The TOPEX/POSEIDON project is a cooperative project between the United States' National Aeronautics and Space Administration (NASA) and the French Space Agency, Centre National d'Etudes Spatiales (CNES). The main goal of the TOPEX/POSEIDON mission is to understand the role of ocean circulation in climate change.

TOPEX/POSEIDON uses advanced satellite radar altimetry to make very precise and accurate observations of sea level for three (nominally) to five (expected) years. The orbit, satellite bus and payload instruments are optimized to measure sea level, to map basin-wide variations of currents, and to monitor the effects of the currents, on global climate change. Altimeter measurements also allow scientists to study ocean tides and waves, marine geophysics and winds.

Launched by Ariane on August 10, 1992, TOPEX/POSEIDON is a core element of the international World Ocean Circulation Experiment (WOCE) and Tropical Ocean Global Atmosphere (TOGA) measurement programs. Analysis of the satellite altimetry data, together with WOCE and TOGA ocean measurements, yields major advances in our quantitative understanding of the ocean circulation and its influence on climate. TOPEX/POSEIDON lays the foundation for long-term ocean monitoring from space.