Pleiades-HR (High-Resolution Optical Imaging Constellation of CNES)
Pleiades-HR is a two-spacecraft constellation of CNES (Space Agency of France), representing a long-term engagement with the introduction of advanced technologies in Earth observation capabilities. Starting with the first launch in 2009, the Pleiades program will follow the SPOT program satellite series services, with the following overall objectives:
• Provision of an optical high-resolution panchromatic (0.7 m) and multispectral (2.8 m) imagery with high quality product standards in terms of resolution, MTF (0.2 at system level), and a high image location accuracy
• Global coverage and a daily observation accessibility to any point on Earth
• Service provision of so-called “level-2 products” to customers consisting of a panchromatic image with a merged multispectral image orthorectified on a DTM (Digital Terrain Matrix)
• Provision of stereo imagery (up to 350 km x 20 km or 150 km x 40 km) and mosaic imagery of size up to 120 km x 120 km
• More than 250 images/day are expected from each spacecraft of the constellation
• The constellation is required to support risk management support services in terms of observation coverage (this requires an agile S/C design, a responsive operational concept, and a sufficient ground segment). 1) 2) 3) 4) 5) 6) 7)
Note: The name Pleiades refers to an open cluster of stars (about 35 light years in diameter) in the zodiacal constellation Taurus, about 400 light years from our solar system. It contains a large amount of bright nebulous material and several hundred stars, of which six or seven can be seen by the unaided eye and have figured prominently in the myths and literature of many cultures. In Greek mythology the Seven Sisters (Alcyone, Maia, Electra, Merope, Taygete, Celaeno, and Sterope, names now assigned to individual stars), daughters of Atlas and Pleione, were changed into the stars. The heliacal (near dawn) rising of the Pleiades in spring of the Northern Hemisphere has marked from ancient times the opening of seafaring and farming seasons, as the morning setting of the group in autumn signified the seasons' ends. Some South American Indians use the same word for ”Pleiades” and ”year.” The cluster was first examined telescopically by Galilei Galileo, who found more than 40 members.
Figure 1: Artist's rendition of the deployed Pleiades spacecraft (image credit: CNES)
Since 1997 CNES is studying the use of smaller satellites (a medium spacecraft size of about 1000 kg instead of 3 tons as for the SPOT-5 S/C), resulting in the “3S” platform concept (Small Satellite System) standing for “Suite de Systeme SPOT” or for “SPOT Successor System.” The focus is on cost reduction, technological innovation, user services, and performance upgrades for a new generation of optical imaging satellites, referred to as Pleiades. 8) 9)
In the same timeframe, the Italian Space Agency (ASI) performed also studies of a new program under the name of COSMO-SkyMed. Both countries had the same objectives in building a dual system, serving the civilian user community (institutional and commercial) and the defense users in each country. Recognizing the very similar goals of both programs, the two nations decided to cooperate to avoid duplications of civil and defense systems in Europe.
On Jan. 29, 2001, an intergovernmental agreement (memorandum of understanding) was signed during the Turin meeting between the heads of government of Italy (Guiliano Amato) and France (Lionel Jospin). The objective of this bilateral agreement, referred to as ORFEO (Optical and Radar Federated Earth Observation), is the cooperation of France and Italy on a “dual high-resolution Earth observation system,” comprising a two-satellite constellation in the optical region under the leadership of France, and a four-satellite constellation under Italian leadership in the microwave region of the spectrum (initially X-band SAR). The intent of this agreement is to provide a long-term perspective on a number of high-quality data products and services on the commercial market for a wide range of applications in the fields of cartography, agriculture, forestry, hydrology, and geological prospecting. The dual service concept is seen in the data requirements of the defense and civilian communities with an option of a daily revisit capability. The agreement calls for funding and development of the space segment by each country and a common sharing of the ground segment. 10) 11)
Figure 2: Overview of organizations in ORFEO agreement (image credit: CNES)
In this framework, the Italian and French Governments started a cooperation with the goal of Earth observation for dual-use applications (military and civil) with SAR and optical instruments based on the on-going COSMO-SkyMed and Pléiades small satellite programs, respectively. This dual-use scenario calls for missions that offer advanced observation capabilities in several modes of operation, permitting to meet the objectives of the military and civil communities at the same time. The development of innovative and complementary instrumentation in the radar field (e.g., multi-mode and flexible-support SAR's offering high-resolution data) and in the optical field (e. g., hyperspectral sensor with capabilities of variable spatial resolutions as well as high detection sensitivities in the visible and infrared spectral regions) are major objectives of the programs.
The earlier memorandum of understanding was followed by a Memorandum of Agreement (MoA), for the ORFEO system definition step, between the French space agency (CNES) and the Italian space agency (ASI). The MoA was signed on June 22, 2002 at the Paris Air Show.
In 2005, CNES signed also Pleiades cooperation agreements with SNSB (Swedish National Space Board) with INTA (Instituto Nacional de Técnica Aeroespacial) of Spain, with ASA (Austrian Space Agency) of Austria, and with BELSPO (Belgian Science Policy Office) of Belgium. Belgium and France are already partners in the long-term SPOT program. Pleiades is with this action a multi-mission concept and a partnership program.
Thematic commissioning phase: 12)
The thematic part of the ORFEO accompaniment program covers a large range of applications, and aims at specifying and validating products and services required by users. An in-depth work of user needs assessments in eight domains (sea and coastline, risks and humanitarian aid, cartography and urban planning, geophysical hazards, hydrology, forestry, agriculture and defense) has given rise to a large number of feasibility studies from 2006 to 2011. Since 2006, more than 40 studies have been led by scientists and thematic experts from French and Belgium institutions, in close link with public end-users such as Ministry of Internal Affairs, Ministry of Ecology, French national cartographic institute, etc. Such studies, generally based on student internship, have been performed with imagery support provided by CNES both in the optical domain (WorldView-2, Formosat-2, QuickBird, Ikonos, GeoEye-1, Kompsat, aerial images) and in the radar domain (TerraSAR-X, COSMO-SkyMed, aerial images).
The third and last phase of the ORFEO program, the PUTC (Pleiades Users Thematic Commissioning) phase, started in March 2012. This phase is a direct follow-up of ORFEO program objectives and philosophy, aiming at supporting and encouraging institutional use of Pleiades, performing research, R&D and demonstration projects required by institutional actors. Free open source image handling and processing tools set up in the ORFEO methodological part will be provided, if needed, for easier access to data. Such activities are performed in complementary and in synergy with Astrium GEO-Information Services marketing activities, aiming at developing a Pleiades market among commercial users and at setting up certified and qualified commercial services.
