# Recent Progress on Vegetation Remote Sensing Using Spaceborne GNSS-Reflectometry
## 2 Research Status
### Theoretical Model Research  
The Z-V scattering model is a GNSS reflected signal model widely used in the GNSS-R field and it was first applied to the ocean surface [38]. This model is a function of delay and Doppler and it establishes a physical connection between the observed geometric parameters, environmental parameters, and the output observations of the receiver. The essence of the model is the integral form of the bistatic radar equation, which uses the geometric optics limit of the Kirchhoff approximation within the framework of the two-scale surface model to obtain the ocean surface scattering properties. Using this model, a detailed numerical simulation of the waveform has been carried out, and the calculation results show that as for the steep and moderate elevation angle, the scattered signal on the opposite RHCP is much lower. However, signals on RHCP and LHCP start to converge for low-grazing angles. In the traditional PNT applications of GNSS, reflected signals are considered as harmful signals and should be eliminated. Therefore, most forward physical models focus on code modulation, or use arbitrary values, or use semi-empirical values to determine and define the values of reflected energy, phase, and delay. For GNSS-IR (GNSS-interferometric reflectometry) remote sensing, [PERSON] and [PERSON] have developed a forward GPS full-polarization multi-path model (N-L model) [39], which is based on the physical model of [PERSON] et al. [38]. This model considers the coherence of the direct signals and the reflected signals, while the response of the receiver antenna and reflecting surface are also fully considered. The model assumes that the received energy comes from the specular reflection of the first Fresnel zone, and the reflectivity of the LR or RR polarization is obtained by changing the dielectric constant. The circular polarization reflectivity is obtained by the linear combination of the horizontal polarization and vertical polarization Fresnel reflection coefficients. Research based on this model and in-situ measurement point out that changes in bare soil, vegetation, and snow parameters will cause changes in GPS interferogram metrics (effective reflectometer height, phase, and amplitude) [40; 41; 42]. Researchers have used GPS site data in the PBO network to study the terrestrial parameters. Based on the Nievinski-Larson forward GPS multi-path model, the article used measured data to test the relationship between SNR data and near-surface soil moisture and pointed out that soil moisture on the top 5 cm surface has a significant impact on SNR data, and soil texture has almost no effect on GPS interference signals [41]. [PERSON] et al. have analyzed the influence of canopy parameters on GPS SNR data, and their theoretical model was successfully verified with measured vegetation data [43]. [PERSON] et al. have used the discrete Tor Vergata model to simulate and analyze the feasibility of GNSS-R for forest biomass [22]. This model assumes an ensemble of dielectric elements above the random rough surface. As for the case of forests, the elementary scatterers are cylinders, disks, and needles: the trunks and branches are modeled as cylinders, while the broad leaves are modeled as disks and the conifer leaves are simulated by needles. It is pointed out that unlike the phenomenon of biomass saturation in mono static radar monitoring, it is not prone to saturation when using GNSS-R technology [22]. The model simulation found that the coherent energy is much larger than the incoherent energy. It is believed that the energy obtained by the receiver mainly comes from the specular reflection of the first Fresnel zone. Therefore, the article focused on the analysis of specular scattering characteristics. A deciduous forest site in Hawaii is employed as the in-situ measurement to test the monostatic model and then the simulations in the specular direction are carried out to analyze the results.