Global altimetric mean sea surface derived from the geodetic phase of the ESA ERS-1 mission utilising a spectral least squares collocation technique

This paper describes a method that enables a high resolution global Mean Sea Surface (MSS) to be derived by a least squares collocation technique within the frequency domain via the use of the fast Fourier Transform (FFT). The MSS is produced from the entire two cycles of the ERS-1 Geodetic Phase consisting of 336 days of off-line Precise Ocean Product (OPR) data. This data is corrected for tidal and environmental/electronic effects. Long-wavelength radial orbit error is minimised via the fitting of cubic spines across ERS-1 and TOPEX/Poseidon dual crossover residual time series. After further editing, the remaining data is gridded at intervals of 0.075 degrees (a limit imposed by the cross-track spacing at the equator allowing a resolution of 16 kilometers) whilst non-oceanic regions are filled (a requirement of the FFT) with a reference geoid derived from the OSU91A gravity field. The spectral least squares solution draws on the method of objective mapping within the spatial domain by transforming the whole process into the frequency domain thus relinquishing the spatial requirement for matrix inversions. Finally the resultant MSS is compared against both an independently derived TOPEX/Poseidon reference MSS revealing a RMS fit of 17.9 cm and 13.6 cm against TOPEX/Poseidon altimetry itself. TOPEX/Poseidon altimetry had a 17.1 cm fit with the reference MSS.