SPOT-5 is the fifth satellite in the SPOT series of CNES (Space Agency of France), placed into orbit by an Ariane launcher. Since the first SPOT satellite was launched in 1986, the SPOT system has sought to provide continuity of service and constantly improved the quality of its products for the global user community.
Background: In 1994 the SPOT-5 program was approved by the French government, consisting initially of two identical S/C in orbit and two new optical imagers for these S/C, called HRG (High Resolution Geometric) and HRS (High Resolution Stereoscopic) to provide a ground resolution of 5m. In 1996, the SPOT-5 program was downsized (to one orbiting S/C) and redefined to improve the spatial resolution of the imagery below 5 m. As a result, an innovative image acquisition and processing scheme has been developed by CNES to obtain spatial resolutions of about 3 m from two 5 m images. In January 1999, a further functional improvement was introduced giving the HRS (High Resolution Stereoscopic) instrument a full stereoscopic capability. It was also decided to continue to fly the Vegetation instrument. The SPOT-5 satellite program continues the partnerships of France (CNES), Belgium (OSTC) and Sweden (SNSB) as established at the beginning of the SPOT program. 1)
• To provide image acquisition and service continuity consistent with previous SPOT satellites to satisfy the user investments. Hence, the same sun-synchronous orbit is used providing the existing functional instrument capabilities with a 26 day repeat cycle, the same off-track viewing capability of ± 27º about nadir, the same spectral band selection, and the same 60 km double swath.
• To improve the spatial resolution of the imagery to < 3 m in the panchromatic band and to 10 m in the multispectral mode. The SWIR band imagery remains at 20 m.
• To offer in parallel a stereoscopic along-track observation capability (instead of the previously provided cross-track capability). The intend is to offer high-resolution imagery to be used for DEM (Digital Elevation Model) generation with an accuracy of 10 m.
Figure 1: View of SPOT-5 spacecraft and its instruments (image credit: CNES)
The SPOT-5 satellite configuration takes advantage of the SPOT-4 bus design, using the extended platform design (SPOT MK2, provided by MMS) and service module accommodates twice the payload of the SPOT 3 bus. The ACS (Attitude Control Subsystem) provides a pointing accuracy of 0.05º and an attitude restitution of 6 x 10-5 radians. This excellent location accuracy [autonomous star tracker SED16 manufactured by EADS- Sodern, France] corresponds to < 50 m without ground control points (instead of 350 m on SPOT 1 to 4). The S/C structure has dimensions of: 3.4 m x 3.1 m x 6 m (excluding the solar array). The OBC (Onboard Processor) consists of a Marconi MDC 31750 processor. The S/C mass is 3030 kg at launch, power = 2400 W (EOL), batteries = 4 x 40 Ah, design life = 5-7 years. Note: In 2000 MMS became part of EADS Astrium. 5) 6) 7)
Figure 2: Schematic view of the SPOT-5 architecture (image credit: CNES)
Figure 3: Illustration of the deployed SPOT-5 spacecraft (image credit: CNES)
Launch: A launch of SPOT-5 on an Ariane-4 vehicle took place May 4, 2002. The secondary payload on this flight consisted of two nanosatellites with the name of IDEFIX. Both nanosatellites were designed, built and funded by AMSAT-France. Each nanosatellite has a mass of 6 kg. They remain attached to the third stage of Ariane-4.
RF communications: Onboard solid-state recording capacity of 90 Gbit (EOL). The onboard memory can store 550 images, compared to 400 on SPOT 4. Although the magnetic recorders provided more capacity (120 Gbit), the file management system allocates available storage capacity more efficiently. Onboard data compression of source data is provided with DCT (Direct Cosine Transform) of HRG and HRS data streams. Compression ratios of 2.28-2.8 are achieved depending on imaging mode.
The downlink is in X-band (QPSK modulation) at a data rate of 2 x 50 Mbit/s. The image transmission assembly consists of: a) Two QPSK modulators, b) two X-band solid-state power amplifiers (SSPAs) capable of delivering 20 watts, c) an output multiplexer (OMUX) filter that filters and combines signals before directing them to the antenna.
The data compression algorithm used is DCT (Discrete Cosine Transform), employing 8 x 8, non-overlapping blocks, derived from the JPEG standard with variable length coding and bit rate control. Compression ratios of 2.8 are used. 8)
Orbit: Sun-synchronous circular orbit (identical to those of SPOT-1 to -4), altitude = 832 km, inclination = 98.7º, period = 101.4 min, equator crossing on descending node at 10:30 AM, repeat cycle = 26 days, revolutions/day = 14 5/26.
• 2014: The SPOT-5 spacecraft and its payload are still operational in early 2014 in its 12th year on orbit (design life of 5 years). However, the SPOT-5 local hour is drifting since maneuvers for inclination correction are no more performed (the hydrazine is saved for end of life operations). The end-of-life of SPOT-5 has been adjusted to the beginning of 2015 (nominally, during Q1 of 2015). 9)
• The SPOT-6 mission will complement the SPOT-5 mission by the end of 2012. 10)
Figure 4: Projected service profile of the SPOT family (image credit: )
• The SPOT-5 spacecraft and its payload are operating nominally in 2012 (10th year of on-orbit operations). The SPOT-5 local hour is slowly drifting since maneuvers for inclination correction are no more performed (hydrazine is saved for end of life operations). The end-of-life of SPOT-5 has been adjusted to: end of 2014/beginning of 2015. 11)
The SPOT-4 local hour is drifting since maneuvers for inclination correction are no more performed (hydrazine is saved for end of life operations). SPOT4 should be de-orbited when the local hour will be too low to maintain the main and VEGETATION missions -estimated to be sometime between December 2012 and March 2013 (Ref. 11).
• The SPOT-5 spacecraft and its payload are operating nominally in 2011.
- On April 1, 2011, a refocusing has been performed on the HRG-2 sensor. The result of this action is to recover the instrument's beginning-of-life performances (for relative and absolute FTM, and auto-test calibration).- The operational life expectancy of SPOT-5 remains unchanged, the estimate is for mid-2015. 12)
- After the devastating Japanese earthquake of March 11, 2011, SPOT-5 was tasked by the International Charter on Space and Major Disasters to cover the coast that took the full force of the subsequent tsunami. 13)
The situation regarding Japan's nuclear power plants recently prompted Astrium GEO-Information Services to activate its “SPOTMonitoring” service over the area. Daily change detection reports were delivered to subscribers free of charge March 17-25. The stricken Fukushima Dai-ichi nuclear power plant and its surroundings are still under surveillance and periodic reports will still be delivered over the next few months.
- 25 Years of SPOT Satellites: Since February 22, 1986, SPOT satellites have been keeping a watchful eye on the Earth. For over 25 years, this series of optical observation satellites has been providing images of our planet for an extensive range of applications, such as cartography, crop forecasts, geological exploration, and disaster management. 14)
All five of the SPOT satellites were developed and built by Astrium as prime contractor, responsible for the platform and high-resolution imaging system. Currently, the SPOT satellites are operated by Astrium GEO-Information Services, formerly Spot Image.
- On 8 January 2011, the SPOT-5 satellite acquired a series of images of the Darling River system in Australia south of St George, a town near the border between Queensland and New South Wales. SIS, an Astrium subsidiary in Australia, re-tasked the SPOT constellation to monitor the floods, in response to requests from Queensland’s civil defence services and Geoscience Australia.
• On Dec. 1, 2010, Astrium Services created a single operational management structure bringing together the imagery and services experts Spot Image and Infoterra to form the new GEO-Information division of Astrium Services. The new unit of GEO-Information division of Astrium Services started business in January 2011 - operating the SPOT-4 and SPOT-5 satellites and providing the data to its customer base. 15)
• The SPOT-5 spacecraft and its payload are operating nominally in 2010. Currently, an operational life to mid-2015 is expected (Ref. 17).
• In January 2010, hours after the devastating January 12 earthquake struck Haiti, Spot Image personnel were in nearly constant communications with key customers and other organizations that needed access to imagery to assist in disaster response efforts. SPOT-5 captured its first image of Port-au-Prince on Jan. 14 and several more over the surrounding parts of Haiti in subsequent days.
The 2.5 m images were used by the U.S. government for logistics planning prior to deploying personnel to Haiti and later to manage resources on the ground. In accordance with the International Charter, which is an agreement by multiple nations to provide access to remote sensing data during natural disasters, Spot Image routed image scenes captured by the SPOT ground receiving stations to the Spot Image headquarters in France. There the data was used to generate maps of the stricken area for distribution to Non-Governmental Organizations aiding in the response. 16)
• The spacecraft and its instruments are operating nominally in 2009. Operational analysis studies conducted by CNES project a life expectancy at least to 2014 of the SPOT-5 mission. Chances are that the mission might continue its services until 2015 without serious anomalies. 17)
• In early 2008 some power problems of the power subsystem occurred. However, the exact cause could not be properly identified. The available power decreased slightly which is still quite sufficient to provide full service to all subsystems for mission operations. The situation became very stable in February 2008 and is continuously monitored - with no indication of any deterioration.
Figure 5: SPOT-5 sample image of Naples (Italy) in 2002 (image credit: CNES)
• The commissioning phase of the spacecraft and its instruments was completed on July 12, 2002. At this point, CNES handed over the responsibility for commercial operation of the SPOT-5 system to Spot Image (Ref. 18).
• July 10, 2002: The SPOT-5 in-orbit checkout review was held on July, 10, 2002. Tests performed during in-orbit checkout have shown that: 18)
- the satellite, ground telemetry and command systems, and passenger instruments are functioning perfectly and system availability is excellent
- geometric and radiometric quality of images from the two HRG (High-Resolution Geometric) instruments and the HRS (High-Resolution Stereoscopic) instrument is excellent, exceeding specifications; indeed, ways of further enhancing performance have already been identified.
• In April 2002, the French arms procurement agency, DGA (Délégation Générale pour l'Armement), signed a broad agreement guaranteeing French military access to the civilian SPOT-5 satellite's high-resolution imagery.
Sensor complement: (2 x HRG, HRS, Vegetation, DORIS)
HRG (High Resolution Geometric)
HRG was built by Astrium SAS of Vélizy, France to continue to improve the HRVIR service of SPOT-4. Two HRG instruments are provided in the conventional SPOT-series double-observation configuration, each with a FOV of 4.13º and the same cross-track pointing capabilities of ±27 º as the HRVIR imager on SPOT-4. The observation coverage of each HRG is 60 km in the nadir direction and >80 km in the oblique configuration (same two-swath coverage as before). The main components of the HRG instrument are:
• Stable structure supporting its mechanisms
• Optical subassembly
• Detection electronics
• Ancillary electronics.
HRG features a new linear detector array configuration geometry for the panchromatic band using two parallel rows (i.e., a dual array) of 12000 silicon CCD detectors (6.5 µm in size) for each instrument. The two PAN detector lines are offset in the focal plane in such as way as to provide coincident imagery of the same instantaneous cross-track area, each at a spatial resolution of 5 m. [Note: The dual array in the focal plane (offset by one half pixel in one direction and 3.5 pixels in the other to avoid overlapping) is sufficient to improve the sampling grid without doubling each array's acquisition rate. The new sampling concept is based on Shannon's theory of information which states that “the sampling frequency must be equal to or greater than twice the maximum signal frequency” to obtain clean images using interpolation.] The CCD integration time is within 0.75 ms for a dual-array observation in cross-track of 60 km in which the S/C is moving 5 m in the along-track direction.
Figure 6: View of the integrated HRG instrument (image credit: CNES)
Table 1: Specification of the HRG instrument
The detection unit, mounted in the instrument's focal plane, converts the light signal from the optical subassembly into electric signals for processing by the video electronics units.
Figure 7: Illustration of the detection unit (image credit: CNES)
Figure 8: Schematic view of HRG observation capabilities (image credit: CNES)
Supermode: The term refers to an acquisition process, specific to the HRG instrument of SPOT-5, through which an image sampled at 2.5 m may be obtained from two 5 m resolution panchromatic images acquired simultaneously, keeping within the same borders as the two 5 m resolution images. It is possible to combine the two 5 m pixel image samples into four new slightly overlapping image samples of about 3 m pixel size.
A specific image processing software, developed by CNES is used to reconstruct the final image after three processing steps: interleaving, interpolation and restoration. This new type of image, simulated with all its geometric and radiometric characteristics, has been compared by several users to other types of digital images at different ground resolutions. These users (cartographers, urban planners, environmental experts, foresters, agronomists,) have concluded that the supermode resolution is about 3 m (between 2.5 m and 3.5 m depending on applications and analyzed features). A new quincunx sampling mode was adopted referred to as THR (Très Haute Résolution) or very high resolution mode. 19) 20) 21)
Figure 9: Supermode processing scheme
Figure 10: Alternate view of the supermode scheme (image credit: SPOT Image)
HRS (High Resolution Stereoscopic)
The HRS instrument was developed and built by EADS Astrium SAS, sponsored by CNES and SPOT IMAGE.
The objective is to provide large-area along-track stereoscopic panchromatic imagery with good altimetric accuracy (5-10 m relative and 10-15 m absolute). Applications of the stereo imagery are seen in various fields such as map making and in the generation of DTMs (Digital Terrain Model) The panchromatic band (0.51-0.73 µm) of SPOT-1, -2, -3 is being reintroduced. The HRS instrument features two telescopes allowing a 20º fore view and a 20º aft view over a 120 km swath, respectively (Figure 12). A spatial resolution of 10 m is provided in cross-track and 5 m (parallax direction) in along-track. The stereo acquisition mode can be sustained for scene lengths of up to 600 km. HRS uses the same CCD line detector design as for the HRG instrument. 22)
Figure 11: Schematic illustration of the HRS instrument (image credit: CNES)
Table 2: Specification of HRS instrument
The HRS instrument consists of:
• A carbon-skin sandwich/aluminium honeycomb instrument panel supporting the mechanical interface with the SPOT 5 satellite and the instrument systems
• A stereo video module (MVS), which handles and processes data from the detection units. MVS actively controls the instrument's temperature and provides the ancillary systems required to operate it (chiefly power and TM/TC)
• A thermal cocoon encloses the temperature-sensitive components vital to achieve the instrument's required performance.
A few months after the commissioning phase of the HRS instrument in 2002, CNES proposed to ISPRS a joint initiative for a photogrammetric assessment the new HRS instrument, in particular with regard to the quality and accuracy of DEM (Digital Elevation Model) generation derived from HRS stereo pairs. This proposal was agreed to by ISPRS and given the name of HRS-SAP (High Resolution Stereoscopic-Scientific Assessment Program). An international study team was set up. As of mid-2004, preliminary results have already confirmed the high quality of the HRS instrument on board of Spot 5. Nevertheless, the DEM/DSM (Digital Elevation Model/Digital Surface Model) accuracy derived from HRS data has been assessed around 5 m (relative) and 10/15 m (absolute). 23) 24) 25) 26)
Figure 12: Illustration of the HRS viewing capability (image credit: CNES)
With some minor improvements regarding instrument operations, the Vegetation-2 instrument is identical in its technical specification to Vegetation flown on SPOT-4.
DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite)
See description of instrument under SPOT-4 and under TOPEX/Poseidon.
