The Ocean Topography Experiment (TOPEX/Poseidon) is a cooperative project between the United States and France to develop and operate an advanced satellite system dedicated to observing the Earth's oceans. The mission provides global seal level measurements with an unprecedented accuracy. The data from TOPEX is used to determine global ocean circulation and to understand how the oceans interact with the atmosphere. This understanding will improve our ability to predict global climate.
For this joint mission, NASA provided the satellite bus and five instruments with their associated ground elements. The Centre National d'Etudes Spatiales (CNES) furnished two instruments with their associated ground elements and a dedicated launch on Ariane rocket. Both CNES and NASA provide precision orbit determination and process and distribute data to 38 science investigators from nine nations, as well as other interested scientists.
In the summer of 1992, TOPEX was launched into orbit by an Ariane rocket from the European Space Agency's Space Center located in Kourou, French Guiana. From its orbit 1,336 kilometers (830 miles) above the Earth's surface, TOPEX makes sea level measurements along the same path every 10 days using the dual frequency altimeter developed by NASA and the CNES single frequency solid-state altimeter. This information is used to relate changes in ocean currents with atmospheric and climate patterns. Measurements from NASA's Microwave Radiometer provide estimates of the total water-vapor content in the atmosphere, which is used to correct errors in the altimeter measurements. These combined measurements allow scientists to chart the height of the seas across ocean basins with an accuracy of 13 centimeters (5 inches).
Three independent techniques determine the satellite altitude within 4 inches. NASA's Laser Retroreflector Array is used with a network of 10 to 15 satellite laser ranging stations to provide the baseline tracking data for precision orbit determination and calibration of the radar altimeter bias. The DORIS system, recently demonstrated by the French SPOT-2 mission, provides an alternate set of tracking data using microwave Doppler techniques. The system is composed of an onboard receiver and a network of 40 to 50 ground transmitting stations, providing all-weather, global tracking of the satellite. NASA's Global Positioning System Demonstration Receiver demonstrates a new technique for precise, continuous tracking of the spacecraft.
TOPEX is a vital part of a strategic research effort to explore ocean circulation and its interaction with the atmosphere. It is timed to coincide with and complement a number of international oceanographic and meteorological programs, including the World Circulation Experiment and the Tropical Ocean and Global Atmosphere Program, both of which are sponsored by the World Climate Research Program.
TOPEX/Poseidon is a three-year mission with the potential for a two-year extension. Results from the TOPEX will build the foundation for a continuing program of long-term observations of ocean circulation from space, and for an extensive ocean monitoring program in the next century.
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