SPOT-6 and SPOT-7
SPOT-6 and SPOT-7 Commercial Imaging Constellation
In 2008, Spot Image of Toulouse, France, and partners (EADS Astrium) started an initiative to build a new commercial SPOT mission series, referred to as SPOT-6 /-7, to continue sustainable wide-swath high-resolution observation services as currently provided by the SPOT-5 mission. Having full support from EADS Astrium (Spot Image's majority shareholder), manufacturing of the twin constellation SPOT-6 & -7 was officially announced in mid 2009 by Astrium Services' CEO Eric Beranger. The first launch is scheduled for 2012.
The current (2011) SPOT constellation of two satellites (SPOT-4, and SPOT-5) offers the broadest range of imagery, including, 2.5, 5, 10, and 20 m resolutions. The goal of the new SPOT-6 and -7 series is to guarantee a sustainable operational service to the end users. 1) 2) 3) 4)
From a technical point of view, SPOT-6 & -7 will inherit SPOT-5's experience, using new developments from Pléiades-1 & -2 program, as well as from the latest validated technology.
The SPOT-6 & -7 satellites will address the SPOT 5 market with improved characteristics:
• Same image swath of 60-km to maintain high level of coverage capability
• Better resolution with 1.5 m ortho image products
• Addition of a blue band to get native natural color images
• Ultimate satellite agility, enabling to achieve efficiently both collection of large coverage and collection of individual targets: more than 3 million km2 per day for each satellite
• Reactive tasking: Advanced programming efficiency with up to 6 programming plans per day and per satellite uploads possible to obtain cloud-free imagery
• Daily revisit capability thanks to the phased constellation of SPOT-6 & SPOT-7
• 10-year lifetime for each satellite, assuring data continuity towards 2023.
The SPOT-6/-7 spacecraft will operate in tandem with the 1 m resolution TerraSAR-X and TanDEM-X radar satellites of DLR, whose commercial data is available to Astrium affiliate Infoterra, and the high-resolution Pleiades-1 and -2 spacecraft. The two Pleiades spacecraft were publicly funded, but will be operated for commercial use by Spot Image. Hence, Spot Image will become the first commercial operator offering two High Resolution satellites (1.5 m imagery) and two Very High Resolution satellites (50 cm range of imagery).
Background: CNES, the French Space Agency, is terminating its SPOT program (which started in 1986 and provided an uninterrupted observation service so far) with an expected retirement of SPOT-5 in the timeframe 2014-15. The SPOT-5 mission (launch May 4, 2002) has already exceeded its nominal design lifetime of 5 years.
In July 2008, EADS's Astrium Services unit has acquired a majority stake in Spot Image S.A. in a move that promises to propel the company to the forefront of the international space imagery market. Astrium agreed to acquire most of French space agency CNES's holding in Spot Image, giving it an 81% stake in the Toulouse-based optical imaging specialist with five subsidiaries and strategic agreements around the world.
In mid-2009, the Astrium Services Division of EADS is committing to the development of the spacecraft, SPOT-6 and SPOT-7, even though it does not yet have any government funding or pre-sales agreements. The satellites, to be built by Astrium Satellites Division and launched in September 2012 and late 2013, respectively, will be the first ever built on a private basis and mark a watershed in the geospatial information service industry. Although U.S. imaging satellites like Ikonos and GeoEye are funded privately, they rely on guaranteed sales by the National Geospatial Intelligence Agency to finance satellite purchase. 5) 6) 7) 8)
Since early 2011, Spot Image, as well as Infoterra, a German-based radar imaging specialist with affiliates in France, the U.K., Spain and Hungary, is now fully integrated into the "Geo Information Division" of Astrium Services. 9)
The SPOT-6/-7 satellites are funded by Astrium alone; the company owns the data and system (satellites and ground segments). Astrium is also the commercial satellite operator. This is the first time in the remote-sensing industry that satellite development costs have been funded entirely with private funds. 10)
Figure 1: Artist's rendition of the SPOT-6 & SPOT-7 satellite constellation (image credit: EADS Astrium)
As of January 1, 2014, former EADS rebranded itself as the Airbus Group, with three divisions that include: 11)
- Airbus, focussing on commercial aircraft activities
- Airbus DS (Airbus Defence & Space), integrating the Group's defence and space activities from Cassidian, Astrium, and Airbus Military
- Airbus Helicopters,comprising all commercial and military helicopter activities.
After the consultation process with the works councils, expected to be concluded by mid-2014, the three entities – Airbus Military, Astrium and Cassidian – will be fully integrated and operational at all levels as Airbus DS. 12)
Spot Image and EADS Astrium in 2007 completed the design work on the SPOT-6 satellite with a launch mass of ~ 800 kg. The spacecraft features CMGs (Control Moment Gyroscopes) instead of reaction wheels to improve pointing maneuverability. The satellite's in-orbit design life will be 10 years.
Initially the SPOT-6 and SPOT-7 program was code named "AstroTerra" and was an initiative of Spot Image to ensure SPOT service continuity. The timely delivery of the satellites is of prime importan13)ce for Spot Image, in order to guarantee, without interruption, the level of service expected by its customers. 14) 15) 16) 17)
Table 1: Comparison of key parameters of the SPOT-5 and SPOT-6 /-7 spacecraft
The SPOT-6/-7 satellites are based on the platform family named AstroSat-250 (also referred to as AstroBus-L), i.e. an upgraded version derived from the FormoSat-2 and THEOS missions, featuring CMGs (Control Moment Gyros) as actuators. This technology will provide the satellites with an unrivalled agility, allowing single pass stereo or tri-stereo, mosaics (up to 5 contiguous strips), as well as multiple acquisitions over a given target area.
AstroBus-L description (Ref. 14):
The AstroBus-L is a standard, modular, ECSS (European Cooperation for Space Standards) compatible satellite platform compatible with an in-orbit lifetime of up to 10 years, with consumables sizeable according to the mission needs. The platform design is one-failure tolerant and the standard equipment selection is based on minimum Class 2 EEE (Electrical, Electronic, and Electromechanical) parts, with compatibility to Class 1 in most cases. It is implemented on AstroTerra (SPOT-6/-7), SEOSat/Ingenio of Spain, GMES/Sentinel-2 of ESA, EarthCARE of ESA, and the Kazakhstan HR imaging satellite of KGS (Kazakhstan Gharysh Sapary), currently referred to as ERSSS (Earth Remote Sensing Space System).
The AstroBus-L platform is designed for direct injection into LEO (Low Earth Orbit). Depending on the selection of standard design options, AstroBus-L can operate in a variety of Low Earth Orbits at different heights and with different inclinations.
Note: The AstroBus-L platform is also referred to as AstroSat-250 (or AS250). The AstroSat-250 standard architecture reflects the large experience gained by Astrium in many Earth Observation missions. AstroSat-250 combines heritage with flexibility for customisation. Through optimization of re-use, it helps to reduce program risk and to achieve reliable schedule and cost commitments. It ensures high quality and robustness due to continuous application of proven solutions and it guarantees continuity of engineering skills.
Both satellites have drawn on technological and operational innovations conceived for some of them for the Pleiades constellation (CMG, FOG) and for others on Astrium's product lines. The satellites are in particular based on an Astrium AS250 new generation avionics, integrated on a small sized structure together with an optical instrument based on the NAOMI product line. Both satellites are agile due to their CMGs. Even if today many satellites are maneuverable, all do not swing and re-target at the same speed. CMGs allow the satellites to pitch and roll forward, backward and sideways up to 45° very quickly - twice as fast as earlier designs such as Momentum Wheels. Why does the user care? Because it increases the number of images that can be collected during the same pass. In a shell, collections opportunities are more numerous, scheduling conflicts between contiguous requests are vastly reduced, and then, the average acquisition window is much narrower. Icing on the cake, the acquisition on the same pass of several targets at the same latitude becomes possible.
Figure 2: Schematic view of coverage with agile CMGs (image credit: Astrium SAS)
Figure 3: Standard electrical architecture of the AstroBus-L platform (image credit: EADS Astrium)
The core of the AstroBus-L electrical platform is the redundant OBC (On-Board Computer). The OBC includes processing, reconfiguration and timing functions while the complete I/O system is allocated in a separate physical unit, the RIU (Remote Interface Unit), which enables for mission customization. The backbone for the on-board communication is formed by two MIL-STD-1553 buses, one serving the platform one serving the payload. Optional SpaceWire interfaces allow for transmission of data at high rates between the OBC and e.g. special payloads.
The LEON3-FT (Fault Tolerant) microprocessor, namely the SCOC3 (Spacecraft Controller On-a- Chip), is being used as OBC. SCOC3 has been developed at EADS Astrium SAS; it is manufactured by Atmel.
A standard S-band TT&C system with full spherical coverage in uplink and downlink is used for satellite command reception and telemetry transmission. Ciphering functions are available either integrated within the OBC or in form of an add-on unit to the OBC called DCU.
ADCS (Attitude Determination and Control Subsystem)): The enhanced 3-axis stabilization attitude control system is based on a set of 4 RW (Reaction Wheels) for fine-pointing with 3 MTQ (Magnetic Torquers) for off-loading. In case of very high agility requirements, the reaction wheels are replaced by Astrium patented CMGs (Control Moment Gyros).
Attitude and orbit measurement is performed with a GPS and a Star Tracker (STR) for nominal operation, providing a pointing accuracy of up to 500 µrad (3σ) and a pointing knowledge of up to 30 µrad (3σ), depending mainly on STR accommodation and alignment accuracy. While standard precise attitude control is performed without the support of a gyro, an optional inertial measurement unit can be added for attitude control improvement. On-board orbit determination accuracies of < 10 m (1σ) are achieved, if the standard 1-frequency GPS design is applied. The 2-frequency GPS option provides on-board orbit determination accuracies of < 3 m (1σ).
Figure 4: Photo of the HYDRA multiple head star tracker based on APS (CMOS) detector technology (image credit: EADS Sodern)
Legend to Figure 4: The SPOT-6 spacecraft is the firat EO mission to utilize Sodern's new generation HYDRA star tracker for guidance and navigation. With HYDRA, Sodern is offering a multi-head design which separates the optical head from the centralized processing unit. This eases the thermal control of the optical heads and allows continuous attitude measurement, whether the satellite is facing Earth, the Sun or other bright objects.
