||TOPEX/Poseidon orbits Earth at a relatively high altitude of 1,340 kilometers (833 miles). This minimizes atmospheric drag on the spacecraft, reduces the influence of errors in measuring Earth's gravity field, and simplifies maneuvers needed to maintain the orbit position. The satellite orbit is inclined 66 degrees from Earth's equator.
Designed for a three- to five-year mission, the satellite's primary science goal is to improve our understanding of how the oceans circulate. There are six instruments on board the satellite: a dual-frequency altimeter, a microwave radiometer, a single-frequency solid state altimeter, a laser reflector array, a dual-frequency system receiver and a Global Positioning System receiver.
As TOPEX/Poseidon orbits Earth, the altimeter bounces radar signals off the ocean's surface. The device records the time it takes the signal to return to the satellite and that gives it a precise measurement of the distance between Topex/Poseidon and the seasurface. These data will be combined with measurements from other instruments that pinpoint the satellite's exact location in space. Scientists will then be able to produce a detailed map of ocean topography, or sea level relative to the Earth's center.
Sea level is directly related to ocean currents, eddies and other features of the ocean surface. Information contained in the radar return signals can also be used to calculate wave height and wind speed, two elements that are important for monitoring the global sea state.
JPL's Ocean Topography Experiment, or TOPEX, has been combined with France's Poseidon mission. Together, the TOPEX/Poseidon satellite is the most sophisticated attempt yet to measure and map sea level from space.