Determining the absolute surface geostrophic circulation of the world's oceans has been a tantalizing prospect of satellite altimetry since the advent of the technique. The difficulties arise from the errors in both the orbit and the geoid. The orbit errors of concern are the systematic (or geographically correlated) component that cannot be reduced by averaging in time. This error must be reduced by improved gravity models or by continuous precision satellite-tracking using the Global Positioning System [ Bertiger et al., 1994]. As part of the TOPEX/POSEIDON Mission, gravity model improvement efforts were initiated in both the U.S. and France in the 1980s. These efforts have progressively produced improved gravity models [ e.g., Marsh et al., 1988, 1990 a; Rapp, 1993], which in turn have also led to improved long-wavelength components of the geoid. For instance, Tai [1988] reported improved estimate of the ocean dynamic topography using the gravity model of Marsh et al. [1988]
The ocean dynamic topography is usually solved as an inverse problem involving simultaneous adjustment of the orbit, the gravity model, and the dynamic topography. Using this approach, a number of groups have estimated the global ocean dynamic topography up to spherical harmonics degree and order 6--10 [ Tapley et al., 1988; Marsh et al., 1990 b; Denker and Rapp, 1990; Nerem et al., 1990; Visser et al., 1993]. For shorter scales, the geoid errors overwhelm the oceanographic signals. Using an inverse model of the North Atlantic circulation involving a large amount of oceanographic data, Martel and Wunsch [1993] evaluated the Geosat altimetric dynamic topography of Nerem et al. [1990] and concluded that the Geosat result was inconsistent with the conventional oceanographic data and that the error in the Geosat result was at least a factor of two too large to improve the existing knowledge of the circulation. Preliminary examination of the data from the TOPEX/POSEIDON Mission have shown promise for using the data to improve estimate of the ocean general circulation [ Nerem et al., 1994; Tapley et al., 1994; Stammer and Wunsch, 1994].