thumb|right|A full-size model of ERS-2.
European Remote Sensing satellite (ERS) was the European Space Agency's first Earth-observing satellite programme using a polar orbit. It consisted of two satellites, ERS-1 and ERS-2, with ERS-1 being launched in 1991.
ERS-1
ERS-1 launched 17 July 1991 from Guiana Space Centre aboard an Ariane 4 rocket. The satellite was put into a Sun-synchronous polar orbit at an altitude of 782–785 km. ERS-1 failed on 10 March 2000 after nine years in orbit.
Instruments
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 (Radar Altimeter) is a single frequency nadir-pointing radar altimeter operating in the K<sub>u</sub> 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 became non-operational shortly after launch; later analysis concluded that the failure was due to a memory latchup caused by radiation.