X band microwave backscattering from ocean waves

Backscattering experiments at microwave frequencies were conducted off the west coast of Scotland in the summer of 1991. Using a dual‐polarization, eight‐frequency, X band, coherent scatterometer mounted on the bow of a boat, we measured time‐resolved backscattering from ocean waves at a range of grazing angles from 10° to 70°. From the grazing‐angle‐dependent signals and their Doppler spectra, we are able to differentiate Bragg scattering from non‐Bragg scattering and resolve “peak separation” between the vertical and horizontal polarizations. We observe instances of “super” events, i.e., instances when the horizontal polarization return power equals or exceeds the vertical polarization power at particular frequencies. We find that “super” events occur not only at low grazing angles but at any grazing angle for upwind viewing directions and obtain statistics for such occurrences as a function of grazing angle. We study the coherence properties of scatterers and find strong evidence that at low grazing angles, lifetime‐dominated, non‐Bragg scattering contributes noticeably to returns of both polarizations, but is dominant in providing returns for the horizontal polarization. We examine “spiking” events and find that they can be related to, but need not be limited to, breaking wave events. By comparing the data of upwind runs with cross‐wind and circle runs, we obtain wind direction dependence of Doppler spectra, which further assists in the identification of scattering mechanisms.