Recent Synthetic Aperture Radar (SAR) work confirms the different ocean surface scattering mechanisms for vertically (VV) and horizontally (HH) co-polarized measurements of the Normalized Radar Cross section (NRCS). In particular, VV modulation is well described by the so-called Bragg scattering of e.m. waves on wind-generated gravity-capillary ocean waves. ASCAT winds, which prove indeed to very closely align with the local wind vector, are based on VV polarization. The HH NRCS is now generally assumed to be the sum of a Bragg part and of a non-Bragg part. The response of the sea surface is then based on the short wave spectrum (wavelength from few millimeters to few meters. As the non-Bragg scattering is associated with breaking waves and hence the ocean mean slope distribution, longer waves than only the short wave spectrum come into play as they modulate the ocean slopes. This implies that the HH NRCS depends both on the local wind and the non-locally generated waves that propagate through the scatterometer Wind Vector Cell (WVC).
The second generation EUMETSAT (EPS-SG) scatterometer, SCA, will however also measure HH and cross-polarized normalized radar cross sections (VH and HV). The aim of this study is to thoroughly analyze sea state conditions in association with HH (and VV) NRCS. The objective is to prepare for EPS-SG SCA, but also to verify all current and past Ku-band scatterometers that actually use HH for wind retrieval. It is expected that the OSI SAF will shortly be processing five Indian and Chinese scatterometers employing HH at Ku- and/or C-band.