Summary of CNES SPOT series:
Table 3: Overview of SPOT series missions
Table 4: Overview of SPOT series spectral continuity and resolution improvement
Table 5: Overview of performance parameters of the SPOT family
Figure 13: Comparison of spacecraft bus sizes of the SPOT series
Improved attitude restitution for SPOT-5 (commissioning phase issues)
An extra attitude restitution process has been developed in order to provide an accurate attitude for the imagery location model. It uses enhanced SPOT-5 gyroscope readings, combined with absolute attitude angle measurements given by a stellar sensor. This stellar sensor, SED16 (built by SODERN, Limeil-Brivannes, France), is having its first in-flight experience on SPOT-5; it is designed around a CCD matrix observing numerous stars on each data take. SED16 has a wide FOV and is autonomous (i.e. it has its own built-in star catalog). SED16 can track up to ten stars simultaneously. The measurements are used by an onboard Kalman filter. 27) 28)
Figure 14: The star tracker on the space-facing side of the satellite between the two HRG instruments (image credit: CNES)
The SPOT-5 location model calibration addresses five main issues:
• The first issue is to get best relative and absolute location performances. It consists of relative orientation calibration for HRG, HRS and stellar location unit reference frames.
• The second issue is to get a model of THR (Très Haute Résolution) pairs relative shifts good enough to deliver the best 2.5 m sampled image. The first ever, true HRG images have been acquired during satellite design, a few months prior to launch. Such images contributed to THR processing validation and allowed ground calibration of THR detection lines relative shifts, way before launch. In-flight measures confirmed that such ground measures are reliable.
• The third item is to turn HRS stereo pairs parallax into a precise enough altitude estimate. It implies that HRS location models have to include an accurate model of objective distortions.
• A fourth issue deals with an evaluation of HRG's steering mirror mechanism calibration. The same location performance is required, regardless of HRG mirror viewing angles.
• The final issue deals with optimization of time delay between two HRG off-nadir image acquisitions. Such time delay depends on mirror damping speed. For a given viewing angle, called “Autotest”, one can acquire HRG images of a designed pattern located in the focal plane. A straightforward processing of this image type indicates if the mirror command can be improved.
1) A. Ammar, A. Baudoin, D. Assemat, M. Arnaud, “The SPOT Programme, An Operational Earth Observation System,” Proceedings 45th Congress of the International Astronautical Federation, October 9-14, 1994, Israel
2) A. Baudoin, “The Current and Future SPOT Program,” Proceedings of the ISPRS Joint Workshop `Sensors and Mapping from Space 1999,' Sept. 27-30, 1999, Hannover, Germany
4) J.-P. Gleyzes, A. Meygret, C. Fratter, C. Panem, S. Ballarin, C. Valorge, “SPOT5 : System overview and image ground segment,” Proceedings of IGARSS 2003, Toulouse, France, Juky 21-25, 2003
5) L. Barre, S. Thomas, P. Jacob, T. Foisneau, D. Vilaire, M. Pochard, “Night Sky Tests and In-Flight Results of SED16 Autonomous Star Sensor,” Proceedings of the 26th AAS Conference on Guidance and Control, Breckenridge, CO, Feb. 5-9, 2003, Vol. 113 Advances in the Astronautical Sciences, Edited by I. J. Gravseth and R. D. Culp, AAS 03-042, pp. 389-398
6) L. Blarre, S. Thomas, et al., “Night Sky Tests and In-Flight Results of SED16 Autonomous Star Tracker,” 5th International Conference on Spacecraft Guidance, Navigation and Control Systems, Frascati, Italy, Oct. 22-25, 2002, ESA SP-516
8) P. Lier, G. Moury, C. Latry, F. Cabot, “Selection of the SPOT-5 Image Compression Algorithm,” Proceedings of SPIE, Vol. 3439,70, 1998
9) Information provided by Alain Lapeyre of CNES, Chef de Service Exploitation des Satellites de Télédétection.
