The unprecedented accuracy of TOPEX/POSEIDON (T/P) altimeter data warrants a new evaluation of the methods typically used to form time series of sea level change. Whereas explicit removal of orbit error has always been required as a first step in altimeter data processing, the T/P analysis presented here is based simply on unadjusted, monthly averages. This approach has the advantage of retaining the large‐scale ocean signal, which would be distorted by orbit adjustment. Using 16 months of data, we have evaluated the T/P monthly means on spatial scales ranging from mesoscale to global. In the tropical Pacific, comparisons with 17 island tide gauge records and dynamic height derived from 36 thermistor moorings show that the altimeter data have an accuracy of approximately 2 cm when averaged over spatial scales of a few hundred kilometers. On basin scales in the northern hemisphere, similar agreement is found between the T/P data and the dynamic height climatology of Levitus (1982). These new altimeter observations are thus providing the first reliable view of global sea level changes on seasonal‐to‐interannual timescales.

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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].

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