Correlation between the Backscatter and Doppler Shift Modulations in SAR Images and Its Effect on the Doppler Centroid Estimation

Paper in proceeding, 2023

A known and challenging effect in the retrieval of ocean surface currents from synthetic aperture radar (SAR), is the so called wave-induced velocity bias. This effect is due to the correlation between the normalized radar cross section (NRCS) and Doppler shift modulations, which are generated by the long modulating waves. This paper analyses two collocated NRCS and Doppler frequency shift images acquired by the along-track interferometric SAR TanDEM-X at two polarizations VV and HH. These images capture a swell traveling nearly in the range direction. First, it is shown that the NRCS and Doppler shift modulations, due to the modulating swell wave, are highly correlated. Second, it is shown that this correlation affects the mean Doppler shift. Consequently, the ocean current estimation can be biased by approximately 0.2 m/s, which can be significant at low current magnitudes and when high-accuracy current retrievals are required.