GNSS-R Modeling Results Obtained with Improved Bistatic Radar Equation

New modeling results obtained with an improved version of the bistatic radar equation (BRE) are discussed. This version of the BRE employs arbitrary signals of opportunity and assumes a wider range of surface scattering and kinematic regimes. It contains two terms, one describing the coherent, and another, noncoherent, diffuse component. The latter term can result from any scattering model, not necessarily from the Kirchhoff Approximation/Geometric Optics model as in the previous version.The another limitation of that version pertains to a formulation of the Woodward Ambiguity Function (WAF). It was based on an assumption that all spectral components of the signal experience the same Doppler shift due to kinematic effects. However, for the signals with a significant off-nadir incidence the Doppler shift will affect various spectral components slightly differently leading to a distortion of the WAF. This phenomenon should be taken into account in altimetric measurements from low-Earth orbit platforms performed at incidence angles significantly deviating from nadir.The previous version of the BRE was limited only to the case of strong diffuse scattering, whereas the improved BRE allows computation of the Delay-Doppler Maps (DDM) also for the case of weak surface scattering which might be accompanied by the coherent component. The latter becomes an important feature in the case of the forward bistatic radar scattering from rivers, lakes, small bodies of water, wetlands, sea ice, etc.The mechanism of weak-to-moderate diffuse scattering differs from that of strong diffuse scattering. It involves scattering from surface roughness with spatial scales shorter than the high-frequency cutoff of strong diffuse scattering. For example, the modeling of ocean GNSS reflections shows sensitivity to rain surface effects at low winds.