Synthetic Aperture Radar for ocean current feature retrievals and surface velocity estimates

Estimating currents from SAR imagery is still research-dominated. Most of the work has been in development of improved forward radar imaging models to predict the SAR signatures of regions with changing oceanic currents. In the latest radar imaging model (RIM) by Kudryavtsev et al., (2005) detailed quantitative characterizations of the surface roughness modulations are provided based on a priori description of the wind and current field. In turn, the total surface scattering properties can be quantified and used to predict the normalized radar cross-section (NRSC) measured in SAR images. A new technique for direct estimation of surface currents based on the Doppler shift in complex-valued SAR imagery has also recently been proposed by Chapron et al., (2 005). The Doppler shift is introduced by the relative motion between the satellite platform, the rotation of the Earth and the velocity of the facets of the sea surface from which the backscattered SAR signal originates. The initial two former effects are well known, particularly for Envisat with its very stable satellite orbit and attitude, and can be subtracted to extract the surface Doppler velocity information. This surface velocity, in turn, is composed of contributions from the wind-wave induced mot ion, and the background surface current. Following the Doppler shift equation, an estimate of the surface current can be obtained, provided the contributions from wave- and wind-induced motion are quantified and removed. This is feasible by joint use of t he RIM that simulates radar cross-section signatures and the total Doppler velocity as well as its respective contribution from the motion of the scattering facets and roughness anomalies. The remaining shift must then come from the surface current projec ted in the line-of-sight of the radar instrument. The full integration, testing and validation of the combined forward and inverse methods form a major part of the SARCURE project.