Pencil beam radar selectivity of ocean wave spectra

摘要

Over the past several years some very interesting data relevant to ocean wave backscatter have been produced by non scanning pencil beam radars with high range resolution. These radars are located-on land which overlooks the water/sea surface and operate at low grazing angles (LGA). Prominent among these are the X-band RSRE-Thorn EMI and the LLNL-Hughes real aperture radars employed in the Joint US/UK ocean wave experiments in Loch Linnhe and the Sound of Sleat (Scotland). The range-time image obtained with the RSRE radar is typical of the many images which have been obtained. The most prominent features of these images are linear regions or strips of increased average RCS having temporal durations up to or exceeding 100 seconds. For HH polarization these features are marked by the presence of periodic spikes in RCS. The features appear quite consistent with wave groups which are moving approximately parallel to the radar boresight. A self-consistent model of this wave group behavior, as manifested in the radar images, has been proposed by Tulin. This model provides estimates of the wavelengths involved and accounts for the periodicity of the spiky returns which appear to be due to the passage of nonlinear or incipiently breaking waves through the wave groups. Nothing in this model precludes the presence of multiple wave groups moving at different velocities. In fact, some images exhibit wave groups moving at different velocities, and in some cases both incoming and outgoing wavegroups are present simultaneously. Obviously, X-band Bragg waves cannot explain these features because the lifetime of these waves is, at best, a few seconds. The association of these features with the scattering from the crests of much larger (e.g., meters) waves is reasonable because the lifetime of these waves is very long. However, it is not reasonable to assume that ambient wavegroups in the field of view of the radar all just happen to be moving along the radar boresight.