to a network of distributed X-band ground stations
2) The systematic acquisition and safeguarding of all spacecraft acquired data, and its processing into higher level products ensuring quality and timeliness targets
3) The recurrent calibration of the instrument as triggered by the quality control processes
4) The automated product circulation across PDGS distributed archives to ensure the required availability and reliability of the data towards users
5) The long-term archiving of all mission data with embedded redundancy over the mission lifetime and beyond.
Figure 54: PGDS context in Sentinel-2 system (image credit: ESA)
Figure 55: The Sentinel-2 ground segment (image credit: ESA)
Figure 56: Physical layout of the PGDS ground stations (image credit: ESA) 71)
• CGS (Core Ground Stations: Matera (Italy), Maspalomas (Spain), Svalbard (Norway), Alaska (USA).
• PAC (Processing/Archiving Center): Farnborough (UK), Madrid (Spain)
• MPC (Mission Performance Center): TBD
• PDMC (Payload Data Management Center): ESA/ESRIN, Frascati, Italy.
Table 15: Sentinel-2 level-1 and level-2 products
Copernicus / Sentinels EDRS system operations:
EDRS (European Data Relay Satellite) will provide a data relay service to Sentinel-1 and -2 and initially is required to support 4 Sentinels simultaneously. Each Sentinel will communicate with a geostationary EDRS satellite via an optical laser link. The EDRS GEO satellite will relay the data to the ground via a Ka-band link. Optionally, the Ka-band downlink is planned to be encrypted, e.g. in support to security relevant applications. Two EDRS geo-stationary satellites are currently planned, providing in-orbit redundancy to the Sentinels. 72)
EDRS will provide the same data at the ground station interface as is available at the input to the OCP (Optical Communications Payload) on-board the satellites, using the same interface as the X-band downlink. The EDRS transparently adapts the Sentinels data rate and format to the internal EDRS rate and formats, e.g. EDRS operates at bit rates of 600 Mbit/s and higher.
With EDRS, instrument data is directly down-linked via data relay to processing and archiving centers, while other data continues to be received at X-band ground stations. The allocation of the data to downlink via X-band or EDRS is handled as part of the Sentinel mission planning system and will take into account the visibility zones of the X-band station network and requirements such as timeliness of data.
Figure 57: Sentinel missions - EDRS interfaces (image credit: ESA)
Copernicus / Sentinel data policy:
The principles of the Sentinel data policy, jointly established by EC and ESA, are based on a full and open access to the data:
• anybody can access acquired Sentinel data; in particular, no difference is made between public, commercial and scientific use and in between European or non-European users (on a best effort basis, taking into consideration technical and financial constraints);
• the licenses for the Sentinel data itself are free of charge;
• the Sentinel data will be made available to the users via a "generic" online access mode, free of charge. "Generic" online access is subject to a user registration process and to the acceptation of generic terms and conditions;
• additional access modes and the delivery of additional products will be tailored to specific user needs, and therefore subject to tailored conditions;
• in the event security restrictions apply to specific Sentinel data affecting data availability or timeliness, specific operational procedures will be activated.
Sen2Coral (SEOM S24Sci Land and Water: Coral Reefs)
The objective of ESA’s SEOM (Scientific Exploration of Operational Missions) Program Sen2Coral is the preparation of the exploitation of the Sentinel-2 mission for coral reefs by developing and validating appropriate, open source algorithm available for the community. The project objectives are the scientific exploitation and validation of the Sentinel-2 MSI (Multispectral Instrument) for mapping (habitat, bathymetry, and water quality) and detection change for coral reef health assessment and monitoring, and algorithm development dedicated to Sentinel-2 capabilities to satisfying these objectives. 78)
To address the extremely interesting and challenging questions posed by this project a consortium of contractors with appropriate background knowledge and skills has been assembled. The consortium comprises:
• ARGANS Limited, UK
• CNR-IREA, Italy
• CS-SI, France
The consortium is complimented by a science team of consultants and partners who are recognized international scientists in the field.
• A critical analysis of feedback from scientists and institutions collected through consultations in ESA and Third Party workshops, symposia and conferences.
• Proposal of potential observation scenarii for Sentinel-2 in terms of required spatial coverage and repeat cycle considering user requirements, existing observation initiatives and synergy with other sensors (Landsat, SPOT sensor families).
