ERS-1 (European Remote-Sensing Satellite-1)
ERS is the first ESA program in Earth observation with the overall objectives to provide environmental monitoring in particular in the microwave spectrum (i.e., regular monitoring of land-surface and ocean-surface processes for change detection). Coverage of a broad range of disciplines and topics: Observation of oceans, polar ice, land ecology, geology, forestry, wave phenomena, bathymetry (water depth), atmospheric physics, meteorology, etc. Scientific research: PIPOR (Program for International Polar Ocean Research); PISP (Polar Ice Sheet Proposal). Demonstration of concept and technology for space and ground segments (performance and operational capability). 1) 2) 3) 4) 5) 6)
Figure 1: Illustration of the deployed ERS-1 spacecraft (image credit: ESA)
The ERS program had a pioneering character in the field of instrument development (active sensors), and the introduction and demonstration of advanced observation technologies - a European success story. The ERS preparatory and development program was initiated at ESA in 1981, the C/D phase started in Dec. 1984.
The ERS-1 spacecraft is a three-axis-stabilized Earth-pointing satellite [zero momentum bias with control to 0.11º (pitch/roll) and 0.21º (yaw)]. The platform is a SPOT program bus (SPOT MK1 bus), modified to meet the needs of the ERS missions. The S/C was built by a consortium with DSS (Dornier Satelliten Systeme GmbH) of DASA (DaimlerChrysler Aerospace AG), Friedrichshafen, Germany as prime contractor (since 2000, EADS Astrium GmbH). The payload support module has dimensions of 2 m x 2 m x 3 m (height).
Attitude is measured by a number of sensors (horizon sensor, narrow-field sun sensors, gyroscopes, wide-field sun sensors). Attitude control by a set of momentum wheels. The payload module consists of PEM (Payload Electronics Module) and ASS (Antenna Support Structure). Thermal control is a passive system complemented by an active heater.
Satellite mass = 2384 kg. The solar array (11.7 m x 2.4 m in size) consists of two 5.8 m x 2.4 m panels and supports peak payload power of 2600 W. The antenna is a slotted-waveguide array made of metallized CFRP (Carbon Fiber Reinforced Plastic). The battery storage capacity is 2650 Wh. The mission requirements call for an operational period of 2 years with a possible extension of a 3rd year (but reduced mission).
The ERS-1 orbit was maintained with a number of monopropellant-type thrusters, aligned about the spacecraft's three primary axes, in which hydrazine dissociated exothermically as it is passed over a hot-platinum catalyst. The thrusters were used in different combinations to maintain and modify the satellite's orbit and to adjust its attitude during non-nominal operations. ERS-1 carried 300 kg of hydrazine for orbit maintenance.
Table 1: Overview of some ERS-1 spacecraft parameters
Figure 2: Illustration of the ERS payload support structure (image credit: ESA)
Figure 3: Photo of the front and rear side of the solar array (the rear view shows the pantograph deployment mechanism), image credit: ESA
Figure 4: Photo of the ERS-1 spacecraft in launch configuration (image credit: ESA)
Figure 5: Block diagram of the IDHT (Instrument Data Handling and Transmission) for X-band (image credit: ESA)
Launch: The launch of the ERS-1 spacecraft took place on July 17, 1991 on an Ariane-4 vehicle from Kourou, French Guiana.
Orbit: Sun-synchronous near-circular polar orbit, 98.52º inclination, altitude of 782 -785 km period of about 100 minutes (14.3 orbits/day). The following orbit coverage cycles are defined:
1) Reference orbit - 3 day repeat cycle (high repetition change monitoring with dedicated calibration sites; this orbit was used during the commissioning phase)
2) Ice-Orbit - similar 3 day repeat cycle with a slightly different longitudinal phase. The main limitations of a 3 day cycle are the restricted global coverage for the imaging SAR and the wide separation of the RA-1 tracks.
3) Mapping-Orbit - 35 days repeat cycle (guaranteeing full Earth coverage). This enables SAR imaging of every part of the Earth's surface, with at least twice the frequency of the coverage at middle and high latitudes.
Mission status: The ERS-1 mission ended on March 10, 2000 by a failure of the onboard attitude control system. ERS-1 was lost when a failed gyro prevented the S/C from maneuvering into an emergency acquisition mode with its solar panels pointed toward the sun to keep the batteries charged.
The following list provides some program highlights:
• High-quality observations of all instruments. The availability of C-band SAR imagery in particular opened a wealth of new applications in many fields of the Earth system. In 1993, ERS-1 differential SAR interferometry demonstrated a range precision of 1 cm; this feature provided the ability to detect small changes on Earth's surface: detection of landslides; evolution of volcanic eruptions; detection of surface movement caused by earthquakes; horizontal displacement along active faults. This in turn triggered new projects which enabled precise land motion monitoring on an operational basis based on long-term time series (1992-2003): subsidence, land slides, seismic risk.
• The most exiting results of the ERS-1 mission have been in the field of SAR interferometry, where for the first time precise topographic information could be produced in a tandem mission with ERS-2 in the time period July 1995 to July 1996. DEMs (Digital Elevation Models) with a 10 m vertical precision could be generated. Also, a strong complementarity of microwave and optical observations was recognized during the tandem mission.
• Fundamental discoveries about the oceans and atmosphere: Global wind and wave fields at high spatial and temporal resolution; global ocean dynamics and climatic instabilities; identification of previously unidentified physical ocean features; sea-surface manifestations of atmospheric phenomena.
Sensor complement: (AMI, RA-1, ATSR-1, LRR, PRARE)
The total payload mass is 888.2 kg. The payload instruments are: AMI, RA-1, ATSR (consisting of MWR and IRR), LRR, and PRARE. 7)
AMI (Active Microwave Instrument):
AMI is a SAR (Synthetic Aperture Radar) instrument, built by MMS (Matra Marconi Space), France. Two separate radars are incorporated within the AMI, a SAR for `Image and Wave mode' operation, and a scatterometer (SCAT) for `Wind mode' operation. This instrument can operate in either one of the following modes: 8)
1) AMI in Imaging mode. Measurement in C-band (frequency = 5.3 GHz (equivalent to 5.66 cm wavelength), bandwidth = 15.55 MHz; polarization = Linear Vertical (LV); PRF range = 1640-1720 Hz in 2 Hz steps; long pulse = 37.12 µs; compressed pulse = 64 ns; peak power = 4.8 kW; antenna size =10 m x 1 m; look angle = 23º; radiometric resolution = 5 Bit on raw data (SAR mode), which corresponds to about 30 m spatial resolution; swath width = 100 km. Data rate = 105 Mbit/s.
2) AMI in Wave Mode. Measurement of the changes in radar reflectivity of the sea surface due to surface waves. Provision of images (5 km x 5 km), also referred to as "imagettes," at regular intervals of 200 km along track. These imagettes are transformed into spectra providing information about the lengths and directions of the ocean wave systems. Characteristics: frequency = 5.3 GHz, polarization = Linear Vertical (LV); incidence (look) angle = 23º; wave direction: 0 - 180º.; wavelength = 100-1000 m; direction accuracy = ±20º; length accuracy =±25%; spatial sampling: 5 km x 5 km every 200-300 km; resolution = 30 m; data rate = 370 kbit/s; duty cycle of 70 %.
3) AMI in Wind Scatterometer Mode (AMI-SCAT). Use of three separate sideways-looking antennas (fore, mid and aft beams, see Figure 8) to measure sea surface wind speed and direction. Characteristics: wind direction range = 0 - 360º; accuracy = ±20º; wind speed range = 4-24 m/s; accuracy = 2m/s or 10%, spatial resolution = 50 km; grid spacing = 25 km; swath width = 500 km (same side as SAR imaging); swath stand-off = 200 km to side of orbital track; frequency = 5.3 GHz ±200 kHz; polarization = LV; peak power = 4.8 kW; incidence angle range = 16-42º (mid), 22-50º (fore), 22-50º (aft); antenna length = 2.3 m (mid), 3.6 m(fore), 3.6 m (aft); data rate = 500 kbit/s. Operation over all oceans. Note: AMI-SCAT cannot be operated in parallel with the AMI SAR imaging mode; however, parallel operation of the wind and waves modes is possible. 9)
Figure 6: Functional block diagram of the AMI instrument (image credit: ESA)
The three antenna beams continuously illuminate a swath of 500 km each measuring the radar backscatter from the sea surface for overlapping 50 km resolution cells using 25 km grid spacing. The result is three independent backscatter measurements relating to cell center nodes on a 25 km grid (three different viewing directions, separated by a very small time delay). This permits surface wind vector determination using `triplets' within the mathematical model. AMI is an instrument providing data for a wide range of research disciplines such as: climatology, oceanography, glaciology, land processes, operational meteorology.
