The gate dependence of geophysical retrievals from the TOPEX altimeter

Altimeter waveform data over the oceans are routinely processed to give sea surface height, h, significant wave height, Hs, and normalized signal strength, σ0 (from which wind speed is inferred). The onboard processing of TOPEX altimeter waveform data makes use of a series of different “gates” (averages of waveform bins) according to the wave height conditions being encountered. This formulation was to facilitate the rapid processing of data on board the satellite. Although the processing aims to make a seamless connection between the use of different gates as appropriate, there are various discrepancies in performance, which become manifest when the users are attempting to detect the fine differences associated with global environmental change. Here a brief overview is given of the various effects of the choice of gate index on the derived geophysical parameters. The Hs distribution is shown to be affected by the transition between gate indices, and the C-band σ0 values exhibit a step change there too. The positioning of waveforms within the reception window is also gate dependent, but the routine TOPEX ground processing compensates for that. Improved editing criteria (based on the standard deviations of h and Hs) are proposed.