The Topex/Poseidon satellite altimeter was launched on 10 August 1992 with a mission "to observe and understand the ocean circulation". Topex/Poseidon is a joint project between the French Space Agency, CNES and the American Space Agency, NASA. The satellite carries onboard 2 radar altimeters, different systems for precise orbit determination (DORIS, GPS, laser) and a radiometer.

Every 10 days, the satellite orbiting at an altitude 1300 km provides a complete coverage of the ocean surface topography with unequalled precision: within 5 cm for an instantaneous measurement and within 2 cm for a monthly average. This precision allows us to study the large-scale ocean circulation, the currents, the tides, the waves, etc. With more than 10 years of repeat data, we can also observe warm and cold variations in the ocean which can influence climate, on a seasonal or interannual time scale (eg: El Nino). The radar altimeter can also provide information on the sea level variations on lakes, on the polar ice cap topography, and various snow and sea-ice parameters.

Its successor, Jason-1, was launched on the same groundtrack on 7 December 2001. After a 6 month period of cross-calibration between the two satellites, Topex/Poseidon was manoeuvred sideways to continue on an orbit halfway between the Jason tracks.

The manoeuvres took 30 days from August 15, 2002 (cycles 365-368). The two satellites then orbited next to each other for 3-5 years, with an inter-track spacing of 157 km at the equator, providing excellent space-time coverage of the earth's surface topography, the ocean mesoscale eddy field and importantly, ocean surface currents.