Between 1985 and 1990, the U.S. Navy's Geosat radar altimeter made direct observations of sea-surface topography. Since the sea surface is nearly an equipotential surface of the Earth's gravity field, variations in sea surface topography reveal variations in marine gravity. At short wavelengths (<200 km), the topography of the sea surface mimics the seafloor or basement topography. Observations from Geosat have been used to map the marine geoid with unprecedented accuracy and resolution (see Figure 1). This mapping of the marine geoid—or gravity field—can revolutionize marine geophysics and geodesy, especially in the southern ocean basins where shipboard coverage is sparse.