Anisotropy in ocean scattering of bistatic radar using signals of opportunity

This paper present experimental results demonstrating the use of “signals of opportunity” from digital communications satellites (XM radio) as bistatic radar for ocean remote sensing. This builds upon the previous work which demonstrated that the shape of the cross-correlation “waveform” of reflected XM radio signals is sensitive to the roughness of the ocean surface. In these new results, we compare this sensitivity between the waveforms produced from the two XM radio satellites, viewed simultaneously at different azimuths, and show that a small discrepancy exists in the mean square slope (MSS) retrievals obtained from each of them. We then investigate the hypothesis that this discrepancy is the result of neglecting anisotropy in the model for the probability density function (PDF) of surface slopes and that this discrepancy might be useful for sensing the wind direction. In order to do so, a two-stage estimation process was applied to data collected on an airborne experiment that recorded the direct line of sight and reflected XM radio signals. In the first step, an isotropic normal distribution was assumed for the PDF and the mean square slope (MSS) was fit to the measured waveform data from each satellite independently. Since the two satellites are located at different azimuths, a difference between the two MSS estimates were observed. The second step involved using a bidirectional normal PDF with MSS constrained to that obtained from the first step, and a value was assumed for the ratio of upwind and crosswind slopes. The direction of the principal axes was varied to minimize the total residuals for both satellites. The results were compared with Chesapeake Lighthouse recordings of the local wind direction.