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].