Safe mode attitude sensing is based on a Magnetometer (MAG) / Sun Sensor (BASS) system or optionally a Magnetometer / Coarse Earth Sun Sensor (CESS) system. This provides two optional Safe mode attitude control principles:
1) Sun oriented, magnetic field spin controlled, or
2) Earth oriented.
Propulsion system: A mono-propellant propulsion system is implemented to allow for orbit maintenance and optionally for rapid rate damping during initial acquisition in case of the Earth oriented safe mode. Different tank sizes and thrusters configurations are available to cover specific mission needs.
Figure 5: Photo of the propulsion module of SPOT-6 (image credit: EADS Astrium)
EPS (Electrical Power Subsystem): The EPS is built around an unregulated 28 V Power Bus. GaAs triple junction solar cell based arrays, which are either composed of standard panels or have to be tailor designed to suit mission needs, are used for power generation. Power control and distribution functions are combined in the PCDU (Power Control and Distribution Unit).
For missions with a low power demand (< 1200 W) and only slight variations in solar array illumination conditions, a standard PCDU with shunt regulation system is foreseen. Higher power demand can be satisfied with MPPT (Maximum Power Point Tracker) regulation based PCDU, which can be selected from a set of different peak power values. Sufficient FCLs and LCLs in different power classes are provided for platform and payload power distribution and protection. Furthermore, a comfortable amount of heater switches and redundant release actuators is available. Electrical energy is stored in Li-ion batteries, for which a large range of different capacities is available.
The AstroBus-L software is based on a modular architecture with a standard mission and hardware independent core consisting of the RTEMS operating system and the Astrium CDHS (Core Data Handling System). A library of reusable hardware-and mission dependent software elements exists which can be used as basis for construction of individual mission customized software versions. The process of mission specific software customization and development is done in compliance with ECSS E-40 and Q-80.
An essential feature of AstroBus-L is the robust standard FDIR (Failure Detection, Isolation and Recovery) concept, which is hierarchically structured and can easily be adapted to specific mission needs.
Payload data are being stored in Flash Memory technology based standard CoReCi (Compression Recording and Ciphering) units, available at various capacities ranging from 1 Tbit to ~ 10 Tbit. Options exist for inclusion of compression and encryption functions within the CoReCi. Several different standard algorithms are available for this purpose. 18)
Figure 6: Photo of the CoReCi system (image credit: EADS Astrium)
CoReCi implementation parameters for SPOT-6: 19)
- Modular architecture adaptable to various data rates and capacities
- Input data rate up to 1.4 Gbit/s
- Capacity 850 Gbit (EOL) with Flash technology
- Embedded Wavelet Image Compression with MRCPB algorithm
- Ciphering based on AES (Advanced Encryption Standard) algorithm with 127 x 128 bit ciphering keys
- Data Formatting according to CCSDS ESA Packet Telemetry Standard
- Instrument mass: 14 kg
- Power consumption 75 W during simultaneous data record / data compression / data replay.
CoReCi on SPOT-6 represents the first commercial use of Flash storage technology in an on-board PDHU (Payload Data Handling Unit).
RF communications: The standard for downlink of payload data is a 300 Mbit/s 2-channel cold redundant X- band. The data transmission system uses QPSK modulation. A single Isoflux antenna provides the necessary ground coverage. Optionally, other systems can be incorporated in case of higher downlink data rate needs. Encryption of downlink data can be implemented as option. A number of different basic mechanical standard configurations are available which can be used as foundation for mission customization in many typical applications. The TT&C data are transmitted in S-band.
Figure 7: Illustration of the deployed SPOT-6 spacecraft (image credit: EADS Astrium)
Figure 8: Photo of SPOT-6 in the clean room of ISRO (image credit: ISRO)
In April 2012, Astrium SAS signed a launch agreement with Antrix Corporation Ltd. (the commercial arm of ISRO). 22)
The secondary payload on this flight was:
• PROITERES, a nanosatellite (15 kg) of the Osaka Institute of Technology, Osaka, Japan
The nominal launch of SPOT-7 is planned for 2014.
Orbit: Sun-synchronous circular orbit, altitude = 694 km, inclination = 98.2º, LTAN = 22:00 hours. When SPOT-7 is in orbit, both spacecraft will be deployed into the same orbital plane phased at 180º.
Note: Astrium Geo-Information Services is the civil operator of the Pleiades constellation (Pleiades-1A and -1B) as well as the operator of the SPOT-6 and SPOT-7 constellation. When SPOT-7 is launched in 2014, there will be 2 x 2 satellites, a true constellation with all spacecraft in the same orbital plane, coherently operated (90° one from the other on the same orbit) through a single interface (Ref. 38).
Figure 9: Astrium GEO-Information Services is operating the Pleiades constellation and the SPOT-6&7 constellation (image credit: Astrium)
Insertion orbit: Sun-synchronous circular orbit, altitude = 660 km, inclination = 98.23º, LTAN = 10:00 hours.
Figure 10: Nominal flight profile of PSLV-C23 (image credit: ISRO, Ref. 25)
The secondary payloads on this flight were:
• CanX-4 and CanX-5, a pair of identical nanosatellites (each of 15 kg) of UTIAS/SFL (University of Toronto, Institute for Aerospace Studies/Space Flight Laboratory), Toronto, Canada.
• AISAT (Automatic Identification System Satellite), a technology demonstration nanosatellite (14 kg) of DLR (German Aerospace Center). 26)
• VELOX-1 is a 3U CubeSat with a mass of 4.5 kg of SaRC (Satellite Research Center) at NTU (Nanyang Technological University), Singapore.
Figure 11: The SPOT-7 integration of solar panels is complete as of March 2014 (image credit: Airbus Defence and Space)
Figure 12: Illustration of the secondary payloads (image credit: ISRO, Ref. 25)
The SPOT-7 spacecraft will join the orbit in which its twin, SPOT 6, and the very-high-resolution observation satellites Pleiades 1A and 1B are located, and will be positioned at 180º in relation to SPOT 6 (Figure 13). After SPOT-7 has undergone a period of tests in orbit, Airbus Defence and Space's optical constellation will be at its full operational capacity.
Figure 13: The SPOT-6/-7 and Pleiades satellite constellation configuration (image credit: Astrium GEO-Information Services)
In addition, PSLV-C23 hosts one attached payload, the AINS (Advanced Inertial Navigation System) which will ride uphill with the launcher, making navigation measurements during ascent that are purely used for the validation of the system for potential use on Indian launch vehicles in the future. Data provided by AINS is not used to navigate the PSLV launcher on this flight and is collected for downlink to the ground to examine the accuracy and performance of the new navigation system that is smaller in size than the old unit, but delivers a higher accuracy (Ref. 24).
Mission Status of SPOT-6 and of SPOT-7 (Azersky):
• December 2, 2014: With the commercial launch of SPOT-7, the Airbus Defence and Space's optical satellite constellation is now complete, offering a uniquely comprehensive combination of resolution and coverage. A strategic cooperation agreement has just been signed with the Azerbaijani satellite operator Azercosmos for SPOT-7 exploitation. 27)
SPOT-7, the Earth-observation satellite designed and built by Airbus Defence and Space, is commercially launched today, making its high-resolution imagery available to all users. Together with SPOT-6, Pleiades-1A & -1B, TerraSAR-X and TanDEM-X, SPOT-7 makes Airbus Defence and Space the first operator in the world to offer customers a full palette of Earth-imaging data spanning multiple resolution and spectral wavelengths.
- The completion of this unique constellation also represents an opportunity for Airbus Defence and Space to establish a new cooperation with the Azerbaijani satellite operator Azercosmos, for a joint use of the SPOT (6 & 7) satellites.
- The contract is a long-term cooperation agreement which includes the transfer of ownership of the SPOT -7 satellite to Azercosmos of Azerbaijan, the provision of a ground segment for the exploitation of the satellite and a sound training program for operational capacities development. It also encloses a commercial agreement related to the distribution of SPOT-6 and SPOT-7 data to respective customers of both parties.
- As an established player in satellite manufacturing and service provider, Airbus Defence and Space is pleased to work with Azerbaijan to build its ambitious Space Program. This collaboration is an opportunity for Azercosmos to build up its own life experience both in operating a satellite and in serving commercial market immediately.
- Dec. 2, 2014: Within the strategic cooperation framework between Azerbaijan and France in the space industry, "Azercosmos" OJC, national satellite operator of Azerbaijan, signed a strategic cooperation agreement with Airbus Defence and Space to take over the rights to operate and commercialize the SPOT -7 high resolution (1.5 m imagery products) optical Earth observation satellite, which was successfully launched on June 30, 2014 and to be renamed as Azersky. The agreement, which is an important step in the development of relations between Azerbaijan and France, was signed at Bakutel 2014 Telecommunications and Information Technologies Exhibition. 28)
- The long term strategic cooperation between the two parties also includes joint operations of Azersky of Azercosmos and the SPOT- 6 high resolution (1.5 m imagery products) optical Earth observation satellite of Airbus Defence and Space in a constellation to provide superior service. Moreover, the ground segment to be constructed will allow Azercosmos to access images acquired from Pleiades-1A and -1B, very high resolution (0.5 m imagery products) optical Earth observation satellites commercialized by Airbus Defence and Space. This satellite constellation opens up opportunities for many applications, providing the latest images within an unprecedented time frame. The combined acquisition capacity of Azersky and SPOT-6 is equal to six million km2 per day – an area ten times the size of France. This means that every day, every point on the globe can be viewed in high-resolution by the Azersky and SPOT-6 satellites.
- Azersky has a broad range of applications including but not limited to defense and security, emergency response, exploration of natural resources, maritime surveillance, sea faring, environmental protection, urban planning, cartography, agriculture, and tourism. During the next 1.5 years, more than 25 Azerbaijani professionals will be trained in France on optical satellite operations, product development, sales, and marketing of Geo-Information services. This agreement also marks Azercosmos' entry into the commercial business of Earth observation services, including Geo-Information services. This will strengthen Azerbaijan's position among the limited number of countries rendering satellite services.
