Geosat was a United States Navy satellite designed to measure sea surface heights to within 5 cm. After a year-and-a-half-long classified mission, Geosat's scientific Exact Repeat Mission (ERM) began on 8 November 1986. When the ERM ended in January 1990 (due to failures of both onboard tape recorders), more than three years of precise altimetry data were available to the scientific community.