# Recent Progress on Vegetation Remote Sensing Using Spaceborne GNSS-Reflectometry
## 4 Model Simulations and Discussion
### Different Vegetation Parameters at Various Polarizations on the Scattering Characteristics  
Whether the vegetation parameters (such as water content and biomass, etc.) can be quantitatively analyzed using GNSS-R is a problem that needs to be solved in the later development of GNSS-R. Based on the observation geometry and polarization characteristics, the corresponding scattering properties (L-band) of different vegetation parameters will be analyzed in this section. Table 4 shows the trunk and branch diameters of four stands. Figure 5 shows the scattering characteristics of various polarizations in the specular scattering. The four stands have different diameters. It can be seen that when the trunk diameter changes, the corresponding scattering characteristics are significantly different. However, the change in the branch diameters has little effect on the scattering characteristics. It is known that the trunk diameter has the most obvious impact on the forest biomass. How to use the GNSS-R receiver terminal signal to effectively separate the forest biomass \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Stand 1** & **Stand 2** & **Stand 3** & **Stand 4** \\ \hline Trunk diameter (cm) & 24 & 30 & 24 & 30 \\ Branch diameter (cm) & 0.7 & 0.7 & 0.9 & 0.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Diameters of Aspen stands. Figure 4: Comparison between coherent and incoherent scattering (**a**) RR polarization, (**b**) LR polarization, (**c**) VR polarization, and (**d**) HR polarization. and use this remote sensing technology to monitor the forest biomass is one of the future development directions. As for different canopy densities (as shown in Table 5), Figure 6 shows the corresponding specular scattering characteristics of various polarizations. Under the above observation geometry, it can be seen from the simulation results in Figure 6 that when the canopy density is different, the scattering characteristics are significantly different. Whether the GNSS-R signal caused by the difference in canopy density can be effectively monitored, and the use of this technology to conduct remote sensing research on vegetation density-related information will be a problem to be solved in the future. Figure 7 shows the specular scattering characteristics of different vegetation water contents (Table 6). For different vegetation water contents, we have employed the vegetation dielectric constant model to calculate its permittivity. The moisture contents of the vegetation scatterer (trunk and branch) are from very high content to low content. It can be seen from the simulation results that when the vegetation water content changes, the  
Figure 5: Specular scattering properties of four stands’ diameters at various polarizations. (**a**) RR polarization; (**b**) LR polarization; (**c**) VR polarization; (**d**) HR polarization. Figure 6: Specular scattering properties of various polarizations at different tree densities. (**a**) RR polarization; (**b**) LR polarization; (**c**) VR polarization; (**d**) HR polarization. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Forest 1** & **Forest 2** & **Forest 3** & **Forest 4** \\  & **(Trees/ha)** & **(Trees/ha)** & **(Trees/ha)** & **(Trees/ha)** \\ \hline Tree density & 2000 & 1000 & 700 & 500 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Tree density for four different forest scenarios. corresponding specular scattering characteristics are different. Whether GNSS-R can be used to effectively detect or invert vegetation water content information will be a technical issue that needs to be broken through in future development. This section has given the bistatic scattering responses corresponding to different vegetation geophysical parameters and observation geometry at various polarizations. From the simulations (Figures 3-7), we can see that as the polarization and scattering geometry change (the same geophysical parameters set constants), there are obvious bistatic scattering properties (Figures 3 and 4).