10) J. N. Hourcastagnou, P. Cales, “GSCB Workshop Presentation SPOT / AstroTerra,” 3rd GSCB (Ground Segment Coordination Body) Workshop, 2012, ESA/ESRIN, Frascati, Italy, June 6-7, 2012, URL: http://earth.esa.int/gscb/papers/2012/16b-ASTRIUM_SPOT-AstoTerra.pdf
11) Information provided by Laurence Houpert of CNES (for the SPOT mission team), Toulouse, France
12) Information provided by Laurence Houpert of CNES, Toulouse, France
13) “SPOT-5 watches over Japan’s coastline after earthquake and tsunami,” Astrium Geo-Information Services, http://www.spotimage.co.jp/web/en/988-spot-5-watches-over-japans-coastline-after-earthquake-and-tsunami.php
14) “25 years of Spot satellites,” Feb. 21, 2011, URL: http://www.interspacenews.com/FeatureArticle/tabid/130/Default.aspx?id=6158
15) “Astrium fully integrates Spot Image and Infoterra into new GEO-Information business division,” Astrium, Dec. 1, 2010, URL: http://www.astrium.eads.net/node.php?articleid=6209
16) “Spot Provides Immediate Response to Haiti Earthquake,” Spot Image, URL: http://archive.constantcontact.com/fs013/1102324034386/archive/1102902578471.html
17) Information provided by Frédéric Tavera, SPOT Mission Exploitation Manager of CNES, Toulouse, France
19) SPOT 5 brochure, “Supermode,” of CNES and SPOT Image, May 1999
20) C. Latry, B. Rougé, “In Flight Commissioning of SPOT5 THR Quincunx Sampling Mode,” Proceedings of SPIE, Vol. 4881, 9th International Symposium on Remote Sensing, Aghia Pelagia, Crete, Greece, Sept. 23-27, 2002
22) G. Planche, C. Masso, L. Maggiori, “HRS Camera: A Development and In-Orbit Success,” Proceedings of the 5th International Conference on Space Optics, March 30-April 2, 2004, Toulouse, France, ESA SP-554
23) A. Baudoin, M. Schroeder, C. Valorge, M. Bernard, V. Rudowski, “The HRS-SAP initiative: A scientific assessment of the High Resolution Stereoscopic instrument on board of SPOT 5 by ISPRS investigators,” Proceedings of ISPRS 2004, Istanbul, Turkey, July 12-23, 2004
24) Z. Li, A. Gruen, “Automatic DSM Generation from Linear Array Imagery Data,” Proceedings of ISPRS 2004, Istanbul, Turkey, July 12-23, 2004
25) P. Reinartz, M. Lehner, R. Müller, M. Schroeder, “Accuracy Analysis from DEM and Orthoimages Derived from SPOT HRS Stereo Data without using GCP,” Proceedings of ISPRS 2004, Istanbul, Turkey, July 12-23, 2004
26) S. Massera , P. Favé, R. Gachet, A. Orsoni, “Toward a Global Bundle Adjustment of SPOT-5 HRS Images,” Proceedings of the 22nd Congress of ISPRS (International Society of Photogrammetry and Remote Sensing), Melbourne, Australia, Aug. 25 - Sept. 1, 2012, International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B1, 2012
27) E. Breton, A. Bouillon, R. Gachet, F. Delussy, “Pre-Flight and In-Flight Geometric Calibration of SPOT-5 HRG and HRS Images,” Pecora 15/Land Satellite Information IV Conference, ISPRS Commission I Mid-term Symposium/FIEOS (Future Intelligent Earth Observing Satellites), Nov. 10-14, 2002, Denver, CO
28) J. F. Salaün, E. Chamontin, G. Moreau, O. Hameury, “The SPOT-5 AOCS in Orbit Performances,” 5th International ESA Conference on Guidance Navigation and Control Systems, Frascati, Italy, Oct. 22-25, 2002
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.
The SPOT series: