Monitoring Ocean Surface Current From Spaceborne SAR Doppler Shift: Progress and perspective

High-precision retrieval of ocean surface current (OSC) is critical for maritime applications. Spaceborne synthetic aperture radar (SAR) provides effective, high-resolution, all-weather, and all-day monitoring of sea-state parameters. The Doppler centroid (DC) recorded by singleantenna SAR data has been extensively used to monitor and map ocean surface radial currents. This article presents a systematic review of the last three decades of research on the DC anomaly (DCA) method, covering key topics including radar imaging mechanisms of ocean current remote sensing, DC estimation, nongeophysical corrections, and the estimation and calibration of the wind-wave-induced artifact surface velocity (WASV). Sentinel-1 Level-2 Ocean Radial Velocity (S1 L2 OCN-RVL) products, together with collocated high-frequency (HF) radar data, are further used to illustrate the workflow and applicability of the DCA method. We recommend that prospective studies focus on enhancing the performance of geophysical model functions (GMFs) under extreme sea states, reconstructing the 2D OSC vector, and the synergistic use of multimission satellite observations. Data-driven artificial intelligence (AI) techniques have enhanced the accuracy of OSC retrieval. However, their robustness remains limited by scarce in situ observations, insufficient physical constraints, and inherent uncertainties. Developing near-real-time high-accuracy OSC products from SAR Doppler shift remains challenging.