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Two new data sets from the U.S. Navy's Geodetic Satellite (Geosat) have been produced by the National Oceanic and Atmospheric Administration's (NOAA's) National Ocean Service. Both were derived principally from classified data collected during the 18-month geodetic mission (April 1985–September 1986). Designed and built by the Johns Hopkins University Applied Physics Laboratory, the Geosat radar altimeter measured global sea level with a precision of 3 cm. The first data set consists of approximately 44-million crossover differences (sea-level differences at ground track intersections). These include the initial 18-month period and the first year of the subsequent exact repeat mission, enabling computation of the continuous 30-month sea level time series spanning the two missions. Thirty-six 6250-bpi magnetic tapes organized in 18 ocean regions document the Geosat crossover differences.

Satellite-borne altimeters have had a profound impact on geodesy, geophysics, and physical oceanography. To first order approximation, profiles of sea surface height are equivalent to the geoid and are highly correlated with seafloor topography for wavelengths less than 1000 km. Using all available Geos-3 and Seasat altimeter data, mean sea surfaces and geoid gradient maps have been computed for the Bering Sea and the South Pacific. When enhanced using hill-shading techniques, these images reveal in graphic detail the surface expression of seamounts, ridges, trenches, and fracture zones. Such maps are invaluable in oceanic regions where bathymetric data are sparse. Superimposed on the static geoid topography is dynamic topography due to ocean circulation. Temporal variability of dynamic height due to oceanic eddies can be determined from time series of repeated altimeter profiles. Maps of sea height variability and eddy kinetic energy derived from Geos-3 and Seasat altimetry in some cases represent improvements over those derived from standard oceanographic observations. Measurement of absolute dynamic height imposes stringent requirements on geoid and orbit accuracies, although existing models and data have been used to derive surprisingly realistic global circulation solutions. Further improvement will only be made when advances are made in geoid modeling and precision orbit determination. In contrast, it appears that use of altimeter data to correct satellite orbits will enable observation of basin-scale sea level variations of the type associated with climatic phenomena.

Since the last quadrennial report, new satellite altimeter data have not been generally available. Nevertheless, important progress has been made from analyses of Geos 3 and Seasat data. The marine gravity field has been determined to an extraordinary resolution and accuracy, and important solid earth geophysical interpretations made. In the area of oceanography, statistical estimates of the variability of dynamic topography have been published, methods for incorporating altimeter data into numerical models have been developed and applied, and tentative, but interesting determinations of the mean circulation have been made. In addition, altimeter waveform analysis has made it possible to use altimeter data to determine elevations of major ice fields. Finally, important new insights into the nature of satellite ephemeris error and its influence on altimetrically derived sea surface topography have been achieved.