TOPEX/Poseidon satellite. Artwork of the TOPEX/Poseidon satellite seen in orbit over the Earth. The American TOPEX (The Ocean Topography Experiment) and the French Poseidon are jointly dedicated to studying ocean currents and sea surface height. Surface height is measured using two radio altimeters which share the single antenna seen at the bottom of the spacecraft. The precise position of the spacecraft in its 1336km high orbit is measured using lasers, microwaves and the Global Positioning System. TOPEX/Poseidon was launched by an Ariane rocket on 10 August 1992.