On the use of sea-level gauge data for satellite altimetry validation - a review

The altimetric sea-level data base supplied by the satellite Geosat has permitted numerous studies in the field of ocean science, some of which have addressed, through comparisons with island gauge observations, the question of the quality of the sea level products inferred from die Geosat dataset. The aim of this paper is to provide an overview of these recent studies, illustrating the usefulness of in situ sea level gauge measurements for satellite altimetry validation, to present briefly the status of the present worldwide sea level network, and to provide some guidelines on what must be done in the near future to improve this network further. It is shown that some of the discrepancies between altimeter products and tide gauge records are due to insufficient corrections of the altimeter signal (orbit error, tropospheric water vapour, tidal corrections), and that improving these corrections greatly increases agreement with in situ observations. It is also demonstrated that, if a maximum misfit of 10 cm is accepted at each sea level gauge location, the two datasets are fully coherent, and that consequently they can be merged to obtain optimized maps of large-scale sea level variability, or of the mean sea level. The major conclusions of this overview are the following: 1) reference to tide gauge measurements is obviously of considerable importance in validating altimeter satellite products; 2) given the accuracy already obtained, and considering the optimal design of the coming Topex/Poseidon mission, it is necessary to improve the a priori error bar estimates on tide gauge data, through systematic combination of tide gauge and altimetric data, through regional numerical modelling and through assimilation methods synthesizing the two approaches; 3) the present global sea level gauge network, as developed in WOCE, is on the way to fulfilling the requirements for global validation and improvement of satellite altimetric products; 4) complementary sites, however, stil