• Identifying scientific priority areas and providing guidance for future scientific data exploitation projects. 79)
Tropical coral reefs are globally important environments both in terms of preservation of biodiversity and for the substantial economic value their ecosystem services provide to human communities. Managing and monitoring reefs under current environmental threats requires information on their composition and condition, i.e. the spatial and temporal distribution of benthos and substrates within the reef area. Determining the relative abundance of biotic types such as coral and macroalgae is the key for detecting and monitoring important biotic changes such as phase or regime shifts due to changes in environmental conditions. Coral bleaching events, where stressed corals expel their symbiotic algae and turn white in color, can provide indications of anthropogenic stressors and climate change impacts, while subsequent coral mortality may be a key determinant of future reef state. In addition to monitoring of current status, maps of benthos have the potential to inform management decisions such as the placement of marine protected areas and could in the future be used to seed models to predict ecosystem dynamics.
Figure 58: Image of the North Palau Reef (Western Pacific), acquired with Sentinel-2A on Feb. 10, 2016 (image credit: ESA, Sen2Coral consortium)
Figure 59: Image of Fatu Huku (Pacific) acquired with Sentinel-2A on Feb. 11, 2016 (image credit: ESA, Sen2Coral consortium)
Figure 60: Image of Heron Island, Great Barrier Reef, acquired with Sentinel-2A on Jan. 31, 2016 (image credit: ESA, Sen2Coral consortium)
Background: The degradation of coral reefs is a fact, with 55% of reefs being affected by overfishing and destructive fishing methods, which as the most pervasive threats, whereas 25% of reefs are affected by coastal development and pollution from land, including nutrients from farming and sewage, while one tenth suffer from marine-based pollution (local pressures are most severe in South-East Asia, where nearly 95 per cent of coral reefs are threatened).
In addition the coral reefs’ ecosystems appear to be the first to respond to global climate changes, such as increased sea surface temperature (SST), ultraviolet radiation (UV) and acidification of seawater that results from higher levels of atmospheric CO2 concentration.
Sentinel-2 MSI Data Acquisition:
The MSI (Multispectral Instrument) of the Sentinel-2A mission offers several potential technical advantages in the remote sensing of coral reefs due to:
• 10 meter spatial resolution allowing improvement in visual interpretation of reef features, classification accuracy and bathymetry.
• Additional water penetrating optical band improving consistency under varying water conditions, reducing uncertainty in bottom type and bathymetric mapping, deeper bathymetric accuracy and ability to determine water optical properties.
• Additional NIR/SWIR bands enabling more consistent and accurate determination of atmospheric and surface glint correction.
• Short re-visit time enabling the use of image series to determine fundamental uncertainties for change detection.
• Addresses the current limited remote sensing acquisition plan covering the coast areas.
Figure 61: Overview of processing steps (image credit: Sen2Coral consortium)
Algorithm Development & Data Processing:
The objective “to develop and validate new algorithms relevant for coral reef monitoring based on Sentinel-2 observations” will be addressed by parameterizing existing models for processing hyper-spectral & multi-spectral data and developing pre-processors for these models to build Sentinel-2 data processing algorithms for the retrieval of coral reefs’ static and dynamic characteristics. The code developed will be made available open source.
Validation and uncertainty analysis will involve both comparing Sentinel MSI performance versus Landsat-8 on coral reef mapping objectives and comparing coral reef monitoring products against in situ data from reef sites representative of different composition and structure.
To design, verify and validate three coral reef monitoring products making the best use of Sentinel-2 MSI mission characteristics:
• Habitat mapping of coral reefs
• Coral reef change detection
• Bathymetry over coral reefs.
A 6 day field campaign around the South Pacific island of Fatu Huku was undertaken by French scientist Antoine Collin to collect in-situ data to test and validate the capabilities of the Sentinel-2 satellite to monitor coral reef bleaching.
Fatu Huku Island in French Polynesia was chosen as the survey site because of the presence of developed coral reefs and it is an area water temperatures are high as a result of the current El Niño event. During the survey, water temperature exceeding 30°C were recorded and coral bleaching, the expulsion of the symbiotic algae that provide energy from sunlight to the coral, was observed to be taking place.
Data collected from this field campaign complements archives of in-situ data collected over previous years from coral reef sites across the globe such as at Heron Island and Lizard Island, in Australia, and reefs around Palau, in the western Pacific Ocean.
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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).