No abstract

In April 1985 the U.S. Navy satellite GEOSAT began generating a remarkable data set that may change the way in which physical oceanographers view the global oceans. GEOSAT (Figure 1) carries a radar altimeter that provides a continuous record of sea level along the satellite ground track. Such records enable determination of sea level variability and have application in many areas of ocean dynamics. Experience with GEOS 3 (Geodynamics Experimental Ocean Satellite 3) and Seasat in the 1970s demonstrated the enormous potential of altimetry for oceanography. Seasat, for example, gathered sufficient altimeter data in its last 25 days alone to yield a global description of the mesoscale eddy field [Cheney et al., 1983], wave number spectra of sea level variability [Fu, 1983], and a global model of the M2 tide [Mazzega, 1985].

Recently, geodesy has witnessed a renaissance in geoid computation. The advances over the past decade have taken place at all wavelengths, and have brought forth major improvements in accuracy. An example of a long‐wavelength global gravitation model is the GEM‐T2 (Goddard Earth Model) solution of Marsh et al. [1989], which is complete to degree and order 36, and incomplete to degree 50. Rapp and Pavlis [1990] have computed a pair of solutions, 0SU89A and 0SU89B (Ohio State University), which are spherical harmonic models of the Earth's geopotential complete to degree and order 360. Although termed high degree global models, these solutions provide the geoid to what we may now consider a medium length scale–about 50‐km resolution. High‐resolution geoid height modeling has shown the greatest advances in accuracy. Forsberg [1990] computed a geoid model for the Nordic area on a 5‐km grid, and obtained 3–7 cm standard deviations when compared to Global Positioning System (GPS) and leveling in local networks of 50–100 km in extent.