• Oct. 1, 2014: SPOT -7 has already completed several major milestones of the calibration phase, which confirmed the excellent image quality performances. Since August 5, 2014, images collected are added to the catalog. The start of commercial operations will take place in just a few weeks. Together with SPOT- 6 and Pleiades- 1A/1B, SPOT -7 will open up opportunities for new applications, providing the fresh images within an unprecedented time frame. 29)
• July 4, 2014: First images from the SPOT-7 satellite were acquired within three days after launch. The SPOT- 6/7 constellation is now in place and considerably improves the capabilities and performance offered by SPOT-5, which has been in operation since 2002 and which is scheduled to be decommissioned from commercial service during the first quarter of 2015. This new constellation offers a higher resolution, greater programming reactivity and a much higher volume of images acquired daily (in monoscopic or stereoscopic mode). The first sample images of SPOT-7 are shown in Figures 14 and 15. 30)
Figure 14: SPOT-7 image of the island of La Reunion in the Indian Ocean (size: 2511 km2, population: 840,000) acquired on July 3, 2014 (image credit: Airbus Defence and Space)
Figure 15: SPOT-7 image of Sydney, Australia acquired on July 3, 2014 (image credit: Airbus Defence and Space)
• June 30, 2014: The SPOT-7 satellite separated from the rocket 18 minutes after launch and went into a 660 km orbit; the other four secondary satellites were deployed over the next two minutes, according to ISRO (Indian Space Research Organization). 31)
• May 2014: The flash memory Solid-State Recorder (SSR) of Airbus Defense and Space has exceeded 20 months of operation in orbit onboard SPOT-6, the first commercial satellite to deploy this technology. Flash Solid-State Mass Memory provides far greater storage capacity at a lower operating cost than SDRAM. The advantages have been confirmed on SPOT-6 where the flash SSR has a flawless track-record in an Earth observation mission. This new flash SSR device of Airbus Defense and Space has achieved both NASA and ESA qualification standards. 32)
• The SPOT-6 spacecraft and its payload are operating nominally in 2014. SPOT-6 is delivering products in the frame of major European programs [Copernicus, MARS (Monitoring of Agriculture with Remote Sensing)] and International Charter Space & Major Disaster. 33)
• A SPOT-6 geometric accuracy assessment was performed on three levels: 34)
- Use of the Landsat IAS (Image Assessment System): developed for radiometric and geometric characterization and calibration of Landsat data.
- Band to Band (B2B) assessment: B2B is performed to test band alignment of the image data; it is typically done by registering each band against every other band.
- Image to Image (I2I) registration assessment tool: I2I is usually performed to compare the relative accuracy between two images.
• Excellent location accuracy of the SPOT-6 imagery was already confirmed at commissioning: 19.3 m CE90 @30º; this is nearly two times better than the satellite requirement (35 m CE90 @30º). These measurements were conducted during the commissioning phase (Sept-Dec 2012). 35)
- The current performance (March 2014) after one year of refinement and temporal drift calibration (Oct 2013) is nearly three times better than satellite requirement: 11.9 m CE90 @30º.
- Focal plane calibration and planimetric accuracy assessment.
Planimetric accuracy: Residual error of all Line Of Sight contributors after geometric model reset on a Ground absolute reference:
Auto calibration without reference site:
Figure 16: Schematic view of the SPOT-6 focal plane (image credit: Airbus Defence and Space)
Figure 17: SPOT-6 sample image of Detroit, MI, USA, ortho pan-sharpened 4 bands at 1.5 m resolution acquired on May 25, 2013 (image credit: Airbus Defence & Space) 36)
• Oct. 2013: The remarkable agility of SPOT-6 (and -7) not only benefit to the global acquisition capacity. They also enable performing a prodigious variety of acquisition scenarios in a single satellite pass. Whatever the need of the user is, the system can equally collect (Ref. 17):
- A multitude of individual scenes in a reduced theater (typically 11 within a 1 000 km long orbit slot, (Figure 18 left)
- Long strips (typical acquisition mode for SPOT 6 and 7, with max length being 600 km, (Figure 18 center)
- Contiguous strips (e.g. to cover an area of 330 km x 300 km, that can be then automatically mosaicked and orthorectified in the ground segment (Figure 18 right).
Figure 18: Various coverage scenarios of the highly agile satellite system (image credit: Astrium SAS)
- And also stereo collection for 3D models computation and mapping edition up to 1 / 15 000. The agility is even enabling easily the acquisition of a third image at nadir, then performing tri-stereo: in addition to collecting two oblique images looking forward and backward over an AOI as SPOT-5 did, each new satellite can pivot fast enough to acquire a third vertically over the target. This nadir image in the tri-stereo dataset vastly enhances the quality of elevation data extracted from the stereo images. Indeed, in standard two-image stereo products acquired over steep terrain or dense metropolitan areas, low-lying features may be hidden from view in the oblique perspective by high mountains or tall buildings. In tri-stereo, the nadir image peers straight down into the natural or man-made valleys to capture all features and ground surfaces, resulting in more complete and accurate DEM (Digital Elevation Model) datasets (Ref. 17).
The resolution is not the only element determining the gains in terms of appearance, sharpness and image quality. Another great improvement with SPOT- 6 /-7 is the 12 bit pixel depth. For each spectral band, it means that each pixel can take one value out of 4 096. SPOT-5 has a pixel depth at acquisition of 8 bits, so a given pixel can take one value out of 256, thus displaying less capacity when distinguishing subtle nuances, especially in the beginning or the end of the spectrum:
- It is more likely with SPOT-6 to detect objects in the darkness of the shadow of a building or a mountain, as more nuances can be taken by each pixel
- Similarly, it is easier to detect pale-colored elements in very light / bright environments (sand, ice, nearly-white ground), according to the same principle, as more saturation problems are avoided.
Another significant improvement of SPOT-6 /-7 compared to SPOT-5 is the intrinsic geolocation accuracy of the collected images. Thanks to a new generation of Attitude and Determination and Control Subsystem (ADCS), the actual performance measured on SPOT-6 image primary products, i.e. without use of ground control point, is better than 20 m CE90 and even 10 m for the automatic orthos. With such a performance, entire country maps can be updated for scales of 1:25 000 and even 1:15 000 (Ref. 17).
• July 2013: SPOT-6 has successfully passed all tests conducted by the Joint Research Centre (JRC), the European Commission's in-house science service, and has been contributing to the MARS-CAP (Monitoring Agricultural ResourceS-Common Agricultural Policy) program since July 1, 2013. MARS-CAP is the European program surveying agricultural land by satellite within the framework of the Common Agricultural Policy. 37)
Following qualification, SPOT-6 joined the other Astrium-operated satellites, SPOT-5, Pleiades-1A and Pleiades-1B, on the program on July 1, as part of the 2013 MARS-CAP campaign. Initiated by the European Union in 1993, the MARS-CAP campaigns involve mapping agricultural land throughout Europe to verify declarations relating to cultivated land areas and fallow land submitted by farmers. The European subsidies granted to farmers are based on these declarations and their verification.
SPOT- 6 brings new performance levels to the MARS-CAP program, including heightened resolution (1.5 m), the blue spectral band (to acquire images directly in natural colors), enhanced localization of images and unequalled agility, enabling extensive areas to be mapped in record time. - Astrium Services has been working alongside the European Commission for 20 years to help implement the Common Agricultural Policy.
• In the first quarter of 2013, SPOT-6 collected imagery covering 152 million km2; 300 million km2 have been collected since launch. 38)
Figure 19: SPOT-6 coverage collected in Q12013 (image credit: Astrium GEO-Information Services)
• Jan. 2013: Bamako, Mali, is located on the Niger River in Africa, which can be seen in the upper section of Figure 20. The river passes through much of Mali and ultimately discharges into the Atlantic Ocean from Nigeria. The city spans both sides of the river and three bridges connect the city. - This image is a SPOTMap 1.5 product produced by Airbus Defence and Space, that aims to provide imagery ideally suited to mapping and planning purposes. 39)
Figure 20: This SPOT-6 image, acquired on January 26, 2013, shows Bamako, the capital city of Mali (image credit: Airbus Defence and Space)
• Coverage of Mali: In 2011, the Mali Cartographic Institute had issued an RFP (Request for Proposal) to cover the entire country, 1,250,000 km2 with no sand wind and no cloud. The first strategy was to leverage 2-year old SPOT -5 archive, trying to fill in the gaps with new SPOT-5 acquisitions. But SPOT-5 mirrors were not enabling the acquisition of contiguous segments, thus limiting the progress at each pass. For over a year, the campaign remained uncompleted (Ref. 17).
On October 17, 2012, a month and a half after its launch, SPOT-6 was added to the campaign to rescue and close up the coverage. The first performances were breathtaking, so the tasking team decided to switch the entire country to SPOT-6. On March 15, 2013, - only 5 months after kicking off the tasking request - 100% was completed on spec and on time. This exploit is even more striking since conflicting with another campaign, started the same October 17 over Senegal: even not all the resources of SPOT-6 had been allocated to Mali .... Full Senegal was cleared on March 12, 2013. Imagine the coverage speed once SPOT-7 is there!
Figure 21: Mali coverage example of SPOT-6 in 5 months (image credit: Astrium SAS, Ref. 17)
• In mid-January 2013, SPOT-6 was officially declared "good to go" (i.e., operational) after the recent completion of technical commissioning. All satellite functions and performance are nominal, in some cases even exceeding specifications. The sensor's agility has been thoroughly tested; with scenarios including imaging large areas in a single pass (e.g. 500 km x 150 km), consecutive long strip acquisitions (e.g. 1500 km in 3 North-South segments) and target acquisitions (e.g. more than 15 scenes -60 km x 60 km- over a same point in one pass). 40)
The system is now fully operational, with daily acquisitions averaging very close to the maximum capacity of 3 million km2. The system has successfully downlinked images on 4 passes over the Toulouse station and 8 passes over the Kiruna station. In the last 2 months, SPOT-6 imaged more than 100 million km2 without even operating at full capacity! System operation is now fully managed by Astrium GEO-Information Services with the support of Astrium Satellites for control of the satellite.
• In October 2012, Astrium Services and the Istanbul Technical University (ITU) signed two agreements in Istanbul , to develop high-resolution and large-area coverage services in Turkey, notably for agriculture. The agreements cover a SPOT New Generation receiving station and reception of data from SPOT-6 and SPOT-7, as well as an extension for SPOT-5 data. 41)
• On Sept. 22. 2012, SPOT-6 reached its final orbital slot. A review of this initial positioning phase was successfully completed on October 12, 2012. The review also included an exhaustive analysis of the in-orbit status of the satellite's subsystems. All systems have been powered up and are functioning nominally. Fuel consumption has been tightly controlled, commensurate with the satellite's planned 10-year operating lifetime. 42)
• Sept. 12, 2012: Three days after launch, SPOT-6 acquired its first image of the Bora Bora atoll (French Polynesia), part of the Society Islands in the Pacific Ocean. The pansharpened image (Figure 22) has a resolution of 1-5 m 43) 44)
Figure 22: Spot-6 image of the Bora Bora atoll observed on Sept. 12, 2012 (image credit: Astrium Services)
Legend to Figure 22: Bora Bora is an atoll island in the wind which means "first born." The island is surrounded by a large lagoon and a fringe of coral reef. The center is occupied by an extinct volcano.
Figure 23: SPOT-6 image of Gibraltar observed a few days after launch on Sept. 12, 2012 (image credit: Astrium, Ref. 19)
Sensor complement: (NAOMI)
NAOMI (New AstroSat Optical Modular Instrument):
NAOMI is a product line of high-resolution imagers designed and developed at EADS Astrium SAS. Several versions of the NAOMI instrument have already been developed with a GSD from 1.5 m to 2.5 m, and swath widths from 10 km up to 60 km. All of them are based on the same telescope concept implementing one or several focal planes units in the same camera, and even two cameras in the same instrument as for the SPOT-6 & SPOT-7 program.
NAOMI instruments can be embarked on small platforms (Myriade class) or larger platforms (Astrosat-250). The IEU (Instrument Electronics Unit) can accommodate internal mass memory functions or can be coupled with high capacities mass memories like CORECI equipment also developed by Astrium SAS.
Figure 24: Photo of the standard NAOMI instrument (image credit: EADS Astrium, Ref. 10)
The main building blocks of the instrument are: 45)
• A highly stable, light and compact telescope built in SiC material, with a simple thermal control.
• A focal plane, embedding TDI (Time Delay Integration) detector, a PAN CCD and four XS (multispectral) detectors equipped with strip filters and coupled with front end electronics. The TDI implementation exhibits an outstanding MTF (Modulation Transfer Function) servicewith an extremely low power consumption. This allows significantly loosening of optical requirements at the telescope level, while keeping the same overall optical quality at system level; in other words, the same optical quality can be reached from smaller and much lighter telescopes. Therefore more performance can be obtained from smaller satellites.
• Back-end electronics, including video Electronics, data storage and services adapted to the mission specificities. The modular video chains are capable of operating at different frequencies up to 15 Msample/s, so that the same hardware can be easily tuned to serve ground resolutions ranging from 0.5 m to say 10 m. The swath width can easily be adjusted by butting together several detectors and associated modular video chains, thus fulfilling the requirements of the most demanding customers.
The telescope is based on a Korsch combination, offering a simple, compact concept. The detector, space qualified, includes on the same die one TDI matrix of 7000 pixels for the panchromatic channel, and four lines of 1750 pixels for the multispectral bands. The detector exhibits excellent characteristics that significantly contribute to the instrument very high optical performance.
The optical assembly is based on a Korsch-type telescope including three aspheric mirrors and two folding mirrors.
Figure 25: Illustration of the optical concept of the Korsch telescope (image credit: EADS Astrium)
The detection chain is made of three main parts: the detectors, the Front End Electronics Module (F2EM) and the Video Electronics (MEV) which are part of the IEU (Imaging and Electronics Unit). The PAN + XS focal planes are the heart of the detection chain.
Focal plane is based on a customized high performance detector architecture developed by e2v for Astrium (proprietary architecture). It takes benefit of all the heritage and skills acquired in CCD architecture definition and in operating with the ultimate conditions of speed and performances. The result of this customization offers an unrivalled level of integration and performances. All the stringent constraints of dynamic range optimization and power consumption reduction have been mastered with less than 1 watt detector dissipation.
The Front-End Electronics Module (F2EM) encompasses all the functions to be implemented close to the detectors. Mounted inside the FPA (Focal Plane Assembly), it provides the detectors with all the necessary biasing and clocking signals and performs preamplification and transmission of the video signal to the MEV.
The MEV (Module Electronique Video) is the backend part of the NAOMI detection electronics. The MEV provides the F2EM with the primary power supplies and clocks necessary to front-end operation. The video signal from the F2EM is received, adapted and digitally converted to 12 bit in the MEV. The resulting data, rounded down to 10 useful bits, are then transmitted to the digital functions of the NIEU to be real-time processed and stored into the mass memory for further downlink.
Figure 26: PAN+XS focal plane architecture (image credit: EADS Astrium)
Figure 27: Overview of NAOMI detection chain (image credit: EADS Astrium)
Figure 28: Mechanical architecture of NAOMI (image credit: EADS Astrium)
Figure 29: NAOMI camera with two focal plane units: SPOT-6/-7 instrument configuration (image credit: EADS Astrium)
Table 2: Specification of the NAOMI instrument
The same logic applies to the variable parameter of GSD in Table 2: The GSD is fixed for a particular mission (the ranges feature the values for different missions using a NAOMI instrument). For SPOT-6/-7, the GSD for PAN = 2 m and for MS = 8 m.
Figure 30: Functional block diagram of the IEU including mass memory functions fitted to small platforms configuration (image credit: EADS Astrium)
Figure 31: Functional block diagram of the instrument electronics in a multi-CCD configuration of NAOMI (image credit: EADS Astrium)
Figure 32: Photo of the NAOMI instrument assembly for the SPOT-6 spacecraft (image credit: EADS Astrium)
The overall control ground segment architecture is based on a well-mastered and efficient design which has been used and improved on several Astrium export programs - like the ground segments for the missions: THEOS (Thailand), AlSat-2 (Algeria), and SSOT (Chile). 47) 48)
The SPOT-6/-7 (AstroTerra) architecture comprises a ground segment divided into two major parts:
• The CGS (Control Ground Segment), in charge of controlling, commanding and monitoring the SPOT-6 and SPOT-7 satellites on their orbit, and maintaining the orbit. The objective of CGS is to gather all the requested means for managing the satellite configuration and for supporting the satellite maintenance.
• The EGS (Exploitation Ground Segment), in charge of programming the satellite mission plan and ingesting the image data and processing it in order to produce, archive and deliver the SPOT-6 and SPOT-7 image products.
The CGS is connected to a full scale S-band service, providing access to one or several S-band stations for telemetry and telecommand data exchanges with the SPOT-6 and SPOT-7 satellites. This S-band service will include at least one polar station, so as to provide maximum access to the satellites.
The AstroTerra system is composed of:
• The Space Segment in charge of collecting the imagery data and including :
- Spot 6 and Spot 7 observation satellites operating on a Low Earth Orbit
- The AstroTerra Control Ground Segment, located at the Astrium Satellite premises, Toulouse, France
• The customer Operator Segment also called Exploitation Ground Segment made of:
- A network of AstroTerra DRS (Direct Receiving Stations)
- An AstroTerra Polar Center, located in Kiruna, Sweden
- The AstroTerra Operator Center located in customer premises at Toulouse, France.
Figure 33: Overview of the AstroTerra ground segment based on a set of operational centers and receiving stations installed at optimized locations (Astrium Satellites)
Figure 34: Schematic view of the AstroTerra CGS (image credit: Astrium Satellites)
Instead of procuring one or two ground stations for commanding and controlling the satellites, the AstroTerra program has decided to rely on a S-band service, contracted to the Swedish Space Corporation. This service will provide access to one or several S-band stations in Kiruna and Inuvik for telemetry and telecommand data exchanges with the SPOT-6 and SPOT-7 satellites.
The main external interfaces of the CGS (Control Ground Segments) are:
• The reception of satellite mission plan from the EGS (Exploitation Ground Segment) and the distribution of orbital information, satellite status and mission follow-up to the EGS.
• The reception of external time reference to synchronize all subsystems
• The link to the S-band service
• The distribution of operational alarms (SMS / email)
• The link to a space debris collision risk evaluation service (provided by CNES, the French space agency)
• The link to the encryption device keys manager
• The link to the satellite simulator.
The solution designed for AstroTerra Control Ground Segment relies on software products developed by Astrium. MOSAIC, is a new software product developed in the frame of the AstroTerra project. MOSAIC has now become part of the Control Ground Segment product line. AstroTerra has integrated the following products:
• Control Command: OPEN CENTER
• Flight dynamics: QUARTZ++
• System database : SIS (Satellite Information System)
• Centralized logbook: C-LOG
• Interface adaptation and automation: MOSAIC
• Off-line Telemetry Trend Analysis : TELMA
The EGS (Exploitation Ground Segment) is built to be fully automatic for processing routine daily tasks. As a consequence, the Mission Plan and the Satellite Mission Commands Plan uploading are designed to be carried out without human intervention.
Operations: With SPOT-6 and SPOT-7, Astrium Satellites will for the first time operate commercial Earth observation satellites. Astrium benefits from the knowledge of the control ground segment and, naturally from intimate mastership of the operations requirement of its own satellites. However, setting up the operations of the ground control center has required defining and implementing infrastructure requirements, operations approach, staffing strategy, which are usually only specified by Astrium and implemented by its customers.
Geo-Information Services of Astrium:
Spot Image and Infoterra joined forces within Astrium Geo-Information Services to offer a consolidated product and services portfolio under the Astrium brand. The merger took place in May 2010. On January 1, 2011, a single operational management structure was implemented. 51)
A single operational management structure started on January 1, 2011, bringing closer together the satellite imagery and geoinformation specialists Spot Image and Infoterra to form the GEO-Information division of Astrium Services. As a global, integrated company, the GEO-Information division of Astrium Services is implementing this new organization with a single vision in mind: to better respond to the needs of customers.
The GEO-Information Services division will offer:
• a one-stop shop for data from the SPOT and TerraSAR-X satellites, and from Pleiades
• a single product and services portfolio covering the entire geographic information value chain from satellite imagery to value-added services and turnkey solutions.
Figure 35: Astrium - GEO-Information Services Worldwide (image credit: EADS Astrium)
ERMEX receiving station: In the spring of 2012, Astrium Services signed an agreement with the government of Mexico to upgrade the ERMEX receiving station south of Mexico City. This station will enter service in September 2012. Equipped with a high-tech antenna and a new-generation SPOT terminal, it will initially receive SPOT-5 imagery and then SPOT-6 imagery when commercial operations get underway. It will also receive data from SPOT-7 in 2014. 52)
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46) Information provided by Michel Pascal of Astrium Services, Toulouse, France
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).
The SPOT series:
To work in concert with: