The third Geodetic and Earth Orbiting Satellite (GEOS 3), later renamed Geodynamics Experimental Ocean Satellite, was launched on April 9, 1975. It was an octahedron spacecraft topped by a truncated pyramid, with a parabolic reflector for a radar altimeter on the flat-bottom side. The spacecraft carried a radar altimeter for mapping the oceans, as well as a laser retroflector and other tracking beacons. 

Mission objectives included performing a satellite altimetry experiment in orbit; supporting the calibration and position determination of C-band radar systems for NASA and other agencies; measuring the gravitational field around the Earth, (specifically, to identify the irregularities and anomalies), and performing a satellite-to-satellite tracking experiment with the ATS 6 spacecraft using an S-band transponder system.

Altimeter data from GEOS-3 became part of the GEM-T3, JGM-1, and JGM-2 gravity models. In addition, satellite laser ranging (SLR) data to GEOS-3 and 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 platform, since it was located near the mirror inclination of TOPEX/POSEIDON (66.6 deg).

The geopotential model EGM96 included GEOS-3 SLR data, as well as combined SLR and satellite-to-satellite tracking data from ATS-6.