Monitoring the Stability of Satellite Altimeters with Tide Gauges

A method is described for using tide gauge sea levels to monitor time-dependent drift in satellite altimetric measurements of sea surface height. The method depends on a careful assessment of the quality of the tide gauge measurements available for this application and also takes into account the degree of independence between the altimeter minus tide gauge differences in order to construct an optimal drift estimate and an accurate error estimate for it. The method is applied to the TOPEX altimeter measurements, and a recently discovered algorithm error, which resulted in a slow drift in the TOPEX sea surface heights, is exploited to evaluate the success of the tide gauge drift estimation. It is important to note that the tide gauge analysis was done without any prior knowledge of this error. The result is that the tide gauge analysis reproduces the drift due to the algorithm error to within 6 mm rms, which is comparable to the 5\textendash6-mm internal estimate of the uncertainty of the drift analysis. The analysis is then made using the TOPEX data that have been corrected for the algorithm error and shows that the satellite heights are stable to better than 10 mm over the nearly 4 years of data available, although a drift on the order of 2 mm yr-1 remains, the source of which is unknown. This inferred stability is more than adequate for the majority of applications, although questions still exist for more demanding applications, such as the calculation of global sea level change. Limitations of the tide gauge analysis are discussed, along with potential improvements that might be possible in the future.