Abstract :
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 - he 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.
Keywords :
Global Positioning System; geodesy; geophysical techniques; polarisation; radiative transfer; remote sensing; scattering; soil; vegetation mapping; GNSS constellation reflected signal; GNSS-R application; GNSS-R measurement interpretation; GNSS-Reflectometry multipath forward scattering model; GPS multipath signal theoretical modeling; GPS observable amplitude; Global Navigation Satellite System-Reflectometry; antenna default input parameter; bare soil; bare soil GPS observable comparison; bare vegetation; bistatic scattering model; circular polarization calculation; circular polarization property; code modulation focusing; dielectric soil constant; direct signal elimination; final simulated multipath signal; first-order radiative transfer equation model; forward multipath model; forward multipath model surface scattering; forward multipath simulator; fully polarimetric forward model; geophysical parameter detection; interference signal; low vegetation; microwave remote sensing technique kind; multipath estimation; multipath factor; multipath interaction physical explanation; multipath reflected signal; original Fresnel coefficient calculation; original Fresnel reflectivity; phase difference; physical forward multipath single-scattering model; receiver default input parameter; reflected signal enhancement; reflected surface scattering property; semiempirical model; smaller elevation angle; special GNSS-R receiver kind; specular reflectivity model; surface coherent scattering model; traditional off-the-shelf geodetic GPS receiver; typical multistatic radar working mode; wave synthesis technique; wheat GPS observable comparison; wheat-covered surface; Global Positioning System; Mathematical model; Reflectivity; Remote sensing; Scattering; Soil; Vegetation mapping; GNSS-R; Multipath; SNR; Specular reflectivity; Wave synthesis; radiative transfer equation model;