- Email: [email protected]
Spot-4 vegetation instrument: Vegetation monitoring on a global scale
Acta Astronmrica Vol. 35, No. 7, pp. 453459,
1995
copyright 0 1995Elsevin .s&na Ltd
Pergamon
00%5765(!&QOO27%7
Printed in Great Britain. All rights mcrved 0094-5765/95 59.50 + 0.00
SPOT-4 VEGETATION INSTRUMENT: VEGETATION MONITORING ON A GLOBAL SCALE? J.-P. DURPAIRE, T. GENTET, T. PHULPIN and M. ARNAUD Centre National d’Etudes Spatiales, 18 av. Fdouard Belin, 31055 Toulouse Cedex, France (Received 16 March 1994; received for publication I November 1994)
Abstract-Vegetation plays a major role in global climatic change. It is a major contributor to the hydrological cycle and carbon exchanges between the Earth’s surface and the atmosphere. A new space-based system dedicated to vegetation would be a boom to climatic and environmental studies. The additional possibilities of evaluating agricultural, pasture and forest production would be major contributions to improved natural resources management and a special benefit to agriculture and the general economy in developing countries. A space mission for monitoring terrestrial vegetation at global and local levels is proposed for inclusion in the Spot-4 payload, scheduled for launch around 1997. The “vegetation” concept is more than just an on-board package; it is a complete system with its own space and ground segments. The vegetation instrument (VI) on-board package is designed as an add-on payload that is quite independent of the host satellite. In addition to the basic imaging instrument, the add-on payload includes a solid-state recorder, an image telemetry subsystem and a computer to manage the work plan. To accommodate future long-term missions and achieve a lifetime in excess of 5 years, no moving parts are included in either the imaging instrument proper or the recorder subsystem. The innovative, large field-of-view (101”) imaging instrument features telecentric lenses and focal-plane illumination compensation. Despite the large FOV, pixel size varies extremely little across the swath. Overall, the instrument offers an excellent revisit capability at the highest resolution. The inclusion of the VI package alongside Spot-4’s prime payload of two HRVIR (high resolution visible and i.r.) imaging instruments will open the way to studies requiring both high accuracy satellite imagery and short revisit intervals. The combination of HRVIR and VI imagery will pave the way to powerful new multi-scale interpretation models, particularly as the instruments will share the same spectral bands, have a common reference frame and be synchronized in time.
1. lNTRODUCTlON
The SPOT satellite-based remote sensing system set up by France became operational in May 1986 when the Spot-l satellite was declared ready for service. Since that date, Spot-l and Spot-2, launched in January 1990, have provided continuity of an Earth observation service for a growing community of users. Spot-3, launched in October 1993, will continue the service with exactly the same mission objectives. A second generation of Spot spacecraft, with improved capabilities including a mid-infrared (MIR) band, will be introduced with Spot-4, currently scheduled for launch in October 1997. The vegetation instrument is a new addition to the Spot-4 payload. Like the prime Spot-4 payload-a pair of HRVIR (high resolution visible and i.r.) imaging instruments-it operates in the visible and mid-i.r. (MIR). The VI offers an excellent revisit capability, a mean spatial resolution of around 1 km IAF-92-98 presented at the 43rd Astronautical Washington D.C., U.S.A., 28 August5 September 1992.
tPaper
Congress,
and excellent radiometric characteristics. The combination of vegetation and HRVIR imagery will be ideal for monitoring terrestrial vegetation on a global scale on a regular basis. The VI package has been designed as a selfcontained add-on system complete with its own imaging, recording and data transmission capabilities. The VI system also includes ground facilities for data reception and processing. After suspension in 1988, development work resumed in 1992 under European co-operation arrangements which are now being finalized. These involve the participation of the European Community and individual contributions by several European countries including France, Sweden and Belgium. Italy and Spain have also expressed strong interest in the project.
2. MISSION
2.1. Overall objectives Vegetation plays a major role in global climatic change. It is a major contributor to the hydrological cycle and carbon exchanges between the Barth’s surface and the atmosphere. A new space-based 453
454
J.-P. Durpaire
system dedicated to vegetation would be a boom to climatic and environmental studies. The additional possibilities of evaluating agricultural, pasture and forest production would be major contributions to improved natural resources management and a special benefit to agriculture and the general economy in developing countries. For several years now, data returned by meteorological satellites such as NOAA/AVHRR and Meteosat have been used to study vegetation. The excellent revisit capabilities mean that images can be acquired at intervals compatible with the seasonal rates of change of different types of vegetation while offering both global coverage and reasonable spatial resolutions (l-l 6 km). These studies have consistently confirmed the usefulness of satellite-based remote sensing for determining surface characteristics. They have also played a key role in developing global-scale projects designed to improve our understanding of the functioning of vegetation, its role in environmental and climate change and its interaction with the atmosphere. These studies helped define the data needs for environmental studies and crop yield forecasting. One of the most important needs identified was for easy-to-interpret, long-term data at the regional and global levels. It is this need that the Vegetation package for Spot-4 is designed to meet. 2.2. Mission requirements The main vegetation
mission
requirements
are:
-to
acquire global-scale data as input for climate models or for global crops. forecasting, deforestation monitoring, etc., -to gather self-consistent data sets, over a long period of time, meeting both minimum and guaranteed accuracy requirements, --excellent revisit capability so that data sets can be generated at intervals compatible with biomass rates of change, -spectral bands with minimum susceptibility to atmospheric effects, -accurate image location. -subpixel analysis capability, centralized reception and archiving of global data with consistent quality monitoring. Concerning the sub-pixel analysis capability, the choice of the same spectral bands as the main Spot-4 instruments (HRVIRs) means it will be possible both to combine vegetation and HRVIR data and to set up multilevel sampling procedures. 3. 3. I. General
The VI on-board operation. l
THE VI SYSTEM
description
package
offers
two modes
of
In the global observation mode, the VI package records data with a nadir spatial resolution of
et al.
l
around 1 km in bands B,, B, and MIR and 4 km in band B, (or 1 km if possible). Data are stored on board then dumped to one or more main ground stations which can thus acquire imagery of any area of the globe as required. Recording and playback (or recorder dump) using the on board solid-state recorder are controlled by programming messages uplinked by the Spot-4 Mission Control Center at Toulouse. In the regional observation mode, image data are acquired and retransmitted to ground in realtime. The nadir resolution is around 1 km. A ground station receiving the RO signal can only acquire imagery within its own coverage area.
Global-mode recorded data are transmitted to main ground stations over an X-band image telemetry channel. RO-mode realtime data to stations within range over an L-band channel. All the hardware for both image telemetry channels is included in the VI package. 3.2. Orbital parameters Spot-4 will be placed in precisely the same orbit as its predecessors. The orbit is Sun-synchronous, nearcircular and quasi-polar. The main characteristics are: -period (min): -local equator-crossing time (descending node): -number of revolutions per day: -cycle duration (days): --number of revolutions per cycle: -speed relative to ground (km/s):
101.46 IO:30 14 + 5/26 26 369 6.6
Equator 2824 108 822
D, (km): Dz (km): H (km):
45 2000 76 832
where: D, = distance Dz = distance
between successive ground tracks between ground tracks separated by
26 days H = orbital
altitude.
3.3. Revisit capability
(excluding
cloud corer)
Given the orbital characteristics of the Spot-4 satellite and the FOV of the VI optics (k50.5 , corresponding to a ground swath of 2200 km), repeat imagery can be acquired at very short intervals, Figure 1 shows the VI’s daily coverage capability. The only areas not covered during the 24 h are the elongated black shapes straddling the equator. Above latitude 35’, all regions can be acquired at least twice a day during successive orbital revolutions. Due to the westward drift of ground tracks from one day to the next (545 km/day at the equator). zones not covered one day are covered the next. Note
Spot-4 vegetation instrument
60°
300
00
-300
-600
Field of view (FOV):
+I-50.5”
Swath width:
Daily coverage
2200 km
capability
Fig. I
also that any given point on the equator is accessible, for the same reason, on 3 or even 4 days in a row (in every five or six), but in different portions of the instrument FOV. 3.4. Radiometric characteristics
3.5: Geometric characteristics
The instrument offers four spectral bands. Bands B,, B, and MIR are the same as for the HRVIR instrument and are well suited to vegetation monitoring. Band B, is used for atmospheric correction. Vegetation imagery offers excellent radiometric characteristics. In order to meet the vegetation monitoring objectives, the specifications demand a capability to discriminate changes in reflectance of the order of 0.001-0.003 (smallest detectable noise-equivalent reflectance change). The basic radiometric characteristics of are: Ai: spectral bandwidth at 50% transmission 0.43-0.47 B,: 0.61-0.67 B,: 0.78-0.89 B,: 1.57-1.70 MIR: NEAp z I x 1O-3-3xIO-'
Good radiometric characteristics also demand a high-quality on-board calibration system. The relative target calibration accuracies are 3% for interband measurements, 5% for multidate measurements and 10% for absolute measurements.
pm pm pm pm
where NEAp = smallest detectable noise-equivalent reflectance change IO,1< 60 where OS= Sun zenith angle. The very high radiometric sensitivity specified for the VI package can only be achieved by designing a detection subsystem offering an excellent signal/noise ratio and by using at least IO-bit coding.
Basic geometric characteristics: field of view (FOV):
* 50.5”
(corresponding to a ground swath width of 2200 km) IFOV (instantaneous field of view of a single CCD 1.4 x 10-3 cell) (radian): (corresponding to a ground area or pixel size of 1.15 km on a side). Each vegetation image is made up of a matrix of pixels arranged in rows and columns that are respectively perpendicular and parallel to the satellite velocity vector. Pixel-to-pixel spacings are highly regular in both directions primarily because each band uses a single linear array as the radiation sensor. Spatial resolution (or the area of ground imaged by a single CCD cell) decreases slightly as one moves from the nadir to larger detector look angles due to the Earth’s curvature. By taking into account both platform parameters (satellite’s orbital position and attitude errors) and instrument parameters (alignment, thermoelastic distortion) pixels can be geographically located to an accuracy of better than 2.5 km. The registration of the different spectral channels is such that, at any given instant, the centers of the four pixels corresponding to the four bands fall within a circle 0.5 pixels in diameter. This parameter is fundamental since ground processing generally involves
456
J.-P. Durpaire et al.
combinations of data from the different bands to yield products of varying degrees of sophistication. 3.6. System constraints The VI add-on payload was designed from the outset to meet system constraints guaranteeing full compatibility with the Spot spacecraft. The interfaces between the VI payload and the host satellite have been kept simple by making the payload as independent as possible. A single monobloc structure supports the entire package. This structure is mounted on the Spot-4 equipment compartment immediately in front of one of the HRVIRs. The VI thermal control subsystem is totally independent of the host satellite. Package and satellite are radiatively and conductively isolated. The VI package is fully self-contained as regards: -Xand L-band image telemetry to ground stations -recording and storage of image and auxiliary data -housekeeping (including both programming and monitoring). The Spot bus accommodates the VI package within a 155-kg mass limit. It also provides the package with 200 W of electrical power. The package is designed for a reliability rating of 0.8 over 4 years. The design lifetime of each item of VI equipment is compatible with an overall mission lifetime of 5 years. 3.7. Image telemetry 3.7.1. Global observation mode. The capacity of the VI on-board recorder and the capabilities of the ground receiving system will be chosen to guarantee the reception of all data recorded during each orbital revolution. The image telemetry format will accommodate l-km-resolution data in the B,, B, and MIR bands (100 kbit/s per band) and band B0 data recorded at a resolution of 4 km or, if possible, 1 km. The same telemetry format will be used to transmit instrument data in the calibration mode. The format accommodates the data required for ground-based image processing including channel gains and parameters for determining the geographical location of each image. If band B0 is stored with a resolution of 4 km, the recorded telemetry format corresponds to a data rate of 320 kbit/s. If B, is recorded at 1 km, the rate will be 410 kbit/s. An on-board storage capacity of 2.2 gigabits combined with a two-station ground segment would guarantee the reception of all data acquired each day in the global mode. A ground segment configuration comprising a main European station at a mid-latitude (say, Aussaguel, near Toulouse, France or Fucino, Italy) and a second main station at a higher latitude (say, Kiruna, Sweden) would guarantee a suitable combination of pass windows.
Studies now in progress are looking at the feasibility of on-board data compression. If a suitable solution (i.e. one that can be reasonably implemented in the time available) can be identified, it will be adopted. In this case, a single station at a mid-latitude would suffice. The nominal survey area covers all land masses between 40”s and 60”N. The system described here (on-board storage plus ground stations) should, under certain circumstances, offer margins compatible with broader survey areas. These areas will be defined by scientific users belonging to the mission group. Thus, it should be possible, at certain times, to extend the observation zone to high Arctic latitudes. X-band image telemetry transmissions will be at a rate of around 2.9 Mbit/s and will use phase-shiftkeying (PSK) modulation. 3.7.2. Regional obserzlation mode. Data are transmitted in the realtime RO mode whenever the instrument can see land and the conditions of solar illumination are compatible with the system image quality specifications. Typically, these conditions are met during 35 min during each IOI-min revolution. All image data acquired by the instrument in the RO mode (B2, B, and MIR bands at 1-km resolution) are retransmitted directly along with the necessary auxiliary data for calibration and image location. RO-mode data will be transmitted in the L-band at around 1.7 GHz which is compatible with the large number of ground stations equipped to receive NOAA/AVHRR data. 4. SPACE SEGMENT
4. I. General description The vegetation payload comprises four main subassemblies: -the imaging instrument, consisting of a CCD electronic scanning radiometer and associated electronics comprising amplifier and analog-todigital converter; -an image -processing sub -assembly performing pixel agglomeration, telemetry formatting and, possibly, data compression; -an on-board management sub-assembly which handles Spot bus/VI dialog and controls and monitors all VI equipment, including the recorder; -an image-telemetry sub-assembly for image telemetry downlinking in the X- and L-bands. All four sub-assemblies are integrated into the add-on VI structure attached at four points to the base of one of the HRVIR instruments. Basic VI package characteristics are: mass (kg): dimensions (m):
155 0.7 x 0.7 x 1
451
Spot-4 vegetation instrument
power consumption (W): reliability (over 4 years): instrument data rate (kbits/s): number of bits/pixel:
200 0.8 320-410 10
4.2. Imaging instrument The VI imaging instrument can be described as a multispectral electronic scanning radiometer designed around linear-array sensors sensitive to visible and MIR light. There is one objective lens for each of the four spectral bands (Fig. 2). A 1728~cell CCD linear array is located in the focal plane of each objective lens. The objective lenses, which offer an FOV of + lOl”, are of a new design known as “telecentric”. Within the specified spectral transmission band, exit beams from a telecentric lens are independent of entrance beam angular effects. Each transmission band is thus highly stable across the full width of the corresponding sensor. Each lens is focused and adjusted until registration across the entire FOV is better than 0.5 pixels. The four objective lenses are mounted directly on the main VI structure. Registration requirements require that the optics be kept at +2O”C to an accuracy of better than 1°C. The operating temperature of the MIR sensor is around +S’C. This must be controlled to within a few hundredths of a degree Celsius during image acquisition. The MIR detector thermal control subsystem includes both a coarse-control passive cooler and a fine-control active cooler. Output signals from the linear arrays are fed to an amplifier located as close as possible to the sensors.
