# Investigation of Potential of GNSS-R Polarization: Theoretical Simulations
## 1 Introduction  
Global navigation satellite system (GNSS) is a general term used for various navigation satellite systems. Following completion of China's Beidou global satellite network, there will be more than 150 in-orbit satellites comprising various GNSSs. The direct signals of GNSSs that are reflected from the ground surface can be used in a new type of remote sensing technique, i.e., GNSS reflectometry (GNSS-R) [1, 2, 3]. Its applications extend from the ocean surface to land surface, such as soil moisture, vegetation biomass, wetland monitoring, and soil freeze and thaw detection [4, 5, 6, 7]. The transmitters and corresponding GNSS-R receivers form a typical bistatic radar employ electromagnetic waves (typically L-band waves) to detect the surface of the earth. It should be noted that polarization, which is one of the most important features of electromagnetic waves, can be defined as the orientation and amplitude of the electric field strength change over time. However, with regard to GNSS-R, the polarization properties remain an open issue [8]. In the earliest soil moisture experiment 02/03 (SMEX02/03), the delay map receiver was used to obtain the surface-reflected signals. It was considered that the surface-reflected signals had left-hand circular polarization (LHCP) and thus the LHCP antenna was used to collect the surface-reflected signals, while the right-hand circular polarization (RHCP) antenna was used to obtain the direct signals [9]. However, in the following BAO tower experiment, an improved delay map receiver was used, and four antennas (RHCP, LHCP, horizontal (H), and vertical (V)) were employed. The intention was to use the polarization information to remove the effects of surface roughness on the final soil moisture retrievals [9]. It was believed that the simplifying assumptions of the theoretical model could explain the differences between the models and the in situ measurements [10]. The researchers adopted improved geodetic-quality receivers, i.e., the soil moisture interference pattern GNSS observations at L-band reflectometer (SMIGOL) and the dual-polarization SMIGOL (PSMIGOL), to perform their airborne experiments, and used the linear polarization (V and H) antennas to obtain the surface-reflected signals. Using the interferometric pattern signals collected by the receivers, the authors employed notch positions and numbers to retrieve soil moisture and vegetation water content [11; 12; 13]. The SAM receiver was used in the land monitoring with navigation signals (LEiMON) experiment, together with RHCP and LHCP antennas. The relationships between the target physical parameters (e.g., surface roughness, soil moisture content, and vegetation water content) and the GNSS-R signal revealed a strong correlation between the LR signal, RR signal, and vegetation water content (correlation coefficient: 0.8) [14]. As for spaceborne GNSS-R missions, the UK-DMC satellite launched in 2004 was equipped with the first spaceborne GPS-R payload. It was initially designed to study ocean surface roughness and to estimate sea surface wind speed, but later studies found that signals from the land surface could also be collected [15]. On 8 July 2014, the TechDemoSat-1 (TDS-1) satellite was successfully launched. It is equipped with a GNSS reflected-signal receiver, i.e., the space GNSS receiver remote sensing instrument (SGR-ReSI). TDS-1 provides a large amount of GNSS reflection data from the land surface [16]. The Cyclone Global Navigation Satellite System (CYGNSS) was launched in 2016. The main scientific goal of CYGNSS is detection of hurricane and ocean parameters [7], but recent studies demonstrated the advantages of CYGNSS with regard to the estimation of typical hydrological essential climate variables [4; 5; 6; 17; 18]. It is worth noting that the UK-DMC, TDS-1, and CYGNSS all operate on the assumption that signals reflected from the surface have LHCP, which is reflected in the polarization of the receiver antenna.