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A full-size model of ERS-2.

European Remote-Sensing satellite (ERS) was the European Space Agency's first Earth-observing satellite. It was launched on July 17, 1991 into a Sun synchronous polar orbit at a height of 782–785 km.

ERS-1 carried an array of earth-observation instruments that gathered information about the Earth (land, water, ice and atmosphere) using a variety of measurement principles. These included:

• RA-1 (Radar Altimeter) is a single frequency nadir-pointing radar altimeter operating in the K_u band.
• ATSR-1 (Along-Track Scanning Radiometer) is a 4 channel infrared radiometer and microwave sounder for measuring temperatures at the sea-surface and the top of clouds.
• SAR (synthetic aperture radar) operating in C band can detect changes in surface heights with sub-millimeter precision.
• Wind Scatterometer used to calculate information on wind speed and direction.
• MWR is a Microwave Radiometer used in measuring atmospheric water, as well as providing a correction for the atmospheric water for the altimeter.

To accurately determine its orbit, the satellite included the PRARE (Precision Range and Range-Rate Equipment) and a Laser Retroreflector. The PRARE was non-operational since launch. The Retroreflector was used for calibrating the Radar Altimeter to within 10 cm.

ERS-1 had various mission phases using 3-day, 35-day and a 336-day repeat cycle. The 336-day mission (Geodetic Mission) allowed for accurate mapping of the Earth's bathymetry and geoid over the seas using the Radar Altimeter.

ERS-1 failed on March 10, 2000, far exceeding its expected lifespan.

Its successor, ERS-2, was launched on April 21, 1995. Largely identical to ERS-1, it added additional instruments and included improvements to existing instruments including:

• GOME (Global Ozone Monitoring Experiment) is a nadir scanning ultraviolet and visible spectrometer.
• ATSR-2 included 3 visible spectrum bands specialized for Chlorophyll and Vegetation

When ERS-2 was launched, ERS-1 shared the same orbital plane. This allowed a tandem mission, with ERS-2 passing the same point on the ground 1 day later than ERS-1. ERS-2 has a repeat cycle of 35 days.

ERS-2 has been operating without gyroscopes since February 2001, resulting in some degradation of the data provided by the instruments. The tape drive aboard failed on June 22, 2003, leaving the instruments operating only within visibility of a ground station. Since the tape drive failure additional ground stations have been brought online to increase the data gathering abilities of the satellite. The Wind Scatterometer and GOME were the only instruments of their kind until the launch of MetOp-A.

The successor to ERS-2 is Envisat containing improved versions of many of the instruments onboard ERS-2; however, its operational life has been increased until 2011.

Side view of full-size ERS-2 model.

• ERS-homepage at ESA
• ERS operations page at ESA