Figure 7: Schematic view of AMI SAR image mode geometry (image credit: ESA)
Figure 8: AMI wind scatterometer observation geometries (image credit: ESA)
Figure 9: Schematic view of the AMI wave mode geometry (image credit: ESA)
Table 2: Overview of some AMI observation parameters
Table 3: AMI instrument characteristics
RA-1 (Radar Altimeter-1):
RA-1 is operating in Ku-band, consisting of a reflector, waveguide feed, tripod plus supporting structure, horn feed and the waveguide (built by Alenia Spazio, Italy). RA-1 is a nadir-pointing pulse radar taking precise measurements of the echos from the ocean and ice surfaces. Frequency = 13.8 GHz; pulse length = 20 μs; pulse repetition frequency = 1020 Hz; chirp bandwidth = 330 MHz (for ocean mode) and 82.5 MHz (for ice mode); RF transmit power = 55 W peak; antenna diameter = 1.2 m; max. data rate = 15 kbit/s; instrument mass = 96 kg; power = 130 W. 10) 11) 12)
RA-1 operates in 2 modes: ocean mode and ice mode. Beam width = 1.3º; foot print = 16 - 20 m (depending on sea state). RA-1 operates by timing the two-way delay for a short duration radio frequency pulse, transmitted vertically downwards. The required level of range measurement accuracy (better than 10 cm) calls for a pulse compression technique (chirp). The instrument employs frequency modulation and spectrum analysis of the pulse shape. RA-1 provides measurements leading to the determination of:
- Precise altitude (ocean surface elevation for the study of ocean currents, the tides and the global geoid)
- Significant wave height
- Ocean surface wind speed
- Various ice parameters (surface topography, ice types, sea/ice boundaries)
Figure 10: Block diagram of the RA-1 instrument (image credit: ESA)
Table 4: Performance characteristics of RA-1
Figure 11: Schematic view of the RA-1 antenna (image credit: ESA)
Figure 12: Schematic swath coverages for ERS-1 sensors (image credit: ESA)
ATSR-1 (Along-Track Scanning Radiometer and Microwave Sounder-1):
ATSR was developed and built by RAL, UK (British Aerospace as prime contractor); CRPE, France, and CSIRO, Australia. It consists of two instruments: the MWR (Microwave Radiometer) and the IRR (Infrared Radiometer). A major objective of ATSR is to measure the global SST (Sea Surface Temperature) with the high accuracy required by the climate change research community. The instrument design employs the use of low-noise infrared detectors, cooled to < 95 K by a Stirling cycle mechanical cooler.
• ATSR/MWR (Microwave Radiometer) characteristics: The MWR instrument uses a 60 cm Cassegrain offset-fed antenna to view the Earth in nadir direction in the frequencies of 23.8 and 36.5 GHz. The signals received are compared with those from a reference source at a known temperature to minimize the effects of short-term variations. Additional features are used to calibrate MWR: the sky-horn antenna is pointed toward cold space; the hot reference is obtained internally. IFOV = 20 km (= resolution); each channel has a bandwidth of 400 MHz. Prime objective of MWR is measurements of atmospheric water-vapor and liquid content in order to improve the accuracy of the sea surface temperature measurements and also to provide accurate tropospheric range correction for the RA-1.
Figure 13: Schematic illustration of the ATSR-1 instrument (image credit: ESA)
• ATSR/IRR (Infrared Radiometer) characteristics: The IRR imager has 4 spectral bands centered at: 1.6 µm, 3.7 µm, 10.8 µm and 12 µm. Spatial resolution = 1 km x 1 km (IFOV at nadir). Radiometric resolution < 0.1 K. Absolute accuracy < 0.5 K by averaging over a 50 km x 50 km area for SST with 80 % cloud cover; radiometric resolution <0.1 K; swath width = 500 km. The conical scanning technique enables the Earth's surface to be viewed at two different angles (0º and 47º) in two curved swaths 500 km wide and separated, along track, by about 800 km. Successive scans in the cross-track direction are displaced by about 1 km (along-track) due to the satellite's motion. A rotating mirror scans the two tracks once every 150 s (total of 2000 pixels per scan, 555 for nadir-view data and 371 for forward-view data) Measurements of: 13) 14) 15) 16) 17)
- cloud-top temperature and cloud cover
- SST (Sea Surface Temperature), prime objective of IRR (accuracy achieved of ±0.3º).
The thermal channels of IRR use an advanced detector cooling system and can be calibrated (two blackbody references are being used during each scan). The IRR instrument features a conical scanning configuration resulting in a dual view observation of the same target area. The sensor records a line of off-nadir pixels at a view zenith angle of about 55º and some 900 km along-track. About 2.5 minutes later, a nadir view is obtained when the S/C is directly over the target. The resultant image data set (after resampling both the nadir and forward view data) consists of two co-registered images with a 1 km spatial resolution on a 500 km swath. The dual-view design of ATSR makes it possible to estimate and correct for these atmospheric effects.
Figure 14: Illustration of the conical scan geometry of the ATSR IRR instrument (image credit: ESA)
Figure 15: Combined footprint geometry of ATSR/MWR and ASTR/IRR (image credit: ESA)
Table 5: Spectral bands of the ATSR-1 and ATSR-2 instruments
Figure 16: Schematic view of the ATSR fore-optics (image credit: ESA)
LRR (Laser Retro-Reflector):
LRR is a passive optical device (corner cube reflectors) for accurate satellite tracking from the ground (laser ranging stations of the SLR network) to support instrument data evaluation. LRR characteristics: wavelength = 350 - 800 nm (optimized for 532 nm), efficiency: > 0.15 at end-of-life, reflection coefficient: > 0.8 end-of-life, FOV: elevation half-cone angle = 60º, azimuth of 360º, diameter: ≤ 20 cm.
Figure 17: Illustration of LRR (image credit: ESA)
The corner cubes are made of the highest-quality fused silica and work in the visible spectrum. Their performance is optimized at the two wavelengths (694 nm and 532 nm) commonly used in SLR stations. The corner cubes are symmetrically-mounted on a hemispherical surface with one nadir-pointing corner cube in the centre, surrounded by an angled ring of eight corner cubes. 18)
PRARE (Precise Range And Range-Rate Equipment):
The PRARE objective is precise satellite range determination leading to higher-accuracy altitude measurements. PRARE utilizes 2.2 GHz and 8.5 GHz transmissions for ionospheric corrections and orbit determination, respectively. This information is needed for ocean circulation studies and geodetic applications such as sea-surface topography and crustal dynamics.
The description of PRARE is part of the ERS-2 documentation.
ERS Data Transmission and Ground Segment
The payload data are transmitted by the IDHT (Instrument Data Handling and Transmission) subsystem. The instruments generate data in the form of source packets which in turn are put into transport frames for transmission. Three data streams are transmitted from the IDHT in X-band: Link 1 contains high-rate real-time SAR data at a rate of 105 Mbit/s (8140 MHz); Link 2 contains low-rate real-time data (AMI wave and wind data, RA-1 and ATSR data) at a rate of 1.093 Mbit/s (8040 MHz); Link 3 contains recorder data (all of Link 2) at a rate of 15 Mbit/s. Link 1 is dedicated onto one X-band link, while Link 2 and 3 share the second X-band link. The modulation scheme for Links 1 is QPSK (Quadrature Phase-Shift Keying). The low-rate link uses UQPSK (Unbalanced Quadrature Phase-Shift Keying) to modulate Link 2 and Link 3 data onto a single link. With no recorder dump data BPSK (Bi-Phase-Shift Keying) is used for the real-time data.
The ERS-1 onboard data recorder (magnetic tape recorder) had a capacity of 6.5 Gbit, equivalent to the LBR data volume acquired in one orbit. Although the recorder capacity was exceptionally large for its time of implementation (1990), the AMI SAR data rate of 105 MBit/s could not be recorded, only a real-time downlink service provision was possible. Data from the tape recorder were downlinked at 15 Mbit/s.
Packetized communications protocol. The ERS-1 spacecraft is the first project anywhere introducing the newly defined communication protocols of CCSDS (Consultative Committee for Space Data Standards), a recommendation for all data formats and transmission protocols to support a range of functional services. In the meantime these protocol recommendations have become a "way of life" and standards for virtually all successive Earth observation missions as well as for many commercial communication missions. A definite advantage of the standard is the provision for inter-operability with stations/segments of other space agencies.
