# Recent Progress on Vegetation Remote Sensing Using Spaceborne GNSS-Reflectometry
## 2 Research Status
### Theoretical Model Research
The relationship between the LR polarization energy and the biomass is analyzed using the bistatic radar formula, and it is pointed out that when the biomass increases, the LR polarization energy decreases [22]. They also pointed out that for the RL polarization, the specular measurements at low incidence angles are more suitable for forest applications. Dr. [PERSON] extended the Z-V model to the application of soil moisture [44], he pointed out that the forward scattering from a land surface was similar to scattering from ocean surface; they thought the theoretical simulations presented in ocean surface were also applicable to land surface. The main difference between ocean surface and land surface is the dielectric constant of the targets, as for land surface, surface roughness and vegetation cover are two main factors that affect the scattering properties. Their simulations indicate that the width of the correlation waveform on a flat and bare soil surface is much narrower than from a rougher ocean surface. They also carry on three aircraft fights specifically to collect land reflections and verify their theoretical fundamentals. During the LEiMON project, SAVERS (The Soil And Vegetation Reflection Simulator) model based on the framework of the Z-V model has been developed [45]. The signal correlator power output is a function of the time delay and the Doppler shift, i.e., DDM. Matlab and Fortran are used to develop the SAVERS model. The model is divided into two parts, one is the observation geometry calculation module, and the other one is the ground reflection characteristic calculation module. The Fung and Eom coherent scattering coefficients and Advanced Integral Equation Models are used to calculate the coherent and noncoherent scattering characteristics of bare soil, respectively. The Tor Vergata model is used to calculate vegetation scattering. After confirming the position and speed information of the transmitter and receiver, parameters are put into the geometry module to get the specular reflection point, meanwhile the antenna pattern and iso-Doppler are obtained. Combined with the scattering coefficient module, i.e., to get the reflected surface scattering properties, the final delay Doppler map module is obtained. From the simulator, we can acquire the DDM parameters and then by extracting the peak and waveform information, we can obtain the reflected surface properties. This simulator can provide the intermediate quantities (such as the geometrical parameters and the bistatic scattering coefficient) and the final DDM, the waveform values for all transmitter positions are also available. At the same time, the article uses LEiMON experimental data to verify the model results. They simulate the influence of surface parameters and observation geometry on GNSS-R received energy and take into account factors such as antenna polarization mismatch and cross-polarization isolation through the method of polarization synthesis. The simulations indicate that SAVERS can reproduce very well the ground surface under different situations at LR polarization and low incidence angles. It should be noted that the results at RR polarizations are less satisfactory. One potential reason is that the instrument produced a saturation effect under a certain value. SCoBi-Veg (Signals of Opportunity Coherent Bistatic Scattering Model for Vegetated Terrains) is a bistatic model for SoOP-R vegetation remote sensing applications [46]. It can simulate the polarization reflection of vegetation canopy on flat ground. Monte Carlo is employed in their work to obtain a large number of realizations. The model assumes that the incident signal source is a navigation satellite or digital communication satellite signal, and the frequency covers the P-band and S-band. Analytical wave theory and discrete Born approximation are used in the model to explain the effects of transmitting and receiving antennas and surface scattering in detail (i.e., polarization crosstalk/mismatch, orientation, and altitude).