Extrapolation of electromagnetic pointing error corrections for Sentinel-1 Doppler currents from land areas to the open ocean

Abstract

By utilizing single-look complex data from synthetic aperture radar (SAR), the Doppler centroid anomaly method (DCA) can be employed to determine the velocity of the sea surface current field along the radar line-of-sight direction. However, due to the influence of platform or antenna perturbations, SAR Doppler shift measurements are subject to electromagnetic pointing error (EPE). These errors can typically be reduced by using echo information from land areas (including islands) within the SAR images to correct the Doppler shift. This allows the successful estimation of Doppler shifts due to sea surface factors and, in turn, the retrieval of ocean currents in the radar line-of-sight. However, in cases where SAR images lack data over land, uncorrectable EPE can result in significant inaccuracies in the retrieval of ocean currents. This study utilizes data captured by Sentinel-1A on four orbits in the Florida Strait region of the Gulf Stream from July 2020 to October 2022 to investigate the variation characteristics of EPE. To address the issues of calculating these in SAR images lacking land data, an EPE fitting method is proposed. The correction significantly reduces the error of S1A retrieved current velocity compared with line-of-sight current velocity from the HYCOM model and in situ HF radar, indicating that the proposed method provides an effective way to reduce the impact of EPE in SAR images, even when land is not directly observed.