Sentinel-2 Optical High Resolution Mission for ... - Semantic Scholar
Sentinel-2 Optical High Resolution Mission for GMES Operational Services Philippe Martimort(1), Olivier Arino(2), Michael Berger(1), Roberto Biasutti(2), Bernardo Carnicero (1), Umberto Del Bello(1), Valérie Fernandez(1), Ferran Gascon(1), Bruno Greco(2), Pierluigi Silvestrin(1), François Spoto(1), Omar Sy(1) ESA/ESTEC(1) Noordwijk, The Netherlands
ESA/ESRIN(2) Frascati, Italy
Abstract— In the frame of the Global Monitoring for Environment and Security programme (GMES) jointly implemented by ESA and EC, ESA is developing the Sentinel-2 system, providing globally with systematic acquisition high resolution (10-20 m) optical observations with a high revisit tailored towards the needs of operational land services. This system will ensure data continuity of SPOT and Landsat satellite series and further enhancement to account of future service evolution. Keywords- Earth Observation, optical payload, multispectral instrument, land monitoring, high resolution, Sentinel missions, GMES
I.
B. Geographic coverage The mission is dedicated to the full and systematic coverage of land surface (including major islands) globally from -56° to +83° latitude with the objective to provide cloudfree products typically every 15 to 30 days, as shown in Fig. 1 over Europe and in Fig. 2 over Africa. In order 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. As a comparison, Landsat-7 provides 16-day geometric revisit time, while SPOT provides 26-day revisit, and neither of them provides systematic coverage of the overall land surface.
MISSION CONCEPT
A. User requirements Sentinel-2 will enable operations of valuable information services to the European Union and its Member States in the frame of the Global Monitoring for Environment and Security programme (GMES) [1], 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.
In order 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 onground as back-up.
In order to meet the user needs, Sentinel-2 satellite data will support the operational generation of the following high level products like (see [2] for more detail about mission requirements): - 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 primary aim drives the design towards a dependable multi-spectral Earth observation system that at the same time ensures continuity to Landsat and SPOT observations and improves data availability for users. That in turn has led to the identification of priority improvements with respect to the past missions.
1-4244-1212-9/07/$25.00 ©2007 IEEE.
2677
Figure 1. Coverage time over Europe in summer with 2 satellites
This mechanism enables to avoid the iterations of the individual ordering, and is made possible by the largely repetitive geographical coverage that makes day-to-day mission planning dispensable. Mission planning is however required to perform the roll-tilt manoeuvre in support to unpredictable emergency requests, due to e.g. natural disaster. E. Mission Description The Sentinel-2 mission definition is thus fully consistent with the user needs, in particular with the GMES Service Elements developed by ESA and the GMES Land & Emergency Fast Track Services developed by EC. Frequent revisit time and high mission availability require two spacecraft operating simultaneously, leading to a small cost-effective low-risk spacecraft. The orbit is sun-synchronous at 786 km altitude (14+3/10 revolutions per day) with 10:30 Local Time at Descending Node. This local time has been selected as the best compromise between cloud cover minimisation and sun illumination. It 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 satellites will be equally spaced in the orbital plane.
Figure 2. Coverage time over Africa in summer with 2 satellites
C. Spectral coverage The Sentinel-2 Multi-Spectral Instrument (MSI) features 13 spectral bands spanning from the visible and near infrared (VNIR) to the short-wave infrared (SWIR), featuring 4 spectral bands at 10 m, 6 bands at 20 m and 3 bands at 60 m spatial resolution, as depicted on Fig. 3. This configuration, selected as the best compromise in terms of user requirements and mission performance, cost and schedule risk, will provide enhanced continuity to SPOT and Landsat, with narrower bands for better feature identification, additional channels in the red edge spectral domain allowing assessing the vegetation status, and dedicated bands for an improved atmospheric correction and cirrus cloud detection.
The baseline ground station network will include four core ground stations for payload observation data downlink and one 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 capability to access more rapidly (within 1 to 3 days) some limited geographical areas in emergency mode by performing a roll-tilt manoeuvre of the spacecraft has also been included in the design and could be used in case to observe major disaster (e.g. floods, earthquakes).
SWIR VNIR
II.
60
20
10 400 nm
600 nm
800 nm
1000 nm
1200 nm
1400 nm
1600 nm
1800 nm
2000 nm
2200 nm
2400 nm
Figure 3. Spectral bands versus spatial sampling distance
D. Processing and Distribution In order to provide operational services, an accurate geolocation (better than 20 m) is required and shall be produced automatically to meet the timeliness requirements. After reception, the data are processed automatically over pre-defined areas in pre-defined time windows, selected on the basis of the user requirements, and are made available on-line and distributed to “subscribing” users, i.e. users having notified their interest for that particular dataset (“subscription”). The dissemination is planned mostly on-line.
1-4244-1212-9/07/$25.00 ©2007 IEEE.
2678
SPACE SEGMENT
A. Satellite definition A compact satellite design (see Fig. 4 showing the spacecraft configuration as established during the Definition Phase) with about 1 ton mass ensures compatibility with small launchers like VEGA as baseline and Rockot as back-up. The lifetime is specified as 7 years and propellant is sized for 12 years, including provision for de-orbiting manoeuvres at endof-life. The roof-shaped configuration with fixed bodymounted solar array leads to a simple mechanical design without solar array deployment and drive mechanisms. The satellite will be three-axis stabilized with an AOCS based on high-rate multi-head star trackers, mounted on the instrument structure for better pointing accuracy and stability, as well as a gyroscope and a GNSS receiver. The power subsystem relies on high efficiency solar cells (GaAs triple junction) and Li-Ion battery. The payload data handling is based on a 2 Tbit solid state mass memory and the payload data downlink is performed at a data rate of 450 Mb/s in X-band with 8PSK modulation and an isoflux antenna, compliant with the spectrum bandwidth allocated by the ITU. Command and control of the spacecraft is performed with omnidirectional Sband antenna coverage using a helix and a patch antenna.
respect to Landsat and SPOT). The telescope structure and the mirrors are made of silicon carbide which allows to minimize thermo-elastic deformations. The VNIR Focal plane is based on monolithic CMOS detectors while the SWIR Focal plane is based on a mercury cadmium telluride (MCT) detector hybridised on a CMOS read-out circuit. A dichroic beamsplitter provides the spectral separation of VNIR and SWIR channels. 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 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.
Figure 4. Sentinel-2 satellite synthetic view
In its orbital configuration (see Fig. 5), the satellite is tilted by 22.5° with respect to the roll axis in order to maximize the solar energy collected by the solar array.
In order to mitigate the development risks and to secure the development schedule, technology pre-development activities have been initiated on critical items, namely on the VNIR and SWIR detectors and on the filter assemblies.
Figure 6. Multi-Spectral Instrument View
Figure 5. Sentinel-2 satellite orbit configuration
B. Payload definition Each satellite carries an optical payload consisting of a Multi-Spectral Imager (MSI) and is sized to accommodate an Infrared Monitor as optional passenger. The MSI depicted on Fig. 6 is based on the pushbroom concept. It features a Three Mirror Anastigmat (TMA) telescope (see Fig. 7) with a pupil diameter of about 150 mm, and achieves a very good imaging quality all across its wide Field of View (290 km swath width, significantly enlarged with
1-4244-1212-9/07/$25.00 ©2007 IEEE.
2679
Figure 7. MSI optical design across view
Leaf Area Index
III. GROUND SEGMENT As depicted on Fig. 8, the ground segment includes the following elements:
Sentinel-2 simulated Level 1 Leaf chlorophyll content
- a Flight Operations Segment for satellite command, monitoring and control;
Fractional cover
- a Payload Data Ground Segment for mission planning, payload data reception, processing, archiving, quality control and dissemination;
Leaf water content
The Service Segment, geographically decentralized, will utilise the satellite data in combination with other data to deliver customised information services to the final users.
Supporting stations (LEOP, emergency)
TMTC station (nominal operations)
Data acquisition
Payload data
Sample data
G/S M&C TT&C Data
Ground Communication Network L0 data
Sensor Performance Products & Algorithms
L0 processing
HK data
IV. CONCLUSION Sentinel-2 wide-swath high-resolution multispectral system will provide enhanced continuity to the SPOT and Landsat satellite series observations – with improved revisit time, coverage area, spectral bands, swath width, radiometric and geometric image quality - contributing significantly to the fulfillment of GMES needs in terms of delivery of operational land and emergency services.
X-band Receiving Stations (nominal operations)
FOS Ground Communication Network
Aux data
Figure 9. Sentinel-2 simulated products for agricultural applications
Cal data Ingestion
Flight Operations Control Centre
Management Facility Spacecraft simulator
S/S manifacturer
Monitoring & Control
Mission Scheduling
Catalogue Order Handling (*)
Flight Dynamics
G/S manifacturer
Mission Planning (*)
User services
Processing Metadata Data distr.
Dissemination Indiv. Orders & subscript.
external agencies NOAA IERS NORAD
In terms of programmatics, ESA has performed the Sentinel-2 Definition Phase in 2005 and 2006 with an industrial consortium led by Astrium GmbH (mission prime, platform, system engineering) with Astrium SAS as major subcontractor (Payload, system support). Following the successful completion of this Phase in January 2007, the Invitation to Tender for the Implementation Phase was released in February 2007. The Implementation Phase is expected to start in October 2007 and the launch of the first satellite is foreseen for 2012.
Archiving
(*) ordering is foreseen mainly for past data and for emergency L1 products
Service segment
END USERS
Figure 8. Sentinel-2 Ground Segment
Sentinel-2 will provide continuity of data for services initiated within the GMES Service Element projects such as: •
Forestry (GSE Forest Monitoring)
•
Soil and water resources mapping
•
Urban mapping and classification (SAGE, Urban Services, Coastwatch, GSE Land, RISK-EOS).
Sentinel-2 will also 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. Some typical Sentinel-2 simulated products for agricultural applications are shown on Fig. 9.
1-4244-1212-9/07/$25.00 ©2007 IEEE.
2680
ACKNOWLEDGMENT The authors would like to thank the members of the industrial consortium who have performed the Sentinel-2 Definition Study, in particular the project teams from AstriumGmbH and Astrium-SAS. The authors would also like to thank all other ESA colleagues who participated to the Sentinel-2 project either at technical or management level, and contributed to the successful start of the programme. REFERENCES [1] [2]
ESA GMES Web Site http://earth.esa.int/gmes/ Sentinel-2 Mission Requirements Document, http://esamultimedia.esa.int/docs/GMES/GMES_Sentinel2_MRD_issue _2.0_update.pdf.
ESA/ESRIN(2) Frascati, Italy
Abstract— In the frame of the Global Monitoring for Environment and Security programme (GMES) jointly implemented by ESA and EC, ESA is developing the Sentinel-2 system, providing globally with systematic acquisition high resolution (10-20 m) optical observations with a high revisit tailored towards the needs of operational land services. This system will ensure data continuity of SPOT and Landsat satellite series and further enhancement to account of future service evolution. Keywords- Earth Observation, optical payload, multispectral instrument, land monitoring, high resolution, Sentinel missions, GMES
I.
B. Geographic coverage The mission is dedicated to the full and systematic coverage of land surface (including major islands) globally from -56° to +83° latitude with the objective to provide cloudfree products typically every 15 to 30 days, as shown in Fig. 1 over Europe and in Fig. 2 over Africa. In order 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. As a comparison, Landsat-7 provides 16-day geometric revisit time, while SPOT provides 26-day revisit, and neither of them provides systematic coverage of the overall land surface.
MISSION CONCEPT
A. User requirements Sentinel-2 will enable operations of valuable information services to the European Union and its Member States in the frame of the Global Monitoring for Environment and Security programme (GMES) [1], 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.
In order 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 onground as back-up.
In order to meet the user needs, Sentinel-2 satellite data will support the operational generation of the following high level products like (see [2] for more detail about mission requirements): - 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 primary aim drives the design towards a dependable multi-spectral Earth observation system that at the same time ensures continuity to Landsat and SPOT observations and improves data availability for users. That in turn has led to the identification of priority improvements with respect to the past missions.
1-4244-1212-9/07/$25.00 ©2007 IEEE.
2677
Figure 1. Coverage time over Europe in summer with 2 satellites
This mechanism enables to avoid the iterations of the individual ordering, and is made possible by the largely repetitive geographical coverage that makes day-to-day mission planning dispensable. Mission planning is however required to perform the roll-tilt manoeuvre in support to unpredictable emergency requests, due to e.g. natural disaster. E. Mission Description The Sentinel-2 mission definition is thus fully consistent with the user needs, in particular with the GMES Service Elements developed by ESA and the GMES Land & Emergency Fast Track Services developed by EC. Frequent revisit time and high mission availability require two spacecraft operating simultaneously, leading to a small cost-effective low-risk spacecraft. The orbit is sun-synchronous at 786 km altitude (14+3/10 revolutions per day) with 10:30 Local Time at Descending Node. This local time has been selected as the best compromise between cloud cover minimisation and sun illumination. It 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 satellites will be equally spaced in the orbital plane.
Figure 2. Coverage time over Africa in summer with 2 satellites
C. Spectral coverage The Sentinel-2 Multi-Spectral Instrument (MSI) features 13 spectral bands spanning from the visible and near infrared (VNIR) to the short-wave infrared (SWIR), featuring 4 spectral bands at 10 m, 6 bands at 20 m and 3 bands at 60 m spatial resolution, as depicted on Fig. 3. This configuration, selected as the best compromise in terms of user requirements and mission performance, cost and schedule risk, will provide enhanced continuity to SPOT and Landsat, with narrower bands for better feature identification, additional channels in the red edge spectral domain allowing assessing the vegetation status, and dedicated bands for an improved atmospheric correction and cirrus cloud detection.
The baseline ground station network will include four core ground stations for payload observation data downlink and one 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 capability to access more rapidly (within 1 to 3 days) some limited geographical areas in emergency mode by performing a roll-tilt manoeuvre of the spacecraft has also been included in the design and could be used in case to observe major disaster (e.g. floods, earthquakes).
SWIR VNIR
II.
60
20
10 400 nm
600 nm
800 nm
1000 nm
1200 nm
1400 nm
1600 nm
1800 nm
2000 nm
2200 nm
2400 nm
Figure 3. Spectral bands versus spatial sampling distance
D. Processing and Distribution In order to provide operational services, an accurate geolocation (better than 20 m) is required and shall be produced automatically to meet the timeliness requirements. After reception, the data are processed automatically over pre-defined areas in pre-defined time windows, selected on the basis of the user requirements, and are made available on-line and distributed to “subscribing” users, i.e. users having notified their interest for that particular dataset (“subscription”). The dissemination is planned mostly on-line.
1-4244-1212-9/07/$25.00 ©2007 IEEE.
2678
SPACE SEGMENT
A. Satellite definition A compact satellite design (see Fig. 4 showing the spacecraft configuration as established during the Definition Phase) with about 1 ton mass ensures compatibility with small launchers like VEGA as baseline and Rockot as back-up. The lifetime is specified as 7 years and propellant is sized for 12 years, including provision for de-orbiting manoeuvres at endof-life. The roof-shaped configuration with fixed bodymounted solar array leads to a simple mechanical design without solar array deployment and drive mechanisms. The satellite will be three-axis stabilized with an AOCS based on high-rate multi-head star trackers, mounted on the instrument structure for better pointing accuracy and stability, as well as a gyroscope and a GNSS receiver. The power subsystem relies on high efficiency solar cells (GaAs triple junction) and Li-Ion battery. The payload data handling is based on a 2 Tbit solid state mass memory and the payload data downlink is performed at a data rate of 450 Mb/s in X-band with 8PSK modulation and an isoflux antenna, compliant with the spectrum bandwidth allocated by the ITU. Command and control of the spacecraft is performed with omnidirectional Sband antenna coverage using a helix and a patch antenna.
respect to Landsat and SPOT). The telescope structure and the mirrors are made of silicon carbide which allows to minimize thermo-elastic deformations. The VNIR Focal plane is based on monolithic CMOS detectors while the SWIR Focal plane is based on a mercury cadmium telluride (MCT) detector hybridised on a CMOS read-out circuit. A dichroic beamsplitter provides the spectral separation of VNIR and SWIR channels. 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 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.
Figure 4. Sentinel-2 satellite synthetic view
In its orbital configuration (see Fig. 5), the satellite is tilted by 22.5° with respect to the roll axis in order to maximize the solar energy collected by the solar array.
In order to mitigate the development risks and to secure the development schedule, technology pre-development activities have been initiated on critical items, namely on the VNIR and SWIR detectors and on the filter assemblies.
Figure 6. Multi-Spectral Instrument View
Figure 5. Sentinel-2 satellite orbit configuration
B. Payload definition Each satellite carries an optical payload consisting of a Multi-Spectral Imager (MSI) and is sized to accommodate an Infrared Monitor as optional passenger. The MSI depicted on Fig. 6 is based on the pushbroom concept. It features a Three Mirror Anastigmat (TMA) telescope (see Fig. 7) with a pupil diameter of about 150 mm, and achieves a very good imaging quality all across its wide Field of View (290 km swath width, significantly enlarged with
1-4244-1212-9/07/$25.00 ©2007 IEEE.
2679
Figure 7. MSI optical design across view
Leaf Area Index
III. GROUND SEGMENT As depicted on Fig. 8, the ground segment includes the following elements:
Sentinel-2 simulated Level 1 Leaf chlorophyll content
- a Flight Operations Segment for satellite command, monitoring and control;
Fractional cover
- a Payload Data Ground Segment for mission planning, payload data reception, processing, archiving, quality control and dissemination;
Leaf water content
The Service Segment, geographically decentralized, will utilise the satellite data in combination with other data to deliver customised information services to the final users.
Supporting stations (LEOP, emergency)
TMTC station (nominal operations)
Data acquisition
Payload data
Sample data
G/S M&C TT&C Data
Ground Communication Network L0 data
Sensor Performance Products & Algorithms
L0 processing
HK data
IV. CONCLUSION Sentinel-2 wide-swath high-resolution multispectral system will provide enhanced continuity to the SPOT and Landsat satellite series observations – with improved revisit time, coverage area, spectral bands, swath width, radiometric and geometric image quality - contributing significantly to the fulfillment of GMES needs in terms of delivery of operational land and emergency services.
X-band Receiving Stations (nominal operations)
FOS Ground Communication Network
Aux data
Figure 9. Sentinel-2 simulated products for agricultural applications
Cal data Ingestion
Flight Operations Control Centre
Management Facility Spacecraft simulator
S/S manifacturer
Monitoring & Control
Mission Scheduling
Catalogue Order Handling (*)
Flight Dynamics
G/S manifacturer
Mission Planning (*)
User services
Processing Metadata Data distr.
Dissemination Indiv. Orders & subscript.
external agencies NOAA IERS NORAD
In terms of programmatics, ESA has performed the Sentinel-2 Definition Phase in 2005 and 2006 with an industrial consortium led by Astrium GmbH (mission prime, platform, system engineering) with Astrium SAS as major subcontractor (Payload, system support). Following the successful completion of this Phase in January 2007, the Invitation to Tender for the Implementation Phase was released in February 2007. The Implementation Phase is expected to start in October 2007 and the launch of the first satellite is foreseen for 2012.
Archiving
(*) ordering is foreseen mainly for past data and for emergency L1 products
Service segment
END USERS
Figure 8. Sentinel-2 Ground Segment
Sentinel-2 will provide continuity of data for services initiated within the GMES Service Element projects such as: •
Forestry (GSE Forest Monitoring)
•
Soil and water resources mapping
•
Urban mapping and classification (SAGE, Urban Services, Coastwatch, GSE Land, RISK-EOS).
Sentinel-2 will also 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. Some typical Sentinel-2 simulated products for agricultural applications are shown on Fig. 9.
1-4244-1212-9/07/$25.00 ©2007 IEEE.
2680
ACKNOWLEDGMENT The authors would like to thank the members of the industrial consortium who have performed the Sentinel-2 Definition Study, in particular the project teams from AstriumGmbH and Astrium-SAS. The authors would also like to thank all other ESA colleagues who participated to the Sentinel-2 project either at technical or management level, and contributed to the successful start of the programme. REFERENCES [1] [2]
ESA GMES Web Site http://earth.esa.int/gmes/ Sentinel-2 Mission Requirements Document, http://esamultimedia.esa.int/docs/GMES/GMES_Sentinel2_MRD_issue _2.0_update.pdf.
