The TOPEX/Poseidon satellite altimeter is a science project to measure the ocean surface topography.
It is a joint project between the CNES (France) and NASA (United States) space agencies.
The program is named after the greek god Poseidon.
It contains two altimeters onboard sharing the same antenna, only one altimeter can be operated at any time, with TOPEX given preference (on average 9 in 10 cycles)
The successor of TOPEX/Poseidon is Jason 1.