On the Correlation Between the SAR Backscatter Modulation and Surface Waves Orbital Velocity

A challenging effect in the retrieval of ocean surface currents from synthetic aperture radar (SAR) is the so-called wave-induced Doppler velocity. This effect arises from the correlation between the modulation of the normalized radar cross section (NRCS) caused by the slopes of the long waves and the modulation of the Doppler shift resulting from their orbital velocities. In this article, we analyzed six acquisitions of high-resolution interferometric SAR images of collocated NRCS and Doppler frequency shift acquired by the along-track interferometric SAR TanDEM-X at two polarizations VV and HH. The unique high-quality wave-resolving interferograms of TanDEM-X allow a detailed investigation of the wave-induced Doppler at scales rarely available from space. These images are acquired in different regions, times, and satellite passes (ascending or descending), thus capture different wave conditions in height and propagation direction. First, it is shown that in cases of range propagating waves, the NRCS and Doppler shift modulations are highly correlated. The sign of the correlation coefficient depends on the direction of propagation of the wave relative to the SAR. Moreover, it is shown that the NRCS-Doppler correlation affects the mean Doppler shift and that the wave-induced Doppler velocity increases with the wave steepness. In cases of azimuth propagating waves, the linear correlation is very low due to large phase difference between the NRCS and the Doppler shift modulations. Consequently, the wave-induced Doppler velocity is negligible. Finally, the wave-induced Doppler, if not taken into account, can bias the ocean current estimation by up to 0.96 m/s.