Validity of the Kirchhoff-Geometric Optics Approach for Modeling of Ocean Bistatic Radar Scattering

The Kirchhoff approximation taken in geometric optics limit (KAGO) is a very robust and fast way to calculate the forward bistatic radar scattering from the rough ocean surface. The normalized bistatic radar cross section (NBRCS) in the KAGO can be characterized by the mean square slope of the rough surface smoothed over spatial scales smaller than some empirically introduced scale. This method produces satisfactory results in many practical cases, although it has certain limitations. Here we use a more accurate method, the small slope approximation of the first order (SSA1) to check the validity of the KAGO approximation.Applying this method, we performed calculations of the NBRCS for the L-band circularly cross-polarized signal in the forward direction for a range of incidence angles and several winds. The empirical Elfouhaily et al. ocean surface spectrum is used here. It is artificially modified by suppressing, or enhancing its spectral density beyond the variable cutoff wave number used in the geometric optics method.The NBRCS vs incidence angle dependencies calculated by the SSA1 in the strong diffuse regime (a large Rayleigh parameter) show some sensitivity to short surface waves which correspond to wavenumbers higher than the cutoff number traditionally used in bistatic scattering models based on the KA-GO.