The most promising studies since 2006 are being assessed with Pleiades imagery, in the eight domains of interest. Several key issues such as response to crisis, urban planning, human pressure on coastlines, watershed cartography, forest management are thoroughly studied. The parameters taken into account for the selection of the thematic studies were:
• Projects ready for operational applications (e.g. monitoring large gathering/summits, updating databases . .. )
• Projects still in demonstration but very mature (e.g. coastline detection and characterization, scrublands detection for fire prevention. . . )
• Projects of interest for institutional actors whatever their technical maturity is: research, Research & Development,demonstration (e.g. Green and Blue corridors, biodiversity, roof reconstruction . . . )
• Projects for operational demonstration capacity acquisition (e.g. IGN studies: department global coverage, MNT-Lidar assessment. . . ).
Major disasters and emergency events are covered with Pleiades imagery for Civil Protection needs. Since the beginning of 2013, Pleiades is fully operational in the International Charter “space and major disasters” process . The PUTC philosophy remains in the association of institutional users in the related projects. Indeed, studies in close interaction with public sector which answer to the requirements of the Ministry of Ecology specified in the Plan d’Applications Satellitaires, but also to other Ministries Requirements (Internal Affairs, Agriculture).
The importance of a collaborative work between the different thematic working groups has been privileged in order to maximize the efficiency for the key thematic studies (i.e Green and Blue Corridors) and to gather the data acquisition over multi-thematic sites. About 70 geographical sites related to almost 130 thematic studies have been selected in the framework of PUTC. More than 500 Pleiades acquisitions have been requested by users.
From its start in March 2012, the PUTC allowed the acquisition of more than 150 Pleiades images with a total surface of 40,000 km2. About 150,000 km2 are planned to be imaged by Pleiades for PUTC until the end of 2013. A total of 69 geographic sites around the world have been selected; their sizes vary from 100 km2 up to 3500 km2.
In the framework of the PUTC, more than 60 scientists or institutional teams are provided with the Pleiades images. Most of them are French but there are also some Belgian partners. As images are bought under DSP licence, institutional partners from France, Belgium, Spain, Sweden and Austria can access those images for free, thanks to their affiliation to ORFEO program.
The design of the Pleiades spacecraft employs a variation of the AstroSat platform, namely AstroSat-1000 of EADS Astrium SAS. The main design drivers for the S/C architecture were “satellite pointing agility and image location accuracy.” Agility requires a very compact S/C design; hence, the imaging instrument is integrated inside the bus. A high degree of image location accuracy is achieved by minimizing the interface between the imager and the bus. The bus structure is of hexagonal shape, with three solar arrays positioned at 120º at the top of the platform (fixed mounting of solar arrays), and three star trackers in a quasi tetrahedron configuration, optimizing the attitude determination accuracy. The solar array size is minimized by using high-efficiency triple-junction cells. The satellite electrical architecture is organized around a central computer, based on a SPARC ERC 32 computer, communicating via MIL-STD-1553B buses to the onboard equipment. The computing, monitoring and reconfiguration functions are centralized in the OBMU (On Board Management Unit). The IMU (Instrument Management Unit) gathers the instrument interfaces: instrument thermal control hardware, mechanisms command, detection unit power conversion. All the other instruments have their own 1553 interface and directly interface with the OBMU. Two 1553 buses are nominally used: the first one for cyclic tasks, mainly allocated to AOCS and thermal control., the second one dedicated to burst exchanges, mainly payload instruments.
Figure 3: Schematic layout of the Pleiades outer structure (image credit: CNES)
Figure 4: Interior structure of the Pleiades spacecraft and instrument accommodation (image credit: CNES)
The S/C is three-axis stabilized; attitude is sensed by star trackers, SED36 of EADS Sodern, (capable of finding attitude starting from a “lost-in-space” condition) and by fiber-optic gyrometers (FOG)- the FOG device, a product of IXSPACE, is also referred to as AstrixTM. The S/C permits a body-pointing capability with roll and pitch maneuvers, each up to 60º, within a period of 25 s. The solar array design (each panel size of 2.3 m x 1.0 m) introduces LPS (Lightweight Panel Structure) and triple-junction GaAs solar cells (RWE-3G with integral diode, 27% efficiency); these new developments were supported by ESA/ESTEC. The spacecraft solar panels (GaAs cells of 5 m2) provides a power of 1.5 kW EOL of 33.6 V at summer solstice (mean power of 850 W). The Lithium-ion batteries have a capacity of 150 Ah. To ensure a balanced power budget over one day, the satellite points its arrays towards the sun before and after every imaging sequence. The S/C propulsion module employs hydrazine (50 l) and four 15 Nms thrusters for orbit control phases. The total dry mass of the S/C is 940 kg (compatible with Rockot, PSLV, and Soyuz launchers) + 75 kg of hydrazine. Hence, the mass of the new generation Pleiades spacecraft is only 1/3 that of its predecessor SPOT-5 with a mass of 3000 kg. 13) 14)
Attitude and orbit determination: An autonomous orbit determination is performed by a DORIS receiver. DORIS is the CNES tracking system, based on measurements between the satellite and dedicated ground stations at two frequencies (400 MHz and 2 GHz). The receiver raw measurements are filtered inside the receiver by a high order navigator, based mainly on Earth gravity potential modeling, to reach an accuracy of about 1m. The receiver can be cold started in any satellite orientation in less than 1 orbit, which eases the satellite operations. It also provides the onboard time and the PPS (Pulse Per Second) count needed to synchronize the system.
Attitude determination is performed by a gyro-stellar system. Three high-accuracy star trackers (STR) are used for attitude sensing, with separate optical heads. STR measures the direction of 12 stars to estimate a 3 axis attitude at a frequency of 8Hz. Each star tracker operates autonomously, its pointing accuracy is better than 2 arcsec in FOV error, and 10 arcsec (max) in noise which translates to 200 m on the ground. Only the axes perpendicular to the boresight axis are used to improve the accuracy. The ground location accuracy of the imagery is 10 m for a 90% probability ground circular error without GCP (Ground Control Points). Solid state FOG (Fiber Optic Gyrometers), Astrix 200 of EADS Astrium-Xspace, are used to ensure high-accuracy attitude determination while maneuvering. 15)
Figure 5: Optical head configuration of the FOG inertial measurement unit (image credit: CNES)
Actuator system: A newly developed CMG (Control Moment Gyro) technology is being introduced to perform the very demanding requirements of rapid slewing spacecraft maneuvers (i.e. body pointing of the S/C within ±60º). A cluster of four CMG actuators is being used, positioned in a pyramid configuration. The patented actuator system, developed by EADS Astrium-Teldix, is referred to as CMG 15-45S (15 Nms, 45 Nm, standard): it delivers a torque up to 45 Nm with a wheel of 15 Nms (angular momentum), sufficient to point a satellite in the 1000 kg class at more than 3º/s of slew rate within < 2 s; the compact architecture can be used for satellites from 1000 kg down to mini- and microsatellites. For Pleiades program applications, the requirements call for CMG fatigue failure modes in excess of 1.8 x 106 cycles under an average output torque of 19 Nm. 16) 17) 18) 19) 20)
Table 1: Characteristics of the CMG 15-45S (for S/C in the 1000 kg class)
Figure 6: Illustration of the CMG 15-45S actuator (image credit: EADS Astrium SAS, CNES)
Figure 7: Pleiades-HR1 undergoes radiometric testing (image credit: CNES)
Figure 8: Photo of the flight payloads in the integration facility in the Kourou Spaceport (image credit: Arianespace)
Legend to Figure 8: The payloads of the Soyuz launch vehicle include the Pleiades 1A satellite, pictured at top, and four ELISA satellites, two of which are seen at bottom. Chile's SSOT satellite is not visible because it is inside the adapter's central cylinder.
New ASAP-S dispenser structure developed for Soyuz:
Arianespace enhanced the Soyuz mission flexibility with its development of a structure to accommodate small secondary payloads. Called the ASAP-S (Arianespace Structure for Auxiliary Payloads-Soyuz) this system continues the ASAP (Arianespace Structure for Auxiliary Payloads) concept previously developed for missions with members of the Ariane family, which have enabled “piggyback” passengers to be flown for the past 20-plus years. With the availability of ASAP-S on Soyuz, the ASAP launch capability on Ariane vehicles has been retired. 21)
The ASAP-S system for Soyuz has external positions for four microsatellites, along with volume inside the center structure for a fifth payload. The ASAP-S external configuration accommodates spacecraft weighing up to 200 kg, while the internal position is designed to accept a payload with a maximum mass of 400 kg.
The launch of the Pleiades-1A spacecraft was the first launch with the new ASAP-S dispenser structure.
Figure 9: Photo of the ASAP-S dispenser structure (image credit: Arianespace)
Legend to Figure 9: The ASAP-S structure is based on a massive rigid low plate made of alloy, which provides the interface with the launcher and supports the propulsion system. This plate provides the vertical stiffness required by the launcher. The structure is completed by four vertical struts at each angle of the satellite and by four lateral panels made of alloy honeycomb, which concur to the lateral stiffness. The ASAP-S structure has a total mass of 425kg, a total height of 1.841 m, and a diameter of 1.875 m. 22) 23)
Launch: The Pleiades-1A spacecraft was launched on December 17, 2011 on a Soyuz ST launcher (with Fregat upper stage) from Europe's spaceport in Kourou, French Guiana (launch provider: Arianespace). The flight was designated as VS02 (Vehicle Soyuz 02) in the Arianespace launcher family numbering system. The mission involved four burns of the Soyuz' Fregat upper stage, which enabled the six satellites to be released for operations at altitudes ranging from 700 km to 610 km. The Fregat upper stage performed a 3 hour, 26 minute flight to deploy its payload. The Fregat upper stage used a new purpose-built payload dispenser developed for Arianespace's Soyuz missions. Flight deployment sequence (Figure 11): 24) 25)
- The Pleiades-HR1 (primary payload) was deployed first after 55 minutes at an altitude of ~694 km
- The 4 ELISA satellites (secondary payloads) were released at ~ 700 km, 59 minutes after lift-off.
- This was followed by another orbit maneuver after which the SSOT satellite (secondary payload) separated from the Fregat stage, 3 hrs. 26 min. after lift-off at an altitude of 610 km.
Figure 10: Image of the Soyuz launch site ELS (Ensemble de Lancement Soyouz) in Kourou French Guinea which launched the Pleiades-HR1 spacecraft (image credit: ESA)
Figure 11: Flight sequence of the Soyuz launcher from the Guiana Space Centre to orbit the Pleiades, ELISA and SSOT satellites (image credit: Arianespace)
Launch: The Pleiades-1B spacecraft was launched on December 02, 2012 (UTC) on a Soyuz STA/Fregat launcher from Europe's spaceport in Kourou, French Guiana (launch provider: Arianespace). The flight was designated as VS04 (Vehicle Soyuz 04) in the Arianespace launcher family numbering system. 26) 27)
Orbit: Sun-synchronous phased orbits (180º phasing, 14 and 15/26 rev./day), altitude = 694 km, inclination = 98.2º, local equator crossing time on a descending node at 10:30 hours. The nominal repeat cycle is 26 days. With a roll ”pointing” capability of 30º with respect to the track, world access can be achieved in 5 days with one satellite, and in 4 days with the system's 2 satellites. 28)
RF communications: An X-band downlink provides a nominal payload transmission rate of 465 Mbit/s in 3 channels, each of 155 Mbit/s capacity. All source data are compressed prior to on-board (mass memory of 600 Gbit EOL). The CCSDS protocol is used for all S/C communications. An S-band link is being used for the support of all TT&C services.
Figure 12: Artist's conception of the Pleiades spacecraft in orbit (image credit: CNES)
Status of the Pleiades-1A and -1B missions:
• 2014: The Pleiades-1A and -1B missions are operating nominally. 30)
• October 2013: More than 1 ½ years after the first satellite’s launch, the Pleiades system is in very good health and the two satellites are fully operational offering a very good image quality. The system offers a very high acquisition capability (more than 500 images per day and per satellite) and its availability is close to 100%. Regional image receiving stations have been installed (Japan, China and Canada). 31) 32)
The use of both satellites simultaneously in orbit shifted by 180° permits to ensure:
- the daily accessibility to any point of the Earth and a prompt imaging required by Defense and Civil Security missions.
- the high coverage capability required for mapping and land planning needs.
Table 2: Data distrubution concept of the Pleiades program
Each Pleiades satellite is capable of targeting images along any ground direction within 47° of vertical viewing position, with very low maneuvers durations between two consecutive images. This high agility permits (Ref. 31):
- to ensure a good reactivity of the system in order to satisfy the urgent needs
- to minimize scheduling conflicts within the dual use framework
- to acquire image in any direction following, for example, coastlines or river routes as so to optimize programming for instance in a crisis case (flooding, tsunami, etc.)
- to enlarge the swath by taking in the same pass adjacent stripes, mosaic images covering up to 140 km x 105km (6 strips) can be acquired
- to acquire in the same pass stereoscopic pairs or tri-stereoscopic triplets even with low base over height ratio, a very important improvement in order to avoid hidden objects in an urban area (occlusions), see Figures 13 and 14 acquired in 2013.
- To perform calibrations on stars or moon images or using “exotics” guidance methods.
Figure 13: Three pictures on the Mecca ”tower clock” acquired by Pléiades 1B every 90 s in a single pass to see the minutes needle moving ! (image credit: CNES)
Figure 14: Three pictures on the Mecca ”tower clock” acquired by Pléiades 1B every 90 s in a single pass to see the minutes needle moving ! (image credit: CNES)
In addition to stereoscopic acquisitions, during commissioning, Pleiades agility has been pushed to its limits in order to acquire more than 30 images in a single pass to produce a sequence of animated images allowing to highlight human activities on the targeted area. This high acquisition capability is accompanied by a revisit interval of less than 24 hours to meet both civil and military requirements.
The orbit has been selected in order to minimize the revisit time with either one or two satellites in orbit.
Table 3: Revisit time scenario
Pléiades has demonstrated its capability to acquire cloud-free images very quickly (more than 50% of the images have a cloud cover better than 10%). As an example on climatologic difficult areas (Figure 15), 94% of Burundi have been acquired and validated within 4 weeks and 3 segments of 90 km have been acquired cloud-free over Amazonia on a single pass....
• Sept. 19, 2013: The 50,000 ton Italian cruise ship, Costa Concordia, which ran aground on January 13, 2012 due to error made by its Captain was righted after massive salvage efforts by a team of engineers and mariners. This has been the most expensive salvage operation in marine history. 33) 34)
- The very high resolution Pleiades satellite, captured the refloating on 17 September of the Costa Concordia (Figure 17), wrecked off the Island of Giglio, Italy.
Figure 16: Pleiades image acquired on July 12, 2013 showing the wrecked Costa Concordia off the coast of Giglio, Italy. (image credit: CNES Distribution Astrium/SPOT Image)
Figure 17: Re-floating operation of the Costa Concordia. This Pleiades-1A image was captured on September 17, 2013 (image credit: CNES Distribution Astrium/SPOT Image)
• March 2013: Pleiades-1B has completed its technical commissioning and is now commercially available. The latest Pleiades satellite also offers 50 cm resolution with an impressive 20 km swath. Like its twin, orthorectified imagery is standard and several tasking options are available. - The Pleiades twins now operate as a true constellation on the same orbit, allowing a daily revisit capability to guarantee the customer the right information at the right time. 35) 36)
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. 37)
Figure 18: Astrium GEO-Information Services is operating the Pleiades constellation and the SPOT-6&7 constellation (image credit: Astrium)
Figure 19: The Pleiades and SPOT-6/-7 Pleiades satellite constellation configuration (image credit: Astrium GEO-Information Services)
• January 2013: The Pleiades-1A spacecraft is operating nominally in 2013. The Pleiades-1B spacecraft is in the commissioning phase.
• On Dec. 5, 2012, Pleiades-1B acquired its first images. 38)
Figure 20: Pleiades-1B acquired this pansharpened, high-resolution image of Istanbul (Turkey) on Dec. 5, 2012 (image credit: Astrium Services)
• December 02.2012: CNES is reporting that operations on the Pleiades-1B are nominal; the solar panels are deployed, the attitude is stabilized, the thermal and power balance is reached. 39)
• In August 2012, Pleiades-1A has already acquired more than 10,000 images, the system is very robust and the availability of the satellite is excellent. The success of the Pleiades mission is now in the hands of operational teams that will focus their energies to satisfy the users needs and keep Pleiades in good health throughout the years to come (Ref. 7).
The assessment of the image quality and the calibration operation have been performed by the CNES Image Quality team during the 6 month commissioning phase that followed the satellite launch. These activities cover many topics gathered in two families : radiometric and geometric image quality. The new capabilities offered by PLEIADES-HR agility allowed to imagine new methods of image calibration and performance assessment. 42)
The radiometric activities dealt with the following main topics:
- MTF assessment and refocusing operations
- Absolute calibration
- Inter-detector normalization
- Signal-to-noise ratio assessment
- Onboard compression bit rate optimization
- Clouds auto-detection
The geometric activities evaluated were:
- Localization performance
- Focal plane cartography
- Line-of-sight dynamic stability
- 3D rendering capabilities (Ref. 42).
• In early June 2012, all the calibrations were performed; the precise measurement of the performances are still ongoing satisfactorily: the end of the Image Quality Commissioning Phase is scheduled for the end of June 2012. 43)
- The launch of the Pleiades-1B spacecraft is planned for the end of 2012 (Ref. 43).
Figure 21: Pleiades-1A image of Melbourne acquired during commissioning (image credit: CNES, Ref. 7)
- In return for financing 90% of the Pleiades program, the French government has reserved access to specific percentages of Pleiades’ output of 900 images per day once both satellites are in their 694 km orbit, spaced 180º apart.
- The French Defense Ministry has priority access to 50 images per day from the two-satellite Pleiades constellation.
- Civil agencies of the French government have access to 40% of the remaining Pleiades output, with Astrium Geo-Information Services claiming 60%.
• The commissioning phase ended on March 2, 2012. The main image quality performances had been assessed and appeared to be very good; even if some of them were not considered to be accurately measured the Pleiades-1A system was declared fully operational (Ref. 43). 45)
• The Pleiades-1A spacecraft is in the commissioning phase in early 2012 (the commissioning phase is led by CNES).
Figure 22: Image of Bora Bora Island in Polynesia observed by Pleiades-1A on January 3, 2012 (image credit: CNES) 46)
• The Pleiades-1A satellite imager captured the first panchromatic satellite images 3 days after its successful launch from the Kourou launch site (French Guiana) via a Russian Soyuz ST rocket on December 16, 2011. 47)
Pleiades-1A represents the first very high-resolution satellite of the SPOT family and will be capable of providing orthorectified color data at 0.7 m resolution and revisiting any point on Earth as it covers a total of 1 million km2 daily. Perhaps most importantly, Pleiades-1A will be capable of acquiring high-resolution stereo imagery in just one pass, accommodating large areas (up to 1,000 km x 1,000 km).
Sensor complement: (HiRI)
HiRI (High-Resolution Imager):
HiRI of CNES with Thales Alenia Space (TAS-F) as the prime contractor for this instrument (formerly Alcatel Alenia Space). The objective is to provide high-resolution multispectral imagery with high geo-location accuracy. The camera design employs a pushbroom imaging concept. Extensive use of existing state-of-the-art technologies is made regarding such items as: a) camera alignment procedures, b) telescope thermal control and mechanical assembly principles, c) video processing techniques. 49) 50) 51) 52) 53)
The industrial team consists of the following partners:
• Thales Alenia Space, France: Instrument, telescope, detection unit, telescope structure & thermal control, video electronics, video power supply, harness
• Thales Alenia Space España: Instrument service module
• Sener: Shutter, detection unit structure & thermal control
• E2V (Chelmsford, UK): Pan & MS CCD imaging detectors
• SESO (Société Européenne de Systèmes Optiques), France: telescope mirror manufacturing
• EADS Sodern: FPA (Focal Plane Assembly)
• Sagem: Spectral filters.
Figure 24: Illustration of the HiRI instrument (image credit: CNES)
Table 5: Introduction of technologies into camera design
Figure 25: Optical configuration of HiRI (image credit: TAS)
Optical assembly: The instrument employs a Korsch all-reflective 4 mirrors telescope design with TMA (Three Mirror Anastigmatic) optics. An additional plane mirror (MR) is used to enhance the instrument compactness. The imaging geometry optimization features a primary mirror size of 650 mm diameter, which suits well to the detectors performance and the orbit characteristics. The instrument architecture chosen is organized around a central plane structure supporting the primary mirror, the tertiary mirror, the plane mirror (MR), and a central cylinder that supports the secondary mirror. The optical assembly consists of an on-axis part (M1 + M2 collector mirrors) and an off-axis part (M3 + MR mirrors) feeding the different focal planes. 54) 55) 56)
Figure 26: Illustration of the FPA (image credit: (TAS)
The optics system of the instrument uses state-of-the-art techniques such as SiC-100 (silicon carbide) material for the mirrors and the telescope structure, specific detectors, and a modular video chain design. EADS Sodern is responsible for the development of the FPA. The FPA is offering a wide variety of new technologies for the imaging function. The size of the observed image is close to 400 mm and is analyzed in 30,000 samples in Pan and 7,500 in MS.. The focal plane assembly consists of two symmetrical arrangements of Pan and MS detectors. The beam splitter is made of a set of mirrors (Figure 26).
The spectral selection is made by optical filters placed very close in front of the detectors. Pan filters and MS stripe line filters are space-qualified multi-layer coatings deposited on glass substrates. Each filter is composed of a high-pass filter and a low-pass filter. An absorbing material deposited between the MS filters isolates each band from the others to avoid interband straylight.
Figure 27: Exploded view of the HiRI camera (image credit: TAS, CNES)
Attitude sensors (star tracker heads and gyroscope heads) are placed on this central instrument structure to improve the performance. A dedicated supporting truss structure ensures the instrument interface with respect to the bus. The detector thermal radiator has its own supporting structure. The instrument design employs carbon material for the structure (the carbide characteristics are: very low coefficient of thermal expansion, very low density, resulting in a light telescope, and a simple thermal control) and Zerodur material for the mirrors.
The instrument focus mechanism is placed onto the tertiary mirror. This position offers an optimum between mechanism and accuracy. The instrument includes also an internal shutter to protect it from sun radiation in non-operational phases such as launch, attitude acquisition, or safe modes. The shutter is placed behind the primary mirror to protect only the tertiary mirror and the detection cavity.
Table 6: Characteristics of the HiRI instrument
The Pan band of the detector array makes use of back-thinned TDI (Time Delay Integration) with 5 selectable levels from 7 to 20. The size of the detector array assembly is 5 x 6000 (30,000 pixels or samples in cross-track) with a pixel size of 13 µm. The Pan band employs PhotoMOS-type CCDs [note: the PhotoMOS relays have LED inputs and MOSFET outputs that provide input-to-output isolation - comparing favorable with the isolation obtained from EMRs (Electromechanical Relay)]. The detector array is clocked continuously to give a time-delay-and-integrate (TDI) function. The transfer of charge along the CCD is made synchronous with the velocity of the scan image. The integration time is m times longer than a single-detector integration time, where m is the number of stages in a row.
MS bands: Five detector line arrays, each of 1,500 elements (7,500 pixels in cross-track), are utilized for the four MS bands (line arrays). The spacing between the centers of two consecutive lines is 936 µm. Each line array has a pixel pitch of 52 µm.
Each TDI detector outputs data at 58 Mpixel/s (or 290 Mpixel/s total), which amounts to a data rate of about 700 Mbit/s per detector (or 3.5 Gbit/s total in worst case). The MS detectors output at the same frequency their video data. Those video signals are converted in numerical data with individual chains at about 7 Mpixel/s, for maximum SNR performance. The focal plane is physically coupled with the detection electronics to form the integrated detection unit. The overall unit realizes the functions of detection and conversion of video signals in numerical data. The highly integrated design allows a simplification of the data transmission between the focal plane and the detection electronics.
Detector electronics: The camera system design employs the newly developed SEDHI (Highly Integrated Detection Electronics Subsystem) architecture to support such functions as: a) integrated video processing in the focal plane, b) provision of a high-speed data link, and c) digital onboard processing. 57) 58)
The modular SEDHI concept is implemented on a single board and provides all electronic functions associated to 1 or 2 detectors, from video signal pre-amplification to the digitization of video frame - including various functions such as video signal processing, detector low noise power supply and polarization generation, detector and video processing chain sequencer, detector clock drivers, synchronization interface, command and control interfaces.
The SEDHI concept uses a number of MVPs (Module Video de Proximité) to organize the detection function in the FPA (Focal Plane Array). One MVP is able to process up to 10 CCD outputs, at a maximum pixel rate of 10 Mpixel/s. The high output data rate of a MVP (up to 1.2 Gbit/s) is transmitted downstream through 2 LNTHD (Liaison Numérique Très Haut Débit), which are serial integrated digital interfaces. Two types of hybrid ASICs were developed, a video processing hybrid, and a CCD phase driver hybrid, in support of all processing functions required.
Figure 28: Overview of elements in the integrated detection unit (image credit: TAS)
Figure 29: Illustration of the focal plane assembly (image credit: CNES)
Figure 30: Views of the Pleiades FPA (top and bottom faces), image credit TAS
The full detection electronics consist in 5 panchromatic MVPs, 3 multispectral MVPs, 3 panchromatic MSP (embedded in a single mechanical module) and 1 multispectral MSP. The Pléiades instrument electronics include the MVP functions, the MSP (Module de Servitude de Proximité) and the MSI (Module de Servitude Instrument) dedicated to command / control. The PAN detection electronics is divided in 3 blocks:
• 1 block which includes 2 MVPs and 1 MSP
• 1 block which includes 1 MVP (associated to the panchromatic detector in the center of the focal plane) and 1 MSP
• 1 block which includes 2 MVPs and 1 MSP
The modular SEDHI concept has the potential to support the new generation of Earth observation optical systems (large swath width, reduced ground sampling distance, number of spectral bands, etc.), the modularity, the compatibility - for a large number of missions. This “photon in - bit out” concept may be summarized by: radiometric performance optimization at lowest costs.
Table 7: Overview of the SEDHI detection electronics
Payload data handling: The MS (Multispectral) video data are output from the instrument, at about 4.5 Gbit/s total output rate, and are compressed in the Payload Data Compression Unit. A wavelet algorithm is used, that enables the compression ratio to go up to 7, while in standard operation the ratio is 4.8. Image data are time-tagged in this unit. The compressed data are stored in SSR (Solid State Recorder) with a capacity of 600 Gbit (EOL). The compression module and the SSR are physically in one equipment referred to as COME (COmpresseur MEmoire). The COME output data rate is nominally 465 Mbit/s on three channels at 155 Mbit/s each. The data cyphering, civil and military, is performed in another equipment, namely the DCU (Deciphering and Ciphering Unit). The data are packetized according to CCSDS standards.
Figure 31: Functional block diagram of the HiRI instrument (image credit: TAS)
The data are coded following a trellis-coding scheme in 8-PSK-type modulators that include their own SSPA (Solid State Power Amplifiers). They are then multiplexed, and downlinked with an X-band antenna. The X-band antenna employs a corrugated horn (omni-directional) with a 64º aperture. It is mounted on a 2-axis gimbal mechanism to ensure image downlink during imaging and maneuvering sequences (gimballed gyroscopic actuators give the S/C a high degree of agility). This mechanism is not used during imaging periods to minimize any dynamic disturbances. A dedicated antenna pointing algorithm is developed to orient the antenna during satellite maneuvers so that the ground station always stays in the antenna lobe. In this way, the complete ground station visibility is used to downlink imagery.
Pleiades data acquisition modes and performance capability:
The agile Pleiades satellites are capable to image targets along any ground direction within 47º of a vertical viewing position (with very low maneuver durations between two consecutive imaging scenes. Several types of acquisition modes are defined:
• One-pass acquisition mode: The satellite agility offers the possibility to locally enlarge (widen) the swath by realizing from the same orbit and thus quasi simultaneously the acquisition of several adjacent swaths within the field of regard (FOR). Hence, it should be possible to acquire imagery with an area of approximately 120 km long, and 110 km wide, using the S/C pointing capability (maximal) in roll of 30º.
• Simultaneous stereoscopic or tri-stereoscopic acquisitions mode: This acquisition mode employs a variation of the one-pass mode. Instead of using the agility capabilities to acquire contiguous imagery, the same scene is acquired with two (or three) different view angles as shown in Figure 32.
• Multi-spot mode: Refers to a support mode in which the S/C acquires many spot targets around the S/C ground track, even fore and aft stereoscopic mode if needed. In this mode, the agility capabilities of the satellite are used to acquire the maximum of scenes on a given surface, in one day.
The basic data products of Pleiades are level 0 and level 1 imagery (up to orthorectified imagery). Intermediate products (level-2) are: Mosaicked imagery, DTM (Digital Terrain Model) extracted from stereo pairs.
The main application fields identified for the Pleiades program data fall into the following categories: Cartography, agriculture, forestry, hydrology, geological prospecting, dynamic geology and risk management.
Pleiades is a multi applications, multi sensor and multi partnership program. The first generation will only be furnished with an optical high-resolution imager. Wide-field, superspectral, hyperspectral and thermal (TIR) system observation capabilities are options for future Pleiades implementations (with launches well beyond 2009).
Table 8: Optical system identification for possible user needs
The Pleiades ground segment is composed of a set of user centers located in France, Sweden, Spain and, optionally, in Italy, all of them being interfaced with a dual center in France. Each user center is able to manage the user image acquisition requests and product generations, they include: 60) 61) 62) 63) 64)
- An X-band antenna for data acquisition, 3-channel signal demodulation and ingestion system
- An image processing unit to inventorize, catalog, and archive the data, and to produce imaging products
- A programming unit to manage the mission planning requests
- A set of access units permitting the users to browse the image catalog, submit requests and to receive the ordered products.
1) Astrium Geo- Information Services (formerly SPOT Image) serves the civilian clients.
• Users can consult the civilian images catalog, submit requests and receive the ordered products
• All civilian requests are centralized by Astrium Geo- Information Services
• Astrium Geo- Information Services insures the civilian antenna management as well as the reception and the archiving of raw data received from satellites
• Astrium Geo- Information Services also manages a network of regional image receiving stations (RIRS). The Kiruna station enables to daily receive the Pleiades images, in complement of the Toulouse station; it is managed as a RIRS.
2) The French and Spanish Defence Mission Centers serve the defence users of each country. The basic functions of the Defence Mission Centers are similar to those of the civilian ground segment with some differences in the interfaces, and the security.
3) CNES hosts the dual control center (civil and military). The control center manages the following functions and service requests:
• Monitoring and control of the Pleiades spacecraft
• Hosting of the defence coordination function in support of an optimized work plan to the different partners, taking into account the priorities of each national work plan
• It realizes the planning of all the programmatic requests, by collecting and analyzing all the requests of the civilian and defence users
• It implements and oversees the periodic calibration function of the imaging instrument, for the benefit of all the users.
A very important operational criteria is the devivery of the data products for the commercial customers in record time. This speed represents a key element of the space and ground segment components, built to comply to the user requirements.
Figure 33: Architecture of the Pleiades ground segment (image credit: CNES)
The ground segments were entrusted to CapGemini and CS-SI (programming system), Thales services (image system) and a consortium involving EADS Astrium, Cap Gemini, CS-SI and Thales services (integration of user centers), while INDRA has been given responsibility for the Spanish Defense mission centre. The receiving stations have been developed by Zodiac.
Figure 34: Overview of the major ground segment partners of the Pleiades program (image credit: CNES)
Figure 35: Alternate view of the Pleiades ground system architecture (image credit: CNES)
HMA (Heterogeneous Mission Access) implementation:
CNES is in the process to implement the HMA data services, a European initiative by the GSCB (Ground Segment Coordination Body), to the following projects of CNES: 65)
• Catalog Service: SPOT & Pleiades
• Ordering Service: Pleiades, SPOT (in planning)
• Programming Service: Pleiades prototype based, SPOT (in planning)
• Online Data Access Service (future): SPOT & Pleiades
The overall objective of HMA is to establish a harmonized access to heterogeneous EO (Earth Observation) missions’ data from multiple mission ground segments, including national missions and the GMES/Sentinel missions. HMA's goal is to standardize the ground segment interfaces of the satellite missions to enable easier access to EO data. HMA is managed by GMES (Global Monitoring for Environment and Security) - the European program for the implementation of information services to support decisions concerning environment and security.
Figure 36: Schematic view of HMA implementation at CNES (image credit: CNES)
The long-term HMA goal are rapid and reliable access to time-sensitive information is a significant strategic resource for any community. The EO interoperability benefits are linked to the following high-level objectives:
- Manage and reduce technical risks in EO systems and operations
- Manage and reduce cost of EO systems and operations
- Establish the baseline for the development of the European Space infrastructure capable of harmonizing and exploiting relevant national initiatives and assets
- Allow interoperability within and across organizations
- Increase competitiveness of European Space (and downstream) industries
- Maintain the leadership in EO systems and operations, and avoid the emergence of undesirable standards
- Ensure that technology drivers for the European Guaranteed Access to Space are lead by European requirements.
The HMA project focuses on defining and providing five interfaces:
1) Catalog Service: browsing and retrieving metadata on collections and products across collaborating HMA catalogs
2) Ordering Service: for products identified in a catalog
3) Programming Service: submitting requests for new acquisitions to the HMA partner missions’ ground segments
4) Mission Planning Service: facilitating preparation of programming requests
5) Online Data Access Services: retrieving products from the online access archives offered through the HMA.
The GMES program, an EU and EC initiative, with its multiple EO missions and data handling concepts by the various national entities within this program, implied no other way but to start the HMA initiative, a coordination effort of standard interfaces for the benefit of all parties involved (ESA, CNES, DLR, EUMETSAT, ASI, INTA, CSA, DMCii, etc.) including TPM (Third Party Missions) and of course the data user community. 66) 67) 68) 69)
• About 15 European Entities (+ their partners)
• More than 40 EO satellites
• More than 60 instruments — and related ground segments, operations, infrastructure
• Plus around 20 data policies.
Calibration methods of the Pleiades program:
For an earth observing satellite with no on-board calibration source like Pleiades, the commissioning phase is a critical quest of well-characterized earth landscapes and ground patterns that have to be imaged by the camera in order to compute or fit the parameters of the viewing models. It may take a long time to get the required scenes with no cloud, whilst atmospheric corrections need simultaneous measurements that are not always possible. 70) 71)
Like other Earth observing space systems, Pleiades has been designed to meet its performance requirements throughout its lifetime. The performance compliance depends on a good knowledge of all parts of the system, from low-level equipment to global chains. This engineering knowledge is used to define useful physical models dedicated to image quality:
• The geometric model is used to compute the location on the Earth of any pixel. It takes into account a geometric modelization of the satellite: orbit determination, attitude estimation, datation, viewing frame determination, focal length adjustment, viewing directions of all detectors, and a ground reference frame.
• The radiometric model is used to convert raw digital counts to TOA (Top-of-Atmosphere) radiance and to normalize the inter-detector sensitivities. It is applied to the pushbroom acquisition principle, since each elementary detector has its own sensitivity to input radiance and its own dark current.
• The resolution model is used to master how details are filtered by the imaging chain, and involves a convolution model by the point-spread function. This model can be handled easily in the Fourier domain where the MTF (Modulation Transfer Function) plays a major role.
Some contributors cannot be modelled and must be minimized by design, like sensor noises. But many parameters can be modelled and calibrated, either during on-ground validation tests or during in-flight operations.
Before launch, the satellite manufacturer runs many tests in order to check the behavior of all the satellite components, and to demonstrate that the requirements are fully compliant. For example, the geometric model is built step by step by combining the different reference frames of the attitude control loop – including sensors and actuators - and the instrument line of sight model. In spite of very accurate measurement instruments like theodolites, this kind of model cannot reach the level of accuracy that is needed for the image location performance where one PAN pixel defines an angle of 1 µrad . That is why a frame correction is scheduled at system or satellite level, in order to compensate for all the uncertainties due to ground conditions or to launch.
Even if a ground sequence test can be considered accurate enough or very representative, the instrument behavior may change after the launch and throughout its lifetime on orbit: on-ground calibration has to be considered as the first characterization before launch. For example, detectors non-uniformity can be measured in front of a integrating sphere during on-ground tests, and the camera best focus can be determined using a collimator on an optical facility, but both of these parameters are expected to vary slightly in orbit and therefore must be updated after launch.
Ground tests are also mandatory to check the right signs of all the signed data : for example, the right sign of the TDI line transfer is compared to the expected velocity of the scene due to the satellite motion.
During the satellite development, the useful parameters of the three image quality models are identified and at least one dedicated calibration procedure has to be defined and validated before launch thanks to simulated images. A lot of experience has been gathered by CNES in the Earth observing systems domain and many calibration procedures rely on this basic principle: if one gets an image of a well-known reference pattern, then one is able to identify the unknown parameters of the model, which links the input scene to the output image.
In radiometric image quality, in-flight normalization and signal-to-noise ratio are classically performed thanks to uniform expanses of snow-covered scenes, absolute calibration is computed on reference photometric sites with a simultaneous atmosphere characterization, the MTF is measured on knife-edge ground patterns.
The geometric model parameters are estimated in two steps. First, the image location biases are tuned thanks to location sites on which many GCPs (Ground Control Points) are available. As these landmarks give the true location of the line of sight, a block adjustment of several images is performed to determine the alignment bias that minimize the location error. Then, the viewing directions of each detector are computed with several methods using inter-images correlation, either relatively between images given by different retinas, or absolutely between an image and a ground orthorectified reference equivalent to thousands of GCPs.
All these calibration methods are very demanding in dedicated images of specific sites, because they may need a lot of images to make an accurate least-square estimation possible, and because it is not easy to get cloud-free images over these sites. The project team can manage the resource sharing during the commissioning phase but the system unavailability during the operational mission must be strictly minimized. Looking for alternate methods that need less usage of ground references is therefore an issue of importance.
Looking from different points of view:
To reduce the in-flight operations, the so-called AMETHIST method has been processed for the first time to be able to compute the parameters of the radiometric model in case of non-linear behavior. An efficient way to bypass the quest of uniformity is to use the satellite agility in order to align the ground projection of the scan-line on the ground velocity. This weird viewing principle (Figure 37) allows all the detectors to view the same landscape. After a pre-processing, that globally shifts each column of the raw image, an image is obtained that contains all needed information. This means that every row contains the set of detector responses to the same landscape. Thus, non-linear normalization coefficients can be computed by a histogram matching method.
Figure 37: AMETHIST acquisition principle (image credit: CNES)
The Pleiades spacecraft is not only capable to be controlled at a specific 3D attitude, but also with a tunable rotation rate. This ability can be used to stop the scanline projection on the ground during the satellite overpass, so that each detector sees the same landscape during the image capture: this is called the steady-mode (Figure 38). This kind of image can be used to compute the SNR, and, combined with a single reference image in the same overpass, can deliver geometrical stability information.
Figure 38: Schematic of the steady-mode guidance principle (image credit: CNES)
Several AMETHIST calibration campaigns were performed during the commissioning phase, over a large collection of landscapes that mix Earth, ocean surfaces and clouds in order to get a large set of pixels at different radiance levels.
Figure 39: Example of AMETHIST PAN and XS images (image credit: CNES)
The AMETHIST calibration method gives the same magnitude of residuals as the direct method that compute normalization parameters on uniform snowy expanses, with far less operational constraints. Moreover, it allows a fine monitoring of the in-flight performance that can be compared to pre-flight expectations: up to now, the normalization monitoring confirms the need for an update of the parameters every 6 months, as expected on-ground.
In addition to images of well-characterized landscapes that have been used for years and continuously improved thanks to previous satellites commissioning, the Pleiades project has been taken many images of the Moon since the launch. In a nominal calibration mode, the moon is observed once a month during the descending phase in order to follow the stability of the absolute calibration of the different spectral bands.
The Pleiades project has also regularly acquired imagery of stars since the launch. It is planned to follow the temporal evolution of the absolute calibration of the instrument with these well-known and stable sources without any access conflict of the mission.
Thanks to its total lack of atmosphere and the perfect stability of its surface and thus optical properties, the Moon constitutes an ideal calibration site for Earth observation missions for which the radiometric quality of the data is crucial. In the framework of the Pleiades- 1A and Pleiades-1B in-flight calibration, studies took place in order to determine the calibration precision that could be reached from the acquisitions realized during the different lunar cycles. The POLO data set (Pleiades Orbital Lunar Observations) was born: over 1000 images of the Moons acquired over 6 months for viewing angles varying from -115º to +115º (0º corresponding to the full Moon, 180º, to the new Moon). 72)
The commissioning phase of Pleiades-1A and Pleiades-1B represented the opportunity: 73)
• To acquire a unique dataset of Moon images with a very high spatial resolution (~300 m)
• More than 800 images of the Moon were acquired in only 6 months (guaranty of the stability of the instrument over this time slot)
• An intensive analysis was performed to determine the sensitivity of the method to the different parameters: precision better than 0.5%.
The short term objective is now to use this POLO data set to improve the ROLO (Robotic Lunar Observatory) lunar model, developed by USGS (United States Geological Survey) which is internationally accepted and used by every space agency for the calibration of satellite which are able to aim at the Moon. This new model should then be available to the international community through the CNES participation to the GSICS (Global Space-based Inter-Calibration System).
SNR (Signal-to Noise Ratio):
The radiometric noise behavior has been modelled for years: its variance is equal to an offset plus a linear function of the input radiance. The offset represents the variance of the noise in darkness and can be directly computed on darkness images. The linear part can be computed by different calibration techniques. One of them is the steady-mode, that provides a full set of couples (i.e., radiance mean, radiance standard deviation).
Figure 40: Variance of the radiometric noise computed on steady-mode image (image credit: CNES)
MTF (Modulation Transfer Function):
Three days after the launch, the Pleiades satellite MTF was computed thanks to images of stars in the Pleiades constellation, as a sign of destiny. MTF is the Fourier Transform of the point-spread function, but the actual images of stars undergo aliasing effects because of the sampling and the high level of MTF, especially in the XS bands. Therefore, a preprocessing is applied on a large number of stars to interlace each elementary response to produce a well sampled one. This method delivers a very accurate MTF in two dimensions, whereas previous methods were able to measure the MTF only along one axis.
In conclusion, the Pleiades technological breakthrough has given the opportunity to imagine new methods in a way that serve not only the calibration accuracy, but also the relaxation of operational constraints. These new calibration methods will reduce many time-consuming activities because they often provide a closer access to the final performance without external data. The Pleiades commissioning phase has been an unforgettable experience in engineering enthusiasm; hence, many new ideas and concepts turned into reality. It is time now to keep on looking ahead and up above in order to face the next challenge of high resolution optics — the design and the calibration process of the next generation cameras with embedded active optics control loops (Ref. 70).
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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.