A known and challenging effect in the retrieval of ocean surface currents from synthetic aperture radar (SAR), is the so called wave-induced Doppler velocity. This effect is due to the correlation between the modulation of the normalized radar cross section (NRCS) due to the slopes of the long waves and the modulation of the Doppler shift due their orbital velocities. In this paper, we analyzed four acquisitions of interferometric SAR (INSAR) 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 swellresolving 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 swell conditions in height and propagation direction. First, it is shown that the NRCS and Doppler shift modulations, due to the modulating swell wave, are highly correlated. The sign of the correlation coefficient depends on the direction of propagation of the swell 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 slope. Consequently, this effect, if not taken into account, can bias the ocean current estimation by up to 0.96 m/s.
