This paper presents a geophysical model function (GMF) that has been developed to describe the relation of the ocean surface wind with the normalized radar cross section (NRCS) at C-band cross polarization. Synthetic aperture radar (SAR) images have been collected simultaneously at co-polarization (co-pol) and cross polarization (cross-pol) at moderate to high wind speeds. Using the SAR co-pol retrieved wind fields as well as an uncertainty estimate of the retrieved wind speeds, the cross-pol dependencies of the NRCS are investigated with respect to wind, incidence angle and polarization pairs. For wind speeds above 10 m/s there is a significant dependence of the NRCS on wind speed. However, the SAR cross-pol data are also significantly affected by the noise floor and cross talk between the channels. Estimates of the noise floor are determined and removed from the NRCS. These observations resulted in the development of three GMFs: the first is for transmission at horizontal (H) polarization and the second at vertical (V) polarization. A third GMF accounts for wind direction dependence. A fourth GMF accounting for the wind direction dependence for V polarization could not be estimated due to the lack of SAR data. Validation of the GMFs is conducted by comparison to collocated Stepped Frequency Microwave Radiometer (SFMR) data. The resulting bias of-0.7 m/s and standard deviation of 3.7 m/s demonstrates the excellent performance for these GMFs for wind speed retrieval between 10 and 35 m/s. Furthermore, comparisons show that SAR cross-pol retrieved wind speeds are of similar quality as those of SFMR and are significantly better in the moderate to high wind speed regime than SAR co-pol retrieved winds.
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