Figure 18: The ERS ground segment for payload data (image credit: ESA)
The ERS Ground Segment includes facilities for the satellite's control and operation, for reception, archiving and processing of the instrument data and provides services to satisfy user requirements for products. It consists of the following elements: 19) 20) 21) 22) 23) 24)
• EECF (Earthnet ERS-1 Central Facility) in Frascati, Italy (ESA/ESRIN), carries out all user interface functions, including cataloging, handling of user requests, payload operation planning, scheduling of data processing and dissemination, quality control of data products and system performance monitoring.
• MMCC (Mission Management and Control Center) in Darmstadt, Germany (ESA/ESOC). MMCC carries out all satellite operations control and functional management, including overall satellite and payload operational scheduling. It also controls the Kiruna ground station.
• Ground stations: 25)
- The ESA ground stations at Kiruna, Fucino (Italy), Maspalomas (Canary Islands, Spain), and Gatineau (Canada);
- National ground station facilities, like the Canadian "Prince Albert" station, the DLR/DFD "O'Higgins" station (in Antarctica) as well as a portable station which can be set up anywhere (DTXS = DFD Transportable X-band Station), the CNES Aussaguel station, the Japanese stations "Hatoyama," "Kumamoto" and "Syowa" (Antarctica), the Indian (ISRO) station Hyderabad, the Alaska SAR Facility, Fairbanks (NASA), Alice Springs and Hobart (Australia), Tromsö (Norway), Cuiaba (Brazil, INPE), Cotopaxi (Ecuador), Miyun (China, CAS), Ryadh (Saudi Arabia), Bangkok (Thailand), Pretoria (South Africa, CSIR), Chung-li, Taoyuan (Taiwan), West Freugh (Scotland, BNSC), Tel Aviv (Israel, ISA), Parepare (Indonesia, LAPAN), Islamabad (Pakistan), Norman (Oklahoma, Eosat), Singapore (University of Singapore), etc.
Ground stations are equipped with "Fast Delivery" SAR processors, capable of generating quicklook images after reception of the pass. These "Fast Delivery Products" (FDP) are directly mailed to the national PAF's (Processing and Archiving Facility).
• Processing and Archiving Facilities (PAFs)
- D-PAF at DLR/DFD in Oberpfaffenhofen, Germany
- F-PAF at CERSAT, Brest, France
- I-PAF at ASI, Matera, Italy
- UK-PAF at RAE, Farnborough, UK
• User centers and individuals, such as national and international meteorological services, oceanographic institutes, various research centers and individual users.
The inability of ERS-1 onboard SAR data recording in the early 1990s created in addition new infrastructures in the form of mobile and stationary ground receiving stations at various sites around the globe. The reason: in spite of the many participating receiving stations (that were gradually added during the ERS program), there was no global coverage for real-time-only reception of SAR data (repetitive observational data of the region of interest). Some remote sites, like Antarctica, were considered to be of great interest by the science community. Other sites (continents), like most of Africa, inner Asia, portions of South America, were simply lacking a high-volume data receiving infrastructure.
Hence, mobile station services were gradually introduced to complement the ERS ground station network. Some of these mobile (or stationary) receiving stations are:
- GARS (German Antarctic Receiving Station O'Higgins) is such a SAR data acquisition station, located at the site of the Chilean Base General Bernardo O'Higgins (Antarctic Peninsula), it was founded in 1989. O'Higgins is located at 57.90º W longitude and 63.32º southern latitude. GARS was installed by DLR/DFD in the southern summer of 1990/91 and is owned by DLR. GARS features an antenna of 9 m diameter and is capable of supporting several projects.
- Syowa Station on Antarctica (Japan, NASDA/NIPR), located at 39.58º E longitude and 69.00º southern latitude.
- Libreville, Gabon (DLR)
- Ulan Bator, Mongolia (DLR)
- McMurdo station at Hut Point Peninsula on Ross Island, Antarctica (NASA/ASF (Alaska SAR Facility), since 1956), located 166.67º E longitude, and 77.85º southern latitude.
All of these stations are capable of downlinking recording of SAR image data (of various missions, ERS-1, JERS-1, RADARSAT, ERS-2, Envisat, etc.). The more recent missions (RADARSAT, Envisat) have onboard recorders permitting at least a limited amount of SAR image data recording.
Commercial ERS Data Distributors
In 1992 ESA selected the first three distributors for ERS-1 data products. These are 26):
- Radarsat International of Ottawa, Canada (responsible for commercial sales in Canada and USA)
- EURIMAGE of Rome, Italy (markets in Europe, North Africa, and the Middle East)
- SPOT Image of Toulouse, France (rest of the world).
1) G. Duchossois, "The ERS-1 Mission Objectives," ESA Bulletin No 65 Feb. 1991, pp. 16-26
2) R. Francis, et al., "The ERS-1 Spacecraft and its Payload," ESA Bulletin No 65 Feb. 1991, pp. 27-48
4) D. Andrews, S. J. Dodsworth, M. H. McKay, "The Control and Monitoring of ERS-1," ESA Bulletin No 65 Feb. 1991, pp. 73-79
5) H. Ege, "Industrial Cooperation on ERS-1," ESA Bulletin No 65 Feb. 1991, pp. 88-94
6) E. Attema, R. Francis, "ERS-1 Calibration and Validation," ESA Bulletin No 65 Feb. 1991, pp. 80- 87
7) "ERS-1 Payload Summary," 1992, URL: http://ceos.cnes.fr:8100/cdrom-97/ceos1/satellit/ers/paysum.htm
8) E. P. W. Attema, "The Active Microwave Instrument On-Board the ERS-1 Satellite," Proceedings of IEEE, Vol. 79, No.6, June 1991, pp. 791- 799
9) ERS-1 User Handbook, ESA SP-1148, May 1992, pp. 6-7
10) G. Schreier, K. Maeda, B. Guindon, "Three Spaceborne SAR Sensors: ERS-1, JERS-1, and RADARSAT- Competition or Synergism?," Geo Informationssysteme, Heft 2/1991, Wichmann Verlag, Karlsruhe, pp. 20 - 27
11) R. Winter, D. Kosmann "Anwendungen von SAR-Daten des ERS-1 zur Landnutzung," Die Geowissenschaften, 9. Jahrgang, Heft 4-5, April-Mai 1991, pp. 128-132
12) W. Kühbauch, "Anwendung der Radarfernerkundung in der Landwirtschaft," Die Geowissenschaften, 9. Jahrgang, Heft 4-5, April-Mai 1991, pp. 122-127
13) T. Edwards, R. Browning, J. Delderfield, D. J. Lee, K. A. Lidiard, R. S. Milborrow, P. H. McPherson, et al., "The Along Track Scanning Radiometer measurement of sea-surface temperature from ERS-1," Journal of the British Interplanetary Society, Vol. 43, 1990, pp.160-180.
14) F. M. Danson, N. A. Higgins, N. M. Trodd, "Measuring Land-Surface Directional Reflectance with the Along-Track Scanning Radiometer," PE&RS, Vol 65, No 12, Dec. 1999, pp. 1411-1417
16) N. C. M. Stricker, A. Hahne, D. L. Smith, J. Delderfield, M. B. Oliver, T. Edwards, "ATSR-2: The Evolution in Its Design from ERS-1 to ERS-2," ESA Bulletin No 83, Aug. 1995
19) M. Fea, "The ERS-1 Ground Segment," ESA Bulletin No 65 Feb. 1991, pp. 49-59
21) W. Markwitz, "Das ERS-1 Bodensegment, Empfang, Verarbeitung und Archivierung von SAR Daten," Die Geowissenschaften, 9. Jahrgang, Heft 4-5, April-Mai 1991, pp. 111-115
22) D. Gottschalk, "ERS-1 Mission and System Overview," Die Geowissenschaften, 9. Jahrgang, Heft 4-5, April-Mai 1991, pp. 100-101
23) M.F. Buchroithner, J. Raggan, D. Strobl "Geokodierung und geometrische Qualitätskontrolle," Die Geowissenschaften, 9. Jahrgang, Heft 4-5, April-Mai 1991, pp. 116-112
24) S. D'Elia, S. Jutz, "SAR Mission Planning for ERS-1 and ERS-2," ESA Bulletin, No 90, May 1997
25) J-C. Bigot, V. Beruti, "The National and Foreign Stations - Key Partners in the ERS Ground Segment," ESA Bulletin No 101, February 2000,
26) `ESA Signs Long-awaited Imagery Sales Deal,' Space News, Feb. 10-16, 1992, p. 4
This description was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" - comments and corrections to this article are welcomed by the author.