Ocean winds from RADARSAT-1 ScanSAR

This paper discusses an algorithm designed to retrieve high-resolution wind fields from scanning synthetic aperture radar (ScanSAR) data acquired on board the Canadian satellite RADARSAT-1. The ScanSAR operates at C-band with horizontal polarization. The wind directions are extracted from wind-induced streaks, e.g., from atmospheric boundary layer rolls or wind shadowing, which are approximately in line with the mean wind direction near the ocean surface. The wind speeds are derived from the normalized radar cross section (NRCS) and image geometry of the calibrated ScanSAR images, together with the local wind direction retrieved from the image. Therefore the semi-empirical C-band model CMOD4, which describes the dependency of the NRCS on wind and image geometry, is used. The CMOD4 was originally developed for the scatterometer of the European remote sensing satellites ERS-1 and ERS-2 operating at C-band with vertical polarization. Consequently, the CMOD4 required modification for horizontal polarization, which is performed by considering the polarization ratio. To demonstrate the applicability of the algorithm, wind fields were computed from 20 RADARSAT-1 ScanSAR wide-swath images and compared to co-located results from the Danish high-resolution limited-area model (HIRLAM). In addition, the error sources in ScanSAR wind retrieval are discussed and sensitivity studies were carried out to estimate wind speed errors due to uncertainties in the NRCS, wind direction, and incidence angles.