A multipath forward scattering model of GNSS-Reflectometry from bare soil and vegetation

In the past two decades, GNSS-R (Global Navigation Satellite System-Reflectometry) has been emerged as a new kind of microwave remote sensing technique. The reflected signals of the GNSS constellations and the direct ones form the typical bistatic or multi-static radar working mode. While, for one side, a new kind of special GNSS-R receivers should be developed to enhance the reflected signals. For the other side, the traditional off-the-shelf geodetic GPS receivers can be used. For the latter ones, their observations contain the interference signals of the direct and reflected signals. The reflected signals as multipath can be used for the detections of the interested geophysical parameters after removing or eliminating the direct signals. However, nowadays, the popular multipath estimations are either empirical ones or focusing on the code modulations, which are not adequate for the physical explanations of the multipath interactions, especially for GNSS-R applications. This paper focuses on developing the physical forward multipath single-scattering model, which is a fully polarimetric forward model. Since the scattering properties of the reflected surface are one of the most important factors for the multipath, it is necessary to develop the surface scattering model for the forward multipath model. However, for the circular polarization properties, the calculation of the original Fresnel coefficient is substituted by the specular reflectivity model, so not only the magnitudes are changed, but also the phase differences are changed. Here, the wave synthesis technique is used to calculate the circular polarization. Dielectric constants of soil are calculated using the semi-empirical model. The surface coherent scattering model is added into the forward multipath model. The default input parameters of the antenna and receivers are used. As for low vegetation, the bistatic scattering model based on the first-order radiative transfer equation model is replaced with the original Fresnel reflectivity and inserted into the forward multipath simulator. Comparisons of GPS observables for bare soil and wheat are also done. The oscillations and the final simulated multipath signals and their dynamic ranges are all acceptable. The amplitudes of GPS observables for wheat-covered surface are lower than bare soil for smaller elevation angles(<;30°) and they become larger while the elevation angles are larger(>30°). Theoretical modeling of the GPS multipath signals shows that it can be an efficient tool for the interpretation of GNSS-R measurements.