Impact of the antenna characteristics on sea surface parameters estimated from low- and high-resolution satellite altimetry

In this study, we present an extension to existing numerical retrackers of synthetic-aperture radar (SAR) altimetry signals considering a non Gaussian antenna characteristic. To our knowledge at the time of writing, this manuscript presents the most consistent retrieval of geophysical parameters compared to Low Resolution Mode (LRM) retracking results. The novelty is an approximation of the theoretical antenna pattern with a sum of three Gaussian functions to mitigate the sea surface height estimation errors for Sentinel-3A and Sentinel-6A in SAR mode. Additionally, we explain offsets in the derived closed-form equation and describe the mean along-track water velocity $u_x$ (later mean line of sight velocity) as function of eastward and northward wind This allows us to mitigate the effects of $u_x$ in a SAR stack and a lookup table is generated to correct the sea surface height estimates in SAR mode. Further on, we discuss how this new approach performs with respect to different antenna pattern implementations by processing five months (cycles 72 to 76) of Sentinel-3A and six months (cycles 17 to 42) of Sentinel-6A level 1A data on a global scale. We observe that the new retracking method is, on average, more accurate with respect to LRM. To ensure that the results of the new retracker are not biased, we retrack using the new and the current state-of-the-art method on Sentinel-3A and Sentinel-6A data produced in a Monte Carlo simulation. We analyze the simulation results with respect to the accuracy of sea level anomalies as function of the significant wave height (SWH), considering as reference the LRM sea level anomalies. We show that the accuracy of the new antenna characterization is higher compared to a single Gaussian approximation.