TOPEX POSEIDON MISSION OVERVIEW

TOPEX/POSEIDON is the first space mission specifically designed and co nducted for studying the circulation of the world's oceans. The missio n is jointly conducted by the United States and France. A state-of-the -art radar altimetry system is used to measure the precise height of s ea level, from which information on the ocean circulation is obtained. The satellite, launched on August 10, 1992, has been making observati ons of the global oceans with unprecedented accuracy since late Septem ber 1999. To meet the stringent measurement accuracy required for ocea n circulation studies, a number of innovative improvements have been m ade to the mission design, including the first dual-frequency space-bo rne radar altimeter capable of retrieving the ionospheric delay of the radar signal, a three-frequency microwave radiometer for retrieving t he signal delay caused by the water vapor in the troposphere, an optim al model of the Earth's gravity field and multiple satellite tracking systems for precision orbit determination. Additionally, the satellite also carries two experimental instruments to demonstrate new technolo gies: a single-frequency solid-state altimeter for the technology of l ow-power, low-weight altimeter and a Global Positioning System receive r for continuous, precise satellite tracking. The performance of the m ission's measurement system has been tested by numerous verification s tudies. The results indicate that the root-sum-square accuracy of a si ngle-pass sea level measurement is 4.7 cm for the TOPEX system and 5.1 cm for the POSEIDON system; both are more than a factor of 2 better t han the requirement of 13.7 cm. This global data set is being analyzed by an international team of 200 scientists for improved understanding of the global ocean circulation as well as the ocean tides, geodesy, and geodynamics, and ocean wind and waves. The mission is designed to last for at least 3 years with a possible extension to 6 years. The mu ltiyear global data set will go a long way toward understanding the oc ean circulation and its variability in relation to climate change. A s ummary of the mission's systems and their performance as well as the m ission's science team is presented in the paper.