Diffraction effects in RADAR illumination of sea waves

Detailed calculations have been undertaken of the effect of diffraction on the measurement of wave profiles using X band RADAR using path integral methods. The focus here is upon determining the surface illumination of the sea surface by the RADAR in the regions classically referred to as wave shadowed segments of the wave profile. This emphasis is because wave shadowing is accepted as a major source of errors in X Band wave RADAR based estimates of sea wave profiles. This work aims to evaluate the difference between the radar illumination intensities of the sea wave surface estimated from ray optics compared to a full diffraction-based calculation. It is found that the simple ray optics approach used in the majority of treatments of wave shadowing produces very misleading results. Both the illumination profile of the sea surface and the absolute level of illumination are markedly changed in wave shadowed regions when diffraction effects are included. No attempt is made to calculate the backscattered signal as there is no known method of determining the detailed spatio-temporal structure of the capillary waves that produce this scattering. Hence, the goal here is to highlight the problems associated with using the traditional geometric optics or related approximation methods to determine the sea surface illumination intensity. Clearly, if the illumination intensity is in error in certain regions then the RADAR backscatter data and hence the wave profile estimates cannot be relied upon.