GNSS-R Land Surface Remote Sensing Scattering Properties Study

GNSS-R is a new promising remote sensing technique, in which the special designed receivers collect the reflected signals of GNSS. As for ocean scenarios, GNSS-R can be used to retrieve ocean surface wind, sea wind height, tide, sea salinity, among others. For land scenarios, low cost, small power, high spatial and time resolutions are the advantages, so this technique is considered to be a promising tool to retrieve land surface geophysical parameters, such as soil moisture and vegetation height or biomass. GNSS-R land surface scattering characteristics are studied in this dissertation in order to make GNSS-R land surface parameters retrievals practical.Constellation of GNSS is transmitters, the special designed GNSS-R receivers collect the land or ocean surface reflected signals. Therefore, it is a kind of bistatic radar, for which bistatic scattering models are needed. For bare surface, the original used scattering model PO, GO and SPM models are modified to bistatic scattering. Bi-mimics model is developed based on the original backscattering Mimics. By adding scattering zenith angles and azimuth angles in Mimics, the corresponding angles in phase matrixes and extinction matrixes are needed to be modified. Meanwhile the angles in ground reflectivtity matrix and scattering matrix are modified. Compared to ocean surface, roughness scales of land surface are small, so it is commonly recognized that the reflected signal is largely coming from the specular reflection point of first Fresnel zone. The diffuse scattering power is too weak to be received. The angles in the above bisatic scattering models are set to specular ones to do research. As for vegetation study, Spec-mimics is developed and utilized as a study tool to analyze vegetation (crop and forest) biomass. Theoretical studies indicate that GNSS-R has the potential to monitor biomass.In order to reduce ionospheric effects, the GNSS transmitted signals are right handed circular polarization, whose polarization characteristics are changed after reflecting from the earth surface. Therefore the corresponding polarized attenna of receiver should be carefully considered to decrease signals loss. This dissertation focuses on the land surface reflected signals polarization charateristics study. Modified stokes vectors are modified in bare surface scattering model and the above Bi-mimics model, Spec-mimics model to calculate bistatic or specular scattering cross sections for any combination of transmitter and receiver polarizations. Aspen in Mimics handbook is used as model input, relationship between scattering coefficients of RHCP transmitted, linear(H,V) and circular(RHCP, LHCP) polarized received versus scattering angles are analyzed. Theoretical study indicates that as for RHCP transmitted signals, the LHCP reflected signals (bare surface or vegetation) exist only in larger incident angles (smaller elevation angles). But the reflected signals exist in any incident angles as for linear polarized (H, V) reflectivity, where V polarization has larger dynamic range. In conclusion, when the receiver can track signals at any elevation angles for one or several satellites(such as SMIGOL), the V polarized receiver antenna is favored; LHCP attenna respond only at some angles when only the highest elevation angle can be tracked(such as DMR used in SMEX02airborne GPS-R experiment).However, further in-situ experiments are needed to calibrate the above theoretical results. And practical constructions of GNSS-R land surface retrieval and monitoring network are the focuses and difficulties of future study.