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ERS

ESA's low altitude Earth observation system debuted in 1991 with the launch of ERS-1. The culmination of a 10-year development project led by Germany, ERS-1 hosts a suite of precision instruments tailored for a comprehensive environmental monitoring program with objectives including:

• " a much more accurate representation of the interactions between ocean and atmosphere in climatic models,
• a major advance in our knowledge of ocean circulation, its variability and the associated energy transfers,
• better monitoring of polar regions, in particular the Arctic and Antarctic ice sheets and sea-ice-covered areas,
• a more comprehensive understanding of coastal processes and surface pollution, including erosion, sedimentation, coastal currents, estuarine fronts and circulation,
• the regular monitoring of land-surface processes on a global scale, and in particular the vegetation cover,
• the monitoring of changing land-use patterns,
• offering a unique all-weather sensing capability for disaster observation and assessment, and
• enhancing the data available for operational meteorology, in particular observation ofwinds near the sea surface, sea-state, seasurface temperature measurements, cloudfields, atmospheric water content, and seaice distribution" (Reference 500).

The 2,384-kg ERS-1 satellite employs a spacecraft bus derived from the French SPOT satellite, measuring 1.8 m by 1.9 m by 3.1 m. The two primary appendages area solar array with two 2.4 m by 5.8 m segments providing more than 2 kW of electrical power and a combination radar antenna for the Active Microwave Instrument. Placed into a sun-synchronous orbit near 780 km with an inclination of 98.5 degrees, ERS-1 carried 300 kg of hydrazine for a mission expected to last three years. The spacecraft's orbit is maintained with a very high degree of accuracy, resulting in a 35-day ground track pattern which is controlled within i1 km (References 501-502).

The five principal scientific instruments include:

(1) Active Microwave Instrument (AMI) consisting of a side-locking synthetic aperture radar and a scatterometer, both operating at 3.5 GHz. The former returns high resolution photographs with a 100 km ground swath in theimage mode or 5 km by 5 km snapshots in the wave mode. With the use of three antennas with 45 degree separation angles, the scatterometer sweeps a 500 km swath to provide surface wind measurements;
(2) Radar Altimeter (RA) with separate"ocean" and "ice" modes operating at 13.8 GHz;
(3) Along-Track Scanning Radiometer(ATSR) and Microwave Sounder (MS) consisting of an Infrared Radiometer and a Microwave Radiometer to measure the global sea-surface temperature and the atmospheric integrated water content, respectively. The Infrared Radiometer operates at the 1.6, 3.7, 11, and 12 µm bands, while the Microwave Radiometer is tuned to 23.8 and 36.5 GHz;
(4) Laser Retro-Reflector (LRR) assembly of corner cubes mounted on the side of the spacecraft bus is used as a target by ground-based laser ranging stations;
(5) Precise Range and Range-rate Equipment (PRARE) utilizes 2.2 GHz and 8.5GHz transmissions for ionospheric corrections and orbit determination, respectively.

Despite some early data distribution difficulties, ERS-1 has performed remarkably well, returning nearly 800,000 radar images in its first three years of operation alone. The AMI images of ocean areas have been of higher quality than expected, and the uses of ERS-1 data - from oil spill detection to salmon tracking - continue to increase. The most serious technical set backs of the mission were the failures of PRARE less than a month after launch and one channel of the ATSR. However, the greatest threat to ERS has been continued wrangling over its annual operating budget. A potential 1994 shut-down was averted, and the program was eventually extended to permit simultaneous operations with ERS-2, scheduled for launch in 1995 (References 503-516).

ERS-2 will resemble its predecessor but will be equipped with a broader array of instruments and more modern support systems. The 2,516-kg spacecraft will retain the 1 m-by-10 m SAR antenna and its 2.4 m-by-11.7 m solar array. In addition to reflights of all the basic ERS-1 payloads, ERS-2 will carry the Global Ozone Monitoring Experiment (GOME) with an absorption spectrometer to measure the presence of ozone, trace gases, and aerosols in the stratosphere and troposphere and an Instrument Data Handling and Transmission (IDHT) device with two 6.5 GB it tape recorders (References 506 and 517).