# Recent Progress on Vegetation Remote Sensing Using Spaceborne GNSS-Reflectometry
###### Abstract  
Vegetation is an important part of the terrestrial ecosystem and plays a vital role in the global carbon cycle. Traditional remote sensing methods have certain limitations in vegetation monitoring, and the development of GNSS-R (Global Navigation Satellite System-Reflectometry) technology provides a new and complimentary method. With the CYGNSS (Cyclone Global Navigation Satellite System) launch and the increased data acquisition, the use of spaceborne GNSS-R for vegetation monitoring has become a research hotspot. However, due to the complex characteristics of vegetation, its application in this field is still in the exploratory research stage. On the basis of reviewing the current research status, this paper points out the weak links of this technology in terms of polarization and observation geometry. Combined with the microwave vegetation scattering model, this paper analyzes the full polarization bistatic scattering characteristics of vegetation and points out the influence of vegetation parameters (density, water content, and vegetation diameters). The potential feasibility of polarization GNSS-R and future development trends of GNSS-R technology in quantitative retrieval (such as vegetation water content and biomass) are also discussed. G GNSS-R; CYGNSS; L-band; angle; polarization GNSS-R; bistatic scattering; vegetation biomass; vegetation water content 2021  
GNSS (Global Navigation Satellite System) is a general term for various navigation satellite systems, including GPS (Global Positioning System) from the United States, GALILEO from Europe, GLONASS from Russia, and BDS (BeiDou Navigation Satellite System) from China. With the completion of the global network of China's BDS in 2020, the number of existing navigation satellite signals in orbit will exceed more than 150. The traditional application of navigation satellites is to use their direct signals for positioning, navigation, and timing (PNT). However, since the direct signals are continuously reflected on the surface, they can be received by a special reflected signal receiver, forming a new remote sensing method: GNSS-R (GNSS-Reflectometry) remote sensing [1; 2; 3]. As for GNSS-R remote sensing, which employs the GNSS constellations as the transmitted signal source, this technique does not need dedicated transmitters. They only need the corresponding receivers to obtain the surface reflected signals. Therefore, the instruments of this kind of remote sensing technique have the salient features of being lightweight and requiring low power consumption. Because the GNSS constellation continuously emits direct signals, the spatiotemporal resolutions of this technique are much better than the traditional remote sensing techniques. Compared with the complex medium on the land surface, the ocean surface is relatively uniform. The research of GNSS-R in the ocean field is earlier and more mature than in land surface. However, a data analysis using the UK-DMC (United Kingdom-Disaster Monitoring Constellation) satellite shows that the reflected signal from the land surface can be received [4]. Although the main scientific goal of CYGNSS (Cyclone Global Navigation Satellite System) launched by NASA in 2016 is to conduct sea surface hurricane research [5; 6; 7], its use for land surface parameter research has also become a hot topic in recent years [8; 9; 10]. On 5 July 2021, the Chinese meteorological satellite FY-3 E was successfully launched. There are eleven payloads on it, and one of them is the GNSS occultation and GNSS-R combined payload, whose potential scientific goal is for land surface remote sensing, and the corresponding research on land surface parameters (such as soil moisture and vegetation biomass) will be carried out in Henan Province simultaneously [11]. The Federated Satellite System mission (FSSCat) was the first Copernicus third-party mission based on CubeSats and it was launched in 2020.