Evolution of the Satellite Radar Altimeter

The radar altimeters flown on Geos-3 (1975), Seasat-l (1978), and Geosat-l (1985) have amply demonstrated the benefits of high-precision measurements of ocean-surface topography, both to geodesy and to oceanography. Two new altimetry missions are planned: the joint NASA/Centre National d'Etudes Spatiales TOPEx/Poseidon mission, to be launched in 1992, and the Navy's Special Purpose Inexpensive Satellite altimeter mission, to be launched in 1990. Although both instruments draw heavily on previous designs and in-flight experience, the mission requirements have led to two very different instruments. These requirements will be discussed, and the resulting designs will be contrasted with their immediate predecessor, Geosat.