*2.1. CYGNSS Data*

The GNSS-R data in this study were produced from the CYGNSS constellation, a bi-static radar system (shown in Figure 1). This constellation is capable of collecting the near-global (between 38◦N and 38◦S latitudes) daily reflected L-1 coarse acquisition GPS signals in the form of DDMs [29–32]. CYGNSS data have been widely employed in GNSS-R scientific research and other practical applications [30]. The mean and median revisit periods of the CYGNSS were 7 h and 3 h, respectively. The spatial resolution is related to surface roughness, which is around 3.5 km × 0.5 km on the land surface and about 25 km × 25 km on the rough sea surface [23]. Compared with conventional microwave remote sensing, the CYGNSS has higher spatial and temporal resolution in observing the Earth's surface, which is conducive to in-depth scientific research by GNSS-R techniques [21,24,33].

The data employed in this study is the 3.0 version CYGNSS product at L1 level, which primarily contains DDMs and a range of metadata describing the geometry and instrument parameters in acquisition. CYGNSS data can be acquired in NetCDF format from https://podaac.jpl.nasa.gov (accessed on 10 September 2021). The primary variables utilized are summarized in Table 1; including DDMs, the transmitter/receiver distance to the specular point, transmitter and receiver, antenna gain, etc.

**Figure 1.** Schematic of the GNