*2.2. CYGNSS Data*

CYGNSS is a constellation of eight satellites, each of which has four channels; i.e., thirty-two observations per second can be received. Due to the increase in CYGNSS sampling frequency after July 2019 (i.e., with 500 ms non-coherent accumulation), the resolution of CYGNSS is 3.5 × 0.5 km at this time. The CYGNSS data used in this study are the L1-level data of version 3.0 from July 2019 to December 2021, which can be downloaded from the official CYGNSS website (https://cygnss.engin.umich.edu/data-products/ (accessed on 6 November 2021)). To improve the quality of CYGNSS data, standard quality control and empirical quality control were performed [19]. The higher launch power of the GPS Block IIF satellite introduces uncertainty at peak power [25]. Removing these data will

reduce the overall data volume by 30%. Therefore, this part of the data was chosen to be retained in this study. The power received by CYGNSS consists of both incoherent and coherent scattering components [26]. Most studies have been based on the assumption that the incoherent component is ignored and only the coherent scattering component is retained [11,12,14,20]. Therefore, based on the land scattering model of GNSS-R, the surface reflectivity Γ(θ) of CYGNSS can be readily derived, following the assumption that the coherent component dominates:

$$\Gamma(\theta) = \frac{P\_r (R\_t + R\_r)^2 (4\pi)^2}{P\_t G\_t G\_r \lambda^2} \tag{1}$$

where *Pr* and *Pt* are, respectively, the reflected power received by the receiver and the transmitted power. λ is the carrier signal wavelength. *Rr* and *Rt* are, respectively, the distances from the receiver and transmitter to the specular reflection point. *Gt* and *Gr* are the transmitting antenna gain and receiving antenna gain, respectively.
