||9 April 1975
GEOS 3 (Geodynamics Experimental Ocean Satellite) was launched in April 1975, and ended its mission in December 1978. It carried the first instrument to yield useful measurements of sea level and its variability over time.
When a satellite flies at higher altitudes, this reduces the strength of the altimeter's return signal (which is proportional to the cube of the height) and alters the size of the footprint on the ground. Because the satellite is incapable of emitting a more powerful signal, the pulse compression technique must be used above a certain altitude to compensate for the weakened return signal. Since GEOS 3 (1974), which flew at an altitude of 840 km, all altimeters have used pulse compression, and the resolution made possible by this compression has been enhanced. GEOS 3 offered significant improvements over Skylab's altimeter, including improved performance as well as greater global coverage, but this performance was still not good enough to enable useful science to be extracted from its measurements.