The ERS satellites's (European Remote Sensing) main mission is to observe Earth, in particular its atmosphere and ocean. Build by Esa, they carry several instruments, including a radar altimeter. ERS-1 was launched in July 1991, switched off in June 1996 and retired in March 2000.

ERS-1 flew on three different orbits:

• a 3-day period for calibration and sea ice observation (from 12/28/1991 to 03/30/1992 and from 12/24/1993 to 04/10/1994),
• a 168-day period for geodetic applications (from 04/10/1994 to 09/28/1994 and from 09/28/1994 to 03/21/1995).
• a 35-day period for multi-disciplinary ocean observations (for the others dates),

ERS-2, the follow-on from ERS-1, was launched in April 1995. It was used in tandem with ERS-1 from August 1995 to June 1996, their identical orbits (35 days) having a one-day shift.