Coherent marine radar measurements of properties of ocean waves and currents

Marine radars offer the capability to image ocean wave propagation by virtue of repetitive coverage of the same scene. With a typical 1.25-s rotation period, ocean wave frequencies of 0.4 Hz can be measured unambiguously. Imaged areas of the order of ten square kilometers, allow the dynamics and kinematics of ocean wave fields to be measured with higher azimuthal resolution than traditional oceanographic instruments, such as buoys or pressure sensors. Using the dispersion relation for shallow waves in coastal regions, ocean wave spectra and bathymetry can be estimated using non-coherent marine radars. The derivation of wave height or wave height spectra from marine radar imagery has had some success by relating the radar echo intensity imagery of waves to wave height using an empirically derived modulation transfer function (MTF). More recently, using a radar scattering model´s dependence of the radar scattering cross section on long wave slope, good results are reported in deep water for shipboard experiments, where winds and waves are typically in the same direction. However, in coastal waters, offshore winds blowing in a direction other than that of the incoming wave field can produces enhanced roughness on the front face of waves, resulting in a modulation of the radar wave field image that is not wave height dependent. A coherent radar can overcome these limitations using the direct measurement of the radial component of orbital wave velocity, accounting for Bragg scatter velocity. Radial orbital velocity will maximize and minimize at similar locations on long wave profiles as do radar echo intensity, so wave patterns should look very similar for the two. Thus, analysis methods similar to those used in non-coherent radar studies should be applicable to coherent radar data as well. We present recent experimental results on the retrieval of coastal ocean wave and current properties due to a storm over the Outer Banks of North Carolina in November of 2009.