Ocean Surface Wind Retrieval From Dual-Polarized SAR Data Using the Polarization Residual Doppler Frequency

An alternative approach to sea surface wind retrieval using synthetic aperture radar (SAR) stripmap (SM) data is explored in this paper, using both the polarization residual Doppler frequency (PRDF)-the difference between the VV and HH Doppler centroids-and the normalized radar cross section (NRCS). The PRDF not only enables the possible elimination of unwanted biases present in both VV and HH Doppler estimates but also helps decrease the number of wind ambiguities down to two. In order to successfully infer the wind field from the PRDF, the use of a geophysical Doppler model function, such as the general curvature model (GCM)-Dop, is necessary. Using such a function, a simulated version of the PRDF at X-band is analyzed in terms of the sea surface wind field at various incidence angles. Simulations first show that the PRDF increases with increasing incidence angle regardless of wind field conditions. Actual PRDF measurements also exhibit strong correlation with radial components of the wind speed. An alternate SAR wind retrieval procedure, incorporating both the PRDF and the NRCS, is tested on a series of dual-polarized SM TerraSAR-X scenes carefully selected along the Norwegian coast. The geophysical model functions used for this analysis are the GCM-NRCS and GCM-Dop. A 1.13-m/s bias, with a correlation of 0.85 and a 1.86-m/s rmse, exists between the mean estimated wind speeds and in situ measurements, while a 15.43 degrees bias, with a 0.93 correlation coefficient and a 34.1 degrees rmse, is found between the mean estimated wind directions and in situ measurements.