Radar altimeters onboard GEOSAT, ERS-1 and TOPEX/POSEIDON satellites measure sea level relative to Earth's center. From more than 15 years of altimetric dataset, maps of the mean sea level have been computed at CLS:

• first, the CLS_SHOM98.2 mean sea surface,
• recently, an improved version: the CLS01 mean sea surface

A Mean Sea Surface (MSS) corresponds to the "permanent" sea level relative to a reference ellipsoid, as measured by satellite altimetry. The MSS contains the marine geoid undulations (±120 meters ) plus the "permanent" elevation of the sea (1-2 meters). Because satellite altimetry reflects the geoid with a high spatial resolution, MSS were first computed by geodesist and geophysicist to study the Earth gravity field and the solid Earth.