Postprocessing of satellite altimetry return signals for improved sea surface topography accuracy

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Large off‐nadir attitude deviations and high surface wave heights cause an alteration in the ocean return signal from a satellite radar altimeter. This leads to an error in the on‐board calculation of the height measurement. The error can be removed by reprocessing the full radar return signal on the ground. In the ground processing, the correct tracking point in the return signal is recomputed through a procedure called retracking. There has been a controversy over whether or not all altimeter data would be retracked. This study analyzes retracked southern ocean data from the first 34 repeat cycles of the Geosat Exact Repeat Mission (ERM), covering November 1986 through April 1988. The final data set consists of over 2.5 million smoothed one‐per‐second measurements of the ocean surface. The significant wave height (SWH) distribution as given on the NOAA geophysical data records (GDRs) for these measurements peaks at around 2.1 m (19% of the measurements) and drops down almost linearly to 2% of the measurements at 5.8 m. There are over 1100 observations with SWH greater than 15 m. The difference between the surface heights calculated from the retracked data and the original on‐board estimates is less than 10 cm for SWH less than 10 m but increases to approximately 1.0 m at a SWH of 18 m. In general, the electromagnetic (EM) bias coefficient calculated using the retracked data is slightly less than that using the unretracked data and does not decrease as much with SWH as do the EM bias coefficients calculated from the unretracked data. A map of the sea surface height variability of the southern ocean created using the retracked data shows differences from variability maps created using the unretracked data in regions of high wave heights. Retracking can be done efficiently on modern UNIX work stations at 0.064 times real‐time acquisition. This study shows that retracking will improve altimeter precision.