MARINE GRAVITY-ANOMALY FROM GEOSAT AND ERS-1 SATELLITE ALTIMETRY

Closely spaced satellite altimeter profiles collected during the Geosa t Geodetic Mission (similar to 6 km) and the ERS 1 Geodetic Phase (8 k m) are easily converted to grids of vertical gravity gradient and grav ity anomaly. The long-wavelength radial orbit error is suppressed belo w the noise level of the altimeter by taking the along-track derivativ e of each profile. Ascending and descending slope profiles are then in terpolated onto separate uniform grids. These four grids are combined to form comparable grids of east and north vertical deflection using a n iteration scheme that interpolates data gaps with minimum curvature. The vertical gravity gradient is calculated directly from the derivat ives of the vertical deflection grids, while Fourier analysis is requi red to construct gravity anomalies from the two vertical deflection gr ids. These techniques are applied to a combination of high-density dat a from the dense mapping phases of Geosat and ERS 1 along with lower-d ensity but higher-accuracy profiles from their repeat orbit phases. A comparison with shipboard gravity data shows the accuracy of the satel lite-derived gravity anomaly is about 4-7 mGal for random skip tracks. The accuracy improves to 3 mGal when the ship track follows a Geosat Exact Repeat Mission track line. These data provide the first view of the ocean floor structures in many remote areas of the Earth. Some app lications include inertial navigation, prediction of seafloor depth, p lanning shipboard surveys, plate tectonics, isostasy of volcanoes and spreading ridges, and petroleum exploration.