# Recent Progress on Vegetation Remote Sensing Using Spaceborne GNSS-Reflectometry
## 3 Existing Problems
### Polarization Characteristics  
Different from the traditional microwave remote sensing techniques, the GNSS-R employs the RHCP to overcome the ionospheric influence. Meanwhile, the polarization characteristics of the electromagnetic wave will change after the direct signal of the navigation satellite interacts with the vegetation parameters. The GNSS-R technique is firstly applied to the ocean surface, which is more uniform than the land surface and the surface polarization characteristics are not obvious. As for the in-situ measurement and spaceborne missions, the LHCP polarized antenna is used for the collection of the reflected signals. Early ground-based, airborne, and other land surface experiments have repeatedly tried to discover the difference in polarization characteristics. In the earliest SMEX02/03 airborne GPS-R experiment [55], it is believed that the reflected energy signal on the surface is mainly LHCP, so the RHCP antenna is used to receive the direct signal from the navigation satellite, and the LHCP is used to receive the reflected signal from the surface; meanwhile, the subsequent BAO (Boulder Atmospheric Observatory) tower experimental receiver uses RHCP to receive direct signals. In order to effectively use the reflected signals and reduce the impact of surface roughness on the accuracy of soil moisture inversion, a low-gain LHCP antenna and four high-gain LHCP, RHCP, H, and V polarized antennas are used to receive ground reflection signals [56]. In order to effectively use the Brewster angle and improve the inversion accuracy, the ground-based experiment of UPC University uses SMIGOL and PSMIGOL receivers, and the antenna adopts V polarization and H polarization [28]. The SAM receiver in the LEiMON experiment uses RHCP and LHCP antennas to receive ground reflection signals [57]. In terms of spaceborne, the receiver in SMAP-R is the radar in the SMAP satellite. Its receiver antenna uses the V and H linear polarizations; both TDS-1 and CYGNSS satellites use LHCP antenna to receive the reflected signal from the surface. It can be seen from the above that the polarization of the receiver antenna is changing. There is no clear determinism to indicate which polarized receiver antenna is most beneficial for vegetation monitoring. Therefore, for the monitoring and retrieval of vegetation biomass, it is urgent to carry out research on the polarization characteristics of GNSS-R [51; 53]. It is necessary to point out the difference in polarization from the theoretical mechanism level, and carry out the parameterized models based on polarization characteristics [58]. Vegetation shows different scattering characteristics to different polarization electromagnetic waves, and the scattering mechanism can be described by surface scattering, dihedral angle scattering, and volume scattering. Flat bare soil is prone to surface scattering, while large tree trunks and rough ground usually have dihedral angle scattering. Vegetation such as forest, crop, and shrub are easy to produce volume scattering. [PERSON] et al. simulated and analyzed eight scattering mechanisms (including G-C-G (ground reflection and cross scattering and ground reflection), C-G (cross scanning and ground reflection), DC (direct cross bistatic scattering), G-C (ground reflection and cross scattering), and SG (the specific ground reflection)) [51]. Different scattering characteristics in different vegetation parameters and observed geometry and system parameters have also been simulated. These research results provide a certain theoretical support for the follow-up study of polarization GNSS-R remote sensing. In the traditional SAR remote sensing, there is a polarimetric SAR imaging system, and with the development of GNSS-R technology, the future polarimetric GNSS-R observation satellite also has potential feasibility. Polarization decomposition is an important technical means to reveal the scattering mechanism of ground objects. Polarization decomposition features play an important role in identifying the types of ground objects.