The GEOS 3 (Geodynamics and Earth Ocean Satellite) was part of a geodetic mission series lead by NASA. It launched on April 9, 1975 and continued operating until July 1979. The spacecraft carried a radar altimeter for mapping of the oceans, as well as a laser retroflector, and other tracking beacons.
The GEOS-3 mission was to measure the gravitational field around the Earth, specifically to identify the irregularities and anomalies. Gravitational anomalies over the ocean also can be used to measure large mesoscale circulation.
Altimeter data from GEOS-3 became part of the GEM-T3, JGM-1 and JGM-2 gravity models. In addition SLR (satellite laser ranging) data to GEOS-3, as well as satellite-to-satellite Doppler tracking data from the geostationary satellite ATS-6 were also included in these models. During the development of the prelaunch TOPEX gravity models, GEOS-3 was considered to be an important satellite, since it was located near the mirror inclination of TOPEX/POSEIDON (66.6 deg).
EGM96 included GEOS-3 SLR data, as well as combined SLR and satellite-to-satellite tracking data from ATS-6.