The general methodology to derive the sea surface wind field from SAR is to apply an empirical Geophysical Model Function (GMF) to the calibrated SAR image using wind direction from external sources or wind streaks visible in SAR

 Abstract— A Geophysical Model Function (GMF), denoted XMOD2, is developed to retrieve sea surface wind field from Xband TerraSAR-X/TanDEM-X (TS-X/TD-X) data. In contrast to the previously developed XMOD1, XMOD2 consists of a nonlinear GMF, and thus, it depicts the difference between upwind and downwind of the sea surface backscatter in X-band SAR imagery. By exploiting 371 collocations with in situ buoy measurements which are used as the tuning dataset together with analysis wind model results, the retrieved TS-X/TD-X sea surface wind speed using XMOD2 shows a close agreement with buoy measurements with a bias of -0.32 m/s, an RMSE of 1.44 m/s and a scatter index (SI) of 16.0%. Further validation using an independent dataset of 52 cases shows a bias of -0.17 m/s, an RMSE of 1.48 m/s, and SI of 17.0% comparing with buoy measurements. To apply XMOD2 to TS-X/TD-X data acquired at HH polarisation, we validate three X-band SAR Polarisation Ratio (PR) models that were tuned using TS-X dual polarisation data by comparing the retrieved sea surface wind speed with buoy measurements.