All four channels are then fed to a single multiplexer followed by a single, IO-bit, analog-to-digital converter. This arrangement guarantees an output signal of the same radiometric quality as the sensor output signals. Pixel width, in the scanline direction-determined by the basic orbit and imaging instrument parameters-corresponds to 1.15 km on the ground. To obtain square pixels, the pixel length, in the flight direction must also correspond to 1.15 km on the ground. Given the satellite’s ground speed, the maximum time available for acquiring an image “scanline” is 176 ms. However, if the full 176 ms were used for photon integration, the detectors would be saturated. The solution adopted by the VI design team was to choose a unit integration time of 8.8 ms and to integrate four times during each 176-ms scanline interval, thereby improving the signal-to-noise ratio. We then proceed to the next band, performing four unit integrations and what is known as “photon accumulation”. Proper registration of the four spectral bands is achieved by mechanically off-setting the viewing axis of each optical channel by a calibrated amount. The sensors for channels B,, B, and B, are silicon CCD linear arrays. Each detector, or CCD cell, is 13 pm on a side. The f-number of the channel B, objective lens is f/4.5, that of the channel B2 and B, lenses f/5.5. All three have a focal length of 9.3 mm. The MIR sensor is an InGaAs linear array. Each detector, or linear array cell, is 30 pm on a side. The MIR objective has an f-number off 15.5 and a focal length of 18.6 mm.
.arth
v
Visible-wavelength
/ ,
Spot-4 bus
objectives
MIR objective
Calibration device shutter
Support structure Vegetatio; payload (with thermal shroud)
Visible-wavelength detection boxes
MIR detection box \ MIR follower unit
Vegetation payload Fig. 2
J.-P. Durpaire et al.
458
The mission assigned to the vegetation instrument is to detect and measure very small variations, in both time and space, in surface cover reflectance. This can only be achieved by including a very-high-accuracy calibration device both as a means of monitoring instrument performance and to permit corrections to be made should instrument performance vary in time. Calibration data are required to normalize detector response in each band and to compute relative bandto-band and absolute sensitivities. The ideal solution would be to measure the response of the sensors and their associated electronics to a source of known uniform radiance over the full 101” FOV. Since no such source exists, the design team opted for an artificial source offering a stable radiance and to scan the entire FOV. The source will be thoroughly calibrated on the ground prior to launch. Any subsequent drift in source performance will be monitored by periodically measuring the instrument response to selected test sites with known stable characteristics, as is currently done with the HRV instruments. 4.3. Image -processing sub -assernbllj The IO-bit words output by the imaging instrument analog-to-digital converter are formatted by the image-processing sub-assembly before being switched to the on-board recorder or the image telemetry sub-assembly. The telemetry format is divided into three parts: -a
header containing synchronization ary data for image processing and VI ing data, -image data (regional or global mode, may be), -a trailer containing auxiliary data not the header.
bits, auxilihousekeepas the case included in
4.4. On-board-management sub-assembly> The on-board-management sub-assembly is the brain and memory of the VI payload. It handles all dialog with the host satellite’s main on-board computer (OBC). The sub-assembly receives programming messages addressed to the VI package from the Spot-4 OBC and transmits VI housekeeping data to the same OBC. By both activating and monitoring all VI electronics, the on-board-management sub-assembly allows the planned acquisition program to be implemented. The sub-assembly also plays a role in the thermal control of the VI package. The VI on-board recorder is part of the on-boardmanagement sub-assembly. This solid-state device offers a storage capacity of 2.2 Gbits. 4.5. Image telemetry sub-assembly The image telemetry sub-assembly transmits to ground stations using the telemetry format described
in Section 4.3. Depending on the mode of operation, the image data encoded in the telemetry format is received either directly from the imaging instrument or played back by the on-board recorder. The sub-assembly features an L-band transmitter for the realtime mode and an X-band transmitter for the recorder-playback mode. Both transmitters will use all-solid-state designs. 5. GROUND SEGMENT
The definition of the vegetation ground segment is not as advanced as that of the space segment. When the project was suspended in 1988, no detailed definition work had been done on the ground segment. Nevertheless, the following points can be made. The VI ground segment will comprise: --a VI mission control center; -one or more main VI imagery receiving stations; and --a VI image processing center (known as CTIV). The mission control center will monitor and program the VI payload via Spot-4 housekeeping telemetry downlinks and command uplinks, respectively. For this reason, the center will be located on the same site as the Spot-4 mission control center. The main imagery receiving stations will receive and record VI imagery downlinked as X-band telemetry. The final decision to use one or two main imagery receiving stations will depend on the final capacity of the VI on-board memory and the introduction or not of data compression. VI ground station facilities will include dedicated receivers, demodulators and recorders. All vegetation image data received by the main imagery receiving stations will be forwarded to the CTIV processing center for processing and archiving. Note that regional receiving stations may also send image data to the center. The center will archive all processed data and compile the VI catalog. It will collect the data relating to image quality (calibration coefficient, location parameters, etc.) provided by the satellite operator. It will also handle data distribution and relations with vegetation program users.
6.
DEVELOPMENT
PLAN
The aim is to have the VI package included in the Spot-4 payload. Spot-3 was launched in October 1993, and Spot-4 is scheduled for launch in 1997. If a start can be made on the industrial development and production phase, it should be possible, despite a fairly tight schedule, to deliver the first model of the VI package in early 1996, which would be compatible with a Spot-4 launch in October 1997. Right now, France and its
Spot-4 vegetation instrument partners are actively engaged in finalizing European cooperation arrangements with a view to making a start on the industrial development and production phase, so the package will be ready to tiy aboard spot-4.
Should Spot-3 run in to launch be brought forward will not be ready in time. In it would, however, be ready in the Spot-5 payload.
459 problems and the Spot-4 to 1995, the VI package this rather unlikely event, in good time for inclusion
1995
copyright 0 1995Elsevin .s&na Ltd
Pergamon
00%5765(!&QOO27%7
Printed in Great Britain. All rights mcrved 0094-5765/95 59.50 + 0.00
SPOT-4 VEGETATION INSTRUMENT: VEGETATION MONITORING ON A GLOBAL SCALE? J.-P. DURPAIRE, T. GENTET, T. PHULPIN and M. ARNAUD Centre National d’Etudes Spatiales, 18 av. Fdouard Belin, 31055 Toulouse Cedex, France (Received 16 March 1994; received for publication I November 1994)
Abstract-Vegetation plays a major role in global climatic change. It is a major contributor to the hydrological cycle and carbon exchanges between the Earth’s surface and the atmosphere. A new space-based system dedicated to vegetation would be a boom to climatic and environmental studies. The additional possibilities of evaluating agricultural, pasture and forest production would be major contributions to improved natural resources management and a special benefit to agriculture and the general economy in developing countries. A space mission for monitoring terrestrial vegetation at global and local levels is proposed for inclusion in the Spot-4 payload, scheduled for launch around 1997. The “vegetation” concept is more than just an on-board package; it is a complete system with its own space and ground segments. The vegetation instrument (VI) on-board package is designed as an add-on payload that is quite independent of the host satellite. In addition to the basic imaging instrument, the add-on payload includes a solid-state recorder, an image telemetry subsystem and a computer to manage the work plan. To accommodate future long-term missions and achieve a lifetime in excess of 5 years, no moving parts are included in either the imaging instrument proper or the recorder subsystem. The innovative, large field-of-view (101”) imaging instrument features telecentric lenses and focal-plane illumination compensation. Despite the large FOV, pixel size varies extremely little across the swath. Overall, the instrument offers an excellent revisit capability at the highest resolution. The inclusion of the VI package alongside Spot-4’s prime payload of two HRVIR (high resolution visible and i.r.) imaging instruments will open the way to studies requiring both high accuracy satellite imagery and short revisit intervals. The combination of HRVIR and VI imagery will pave the way to powerful new multi-scale interpretation models, particularly as the instruments will share the same spectral bands, have a common reference frame and be synchronized in time.
1. lNTRODUCTlON
The SPOT satellite-based remote sensing system set up by France became operational in May 1986 when the Spot-l satellite was declared ready for service. Since that date, Spot-l and Spot-2, launched in January 1990, have provided continuity of an Earth observation service for a growing community of users. Spot-3, launched in October 1993, will continue the service with exactly the same mission objectives. A second generation of Spot spacecraft, with improved capabilities including a mid-infrared (MIR) band, will be introduced with Spot-4, currently scheduled for launch in October 1997. The vegetation instrument is a new addition to the Spot-4 payload. Like the prime Spot-4 payload-a pair of HRVIR (high resolution visible and i.r.) imaging instruments-it operates in the visible and mid-i.r. (MIR). The VI offers an excellent revisit capability, a mean spatial resolution of around 1 km IAF-92-98 presented at the 43rd Astronautical Washington D.C., U.S.A., 28 August5 September 1992.
tPaper
Congress,
and excellent radiometric characteristics. The combination of vegetation and HRVIR imagery will be ideal for monitoring terrestrial vegetation on a global scale on a regular basis. The VI package has been designed as a selfcontained add-on system complete with its own imaging, recording and data transmission capabilities. The VI system also includes ground facilities for data reception and processing. After suspension in 1988, development work resumed in 1992 under European co-operation arrangements which are now being finalized. These involve the participation of the European Community and individual contributions by several European countries including France, Sweden and Belgium. Italy and Spain have also expressed strong interest in the project.
2. MISSION
2.1. Overall objectives Vegetation plays a major role in global climatic change. It is a major contributor to the hydrological cycle and carbon exchanges between the Barth’s surface and the atmosphere. A new space-based 453
454
J.-P. Durpaire
system dedicated to vegetation would be a boom to climatic and environmental studies. The additional possibilities of evaluating agricultural, pasture and forest production would be major contributions to improved natural resources management and a special benefit to agriculture and the general economy in developing countries. For several years now, data returned by meteorological satellites such as NOAA/AVHRR and Meteosat have been used to study vegetation. The excellent revisit capabilities mean that images can be acquired at intervals compatible with the seasonal rates of change of different types of vegetation while offering both global coverage and reasonable spatial resolutions (l-l 6 km). These studies have consistently confirmed the usefulness of satellite-based remote sensing for determining surface characteristics. They have also played a key role in developing global-scale projects designed to improve our understanding of the functioning of vegetation, its role in environmental and climate change and its interaction with the atmosphere. These studies helped define the data needs for environmental studies and crop yield forecasting. One of the most important needs identified was for easy-to-interpret, long-term data at the regional and global levels. It is this need that the Vegetation package for Spot-4 is designed to meet. 2.2. Mission requirements The main vegetation
mission
requirements
are:
-to
acquire global-scale data as input for climate models or for global crops. forecasting, deforestation monitoring, etc., -to gather self-consistent data sets, over a long period of time, meeting both minimum and guaranteed accuracy requirements, --excellent revisit capability so that data sets can be generated at intervals compatible with biomass rates of change, -spectral bands with minimum susceptibility to atmospheric effects, -accurate image location. -subpixel analysis capability, centralized reception and archiving of global data with consistent quality monitoring. Concerning the sub-pixel analysis capability, the choice of the same spectral bands as the main Spot-4 instruments (HRVIRs) means it will be possible both to combine vegetation and HRVIR data and to set up multilevel sampling procedures. 3. 3. I. General
The VI on-board operation. l
THE VI SYSTEM
description
package
offers
two modes
of
In the global observation mode, the VI package records data with a nadir spatial resolution of
et al.
l
around 1 km in bands B,, B, and MIR and 4 km in band B, (or 1 km if possible). Data are stored on board then dumped to one or more main ground stations which can thus acquire imagery of any area of the globe as required. Recording and playback (or recorder dump) using the on board solid-state recorder are controlled by programming messages uplinked by the Spot-4 Mission Control Center at Toulouse. In the regional observation mode, image data are acquired and retransmitted to ground in realtime. The nadir resolution is around 1 km. A ground station receiving the RO signal can only acquire imagery within its own coverage area.
Global-mode recorded data are transmitted to main ground stations over an X-band image telemetry channel. RO-mode realtime data to stations within range over an L-band channel. All the hardware for both image telemetry channels is included in the VI package. 3.2. Orbital parameters Spot-4 will be placed in precisely the same orbit as its predecessors. The orbit is Sun-synchronous, nearcircular and quasi-polar. The main characteristics are: -period (min): -local equator-crossing time (descending node): -number of revolutions per day: -cycle duration (days): --number of revolutions per cycle: -speed relative to ground (km/s):
101.46 IO:30 14 + 5/26 26 369 6.6
Equator 2824 108 822
D, (km): Dz (km): H (km):
45 2000 76 832
where: D, = distance Dz = distance
between successive ground tracks between ground tracks separated by
26 days H = orbital
altitude.
3.3. Revisit capability
(excluding
cloud corer)
Given the orbital characteristics of the Spot-4 satellite and the FOV of the VI optics (k50.5 , corresponding to a ground swath of 2200 km), repeat imagery can be acquired at very short intervals, Figure 1 shows the VI’s daily coverage capability. The only areas not covered during the 24 h are the elongated black shapes straddling the equator. Above latitude 35’, all regions can be acquired at least twice a day during successive orbital revolutions. Due to the westward drift of ground tracks from one day to the next (545 km/day at the equator). zones not covered one day are covered the next. Note
Spot-4 vegetation instrument
60°
300
00
-300
-600
Field of view (FOV):
+I-50.5”
Swath width:
Daily coverage
2200 km
capability
Fig. I
also that any given point on the equator is accessible, for the same reason, on 3 or even 4 days in a row (in every five or six), but in different portions of the instrument FOV. 3.4. Radiometric characteristics
3.5: Geometric characteristics
The instrument offers four spectral bands. Bands B,, B, and MIR are the same as for the HRVIR instrument and are well suited to vegetation monitoring. Band B, is used for atmospheric correction. Vegetation imagery offers excellent radiometric characteristics. In order to meet the vegetation monitoring objectives, the specifications demand a capability to discriminate changes in reflectance of the order of 0.001-0.003 (smallest detectable noise-equivalent reflectance change). The basic radiometric characteristics of are: Ai: spectral bandwidth at 50% transmission 0.43-0.47 B,: 0.61-0.67 B,: 0.78-0.89 B,: 1.57-1.70 MIR: NEAp z I x 1O-3-3xIO-'
Good radiometric characteristics also demand a high-quality on-board calibration system. The relative target calibration accuracies are 3% for interband measurements, 5% for multidate measurements and 10% for absolute measurements.
pm pm pm pm
where NEAp = smallest detectable noise-equivalent reflectance change IO,1< 60 where OS= Sun zenith angle. The very high radiometric sensitivity specified for the VI package can only be achieved by designing a detection subsystem offering an excellent signal/noise ratio and by using at least IO-bit coding.
Basic geometric characteristics: field of view (FOV):
* 50.5”
(corresponding to a ground swath width of 2200 km) IFOV (instantaneous field of view of a single CCD 1.4 x 10-3 cell) (radian): (corresponding to a ground area or pixel size of 1.15 km on a side). Each vegetation image is made up of a matrix of pixels arranged in rows and columns that are respectively perpendicular and parallel to the satellite velocity vector. Pixel-to-pixel spacings are highly regular in both directions primarily because each band uses a single linear array as the radiation sensor. Spatial resolution (or the area of ground imaged by a single CCD cell) decreases slightly as one moves from the nadir to larger detector look angles due to the Earth’s curvature. By taking into account both platform parameters (satellite’s orbital position and attitude errors) and instrument parameters (alignment, thermoelastic distortion) pixels can be geographically located to an accuracy of better than 2.5 km. The registration of the different spectral channels is such that, at any given instant, the centers of the four pixels corresponding to the four bands fall within a circle 0.5 pixels in diameter. This parameter is fundamental since ground processing generally involves
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combinations of data from the different bands to yield products of varying degrees of sophistication. 3.6. System constraints The VI add-on payload was designed from the outset to meet system constraints guaranteeing full compatibility with the Spot spacecraft. The interfaces between the VI payload and the host satellite have been kept simple by making the payload as independent as possible. A single monobloc structure supports the entire package. This structure is mounted on the Spot-4 equipment compartment immediately in front of one of the HRVIRs. The VI thermal control subsystem is totally independent of the host satellite. Package and satellite are radiatively and conductively isolated. The VI package is fully self-contained as regards: -Xand L-band image telemetry to ground stations -recording and storage of image and auxiliary data -housekeeping (including both programming and monitoring). The Spot bus accommodates the VI package within a 155-kg mass limit. It also provides the package with 200 W of electrical power. The package is designed for a reliability rating of 0.8 over 4 years. The design lifetime of each item of VI equipment is compatible with an overall mission lifetime of 5 years. 3.7. Image telemetry 3.7.1. Global observation mode. The capacity of the VI on-board recorder and the capabilities of the ground receiving system will be chosen to guarantee the reception of all data recorded during each orbital revolution. The image telemetry format will accommodate l-km-resolution data in the B,, B, and MIR bands (100 kbit/s per band) and band B0 data recorded at a resolution of 4 km or, if possible, 1 km. The same telemetry format will be used to transmit instrument data in the calibration mode. The format accommodates the data required for ground-based image processing including channel gains and parameters for determining the geographical location of each image. If band B0 is stored with a resolution of 4 km, the recorded telemetry format corresponds to a data rate of 320 kbit/s. If B, is recorded at 1 km, the rate will be 410 kbit/s. An on-board storage capacity of 2.2 gigabits combined with a two-station ground segment would guarantee the reception of all data acquired each day in the global mode. A ground segment configuration comprising a main European station at a mid-latitude (say, Aussaguel, near Toulouse, France or Fucino, Italy) and a second main station at a higher latitude (say, Kiruna, Sweden) would guarantee a suitable combination of pass windows.
Studies now in progress are looking at the feasibility of on-board data compression. If a suitable solution (i.e. one that can be reasonably implemented in the time available) can be identified, it will be adopted. In this case, a single station at a mid-latitude would suffice. The nominal survey area covers all land masses between 40”s and 60”N. The system described here (on-board storage plus ground stations) should, under certain circumstances, offer margins compatible with broader survey areas. These areas will be defined by scientific users belonging to the mission group. Thus, it should be possible, at certain times, to extend the observation zone to high Arctic latitudes. X-band image telemetry transmissions will be at a rate of around 2.9 Mbit/s and will use phase-shiftkeying (PSK) modulation. 3.7.2. Regional obserzlation mode. Data are transmitted in the realtime RO mode whenever the instrument can see land and the conditions of solar illumination are compatible with the system image quality specifications. Typically, these conditions are met during 35 min during each IOI-min revolution. All image data acquired by the instrument in the RO mode (B2, B, and MIR bands at 1-km resolution) are retransmitted directly along with the necessary auxiliary data for calibration and image location. RO-mode data will be transmitted in the L-band at around 1.7 GHz which is compatible with the large number of ground stations equipped to receive NOAA/AVHRR data. 4. SPACE SEGMENT
4. I. General description The vegetation payload comprises four main subassemblies: -the imaging instrument, consisting of a CCD electronic scanning radiometer and associated electronics comprising amplifier and analog-todigital converter; -an image -processing sub -assembly performing pixel agglomeration, telemetry formatting and, possibly, data compression; -an on-board management sub-assembly which handles Spot bus/VI dialog and controls and monitors all VI equipment, including the recorder; -an image-telemetry sub-assembly for image telemetry downlinking in the X- and L-bands. All four sub-assemblies are integrated into the add-on VI structure attached at four points to the base of one of the HRVIR instruments. Basic VI package characteristics are: mass (kg): dimensions (m):
155 0.7 x 0.7 x 1
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power consumption (W): reliability (over 4 years): instrument data rate (kbits/s): number of bits/pixel:
200 0.8 320-410 10
4.2. Imaging instrument The VI imaging instrument can be described as a multispectral electronic scanning radiometer designed around linear-array sensors sensitive to visible and MIR light. There is one objective lens for each of the four spectral bands (Fig. 2). A 1728~cell CCD linear array is located in the focal plane of each objective lens. The objective lenses, which offer an FOV of + lOl”, are of a new design known as “telecentric”. Within the specified spectral transmission band, exit beams from a telecentric lens are independent of entrance beam angular effects. Each transmission band is thus highly stable across the full width of the corresponding sensor. Each lens is focused and adjusted until registration across the entire FOV is better than 0.5 pixels. The four objective lenses are mounted directly on the main VI structure. Registration requirements require that the optics be kept at +2O”C to an accuracy of better than 1°C. The operating temperature of the MIR sensor is around +S’C. This must be controlled to within a few hundredths of a degree Celsius during image acquisition. The MIR detector thermal control subsystem includes both a coarse-control passive cooler and a fine-control active cooler. Output signals from the linear arrays are fed to an amplifier located as close as possible to the sensors.
All four channels are then fed to a single multiplexer followed by a single, IO-bit, analog-to-digital converter. This arrangement guarantees an output signal of the same radiometric quality as the sensor output signals. Pixel width, in the scanline direction-determined by the basic orbit and imaging instrument parameters-corresponds to 1.15 km on the ground. To obtain square pixels, the pixel length, in the flight direction must also correspond to 1.15 km on the ground. Given the satellite’s ground speed, the maximum time available for acquiring an image “scanline” is 176 ms. However, if the full 176 ms were used for photon integration, the detectors would be saturated. The solution adopted by the VI design team was to choose a unit integration time of 8.8 ms and to integrate four times during each 176-ms scanline interval, thereby improving the signal-to-noise ratio. We then proceed to the next band, performing four unit integrations and what is known as “photon accumulation”. Proper registration of the four spectral bands is achieved by mechanically off-setting the viewing axis of each optical channel by a calibrated amount. The sensors for channels B,, B, and B, are silicon CCD linear arrays. Each detector, or CCD cell, is 13 pm on a side. The f-number of the channel B, objective lens is f/4.5, that of the channel B2 and B, lenses f/5.5. All three have a focal length of 9.3 mm. The MIR sensor is an InGaAs linear array. Each detector, or linear array cell, is 30 pm on a side. The MIR objective has an f-number off 15.5 and a focal length of 18.6 mm.
.arth
v
Visible-wavelength
/ ,
Spot-4 bus
objectives
MIR objective
Calibration device shutter
Support structure Vegetatio; payload (with thermal shroud)
Visible-wavelength detection boxes
MIR detection box \ MIR follower unit
Vegetation payload Fig. 2
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The mission assigned to the vegetation instrument is to detect and measure very small variations, in both time and space, in surface cover reflectance. This can only be achieved by including a very-high-accuracy calibration device both as a means of monitoring instrument performance and to permit corrections to be made should instrument performance vary in time. Calibration data are required to normalize detector response in each band and to compute relative bandto-band and absolute sensitivities. The ideal solution would be to measure the response of the sensors and their associated electronics to a source of known uniform radiance over the full 101” FOV. Since no such source exists, the design team opted for an artificial source offering a stable radiance and to scan the entire FOV. The source will be thoroughly calibrated on the ground prior to launch. Any subsequent drift in source performance will be monitored by periodically measuring the instrument response to selected test sites with known stable characteristics, as is currently done with the HRV instruments. 4.3. Image -processing sub -assernbllj The IO-bit words output by the imaging instrument analog-to-digital converter are formatted by the image-processing sub-assembly before being switched to the on-board recorder or the image telemetry sub-assembly. The telemetry format is divided into three parts: -a
header containing synchronization ary data for image processing and VI ing data, -image data (regional or global mode, may be), -a trailer containing auxiliary data not the header.
bits, auxilihousekeepas the case included in
4.4. On-board-management sub-assembly> The on-board-management sub-assembly is the brain and memory of the VI payload. It handles all dialog with the host satellite’s main on-board computer (OBC). The sub-assembly receives programming messages addressed to the VI package from the Spot-4 OBC and transmits VI housekeeping data to the same OBC. By both activating and monitoring all VI electronics, the on-board-management sub-assembly allows the planned acquisition program to be implemented. The sub-assembly also plays a role in the thermal control of the VI package. The VI on-board recorder is part of the on-boardmanagement sub-assembly. This solid-state device offers a storage capacity of 2.2 Gbits. 4.5. Image telemetry sub-assembly The image telemetry sub-assembly transmits to ground stations using the telemetry format described
in Section 4.3. Depending on the mode of operation, the image data encoded in the telemetry format is received either directly from the imaging instrument or played back by the on-board recorder. The sub-assembly features an L-band transmitter for the realtime mode and an X-band transmitter for the recorder-playback mode. Both transmitters will use all-solid-state designs. 5. GROUND SEGMENT
The definition of the vegetation ground segment is not as advanced as that of the space segment. When the project was suspended in 1988, no detailed definition work had been done on the ground segment. Nevertheless, the following points can be made. The VI ground segment will comprise: --a VI mission control center; -one or more main VI imagery receiving stations; and --a VI image processing center (known as CTIV). The mission control center will monitor and program the VI payload via Spot-4 housekeeping telemetry downlinks and command uplinks, respectively. For this reason, the center will be located on the same site as the Spot-4 mission control center. The main imagery receiving stations will receive and record VI imagery downlinked as X-band telemetry. The final decision to use one or two main imagery receiving stations will depend on the final capacity of the VI on-board memory and the introduction or not of data compression. VI ground station facilities will include dedicated receivers, demodulators and recorders. All vegetation image data received by the main imagery receiving stations will be forwarded to the CTIV processing center for processing and archiving. Note that regional receiving stations may also send image data to the center. The center will archive all processed data and compile the VI catalog. It will collect the data relating to image quality (calibration coefficient, location parameters, etc.) provided by the satellite operator. It will also handle data distribution and relations with vegetation program users.
6.
DEVELOPMENT
PLAN
The aim is to have the VI package included in the Spot-4 payload. Spot-3 was launched in October 1993, and Spot-4 is scheduled for launch in 1997. If a start can be made on the industrial development and production phase, it should be possible, despite a fairly tight schedule, to deliver the first model of the VI package in early 1996, which would be compatible with a Spot-4 launch in October 1997. Right now, France and its
Spot-4 vegetation instrument partners are actively engaged in finalizing European cooperation arrangements with a view to making a start on the industrial development and production phase, so the package will be ready to tiy aboard spot-4.
Should Spot-3 run in to launch be brought forward will not be ready in time. In it would, however, be ready in the Spot-5 payload.
459 problems and the Spot-4 to 1995, the VI package this rather unlikely event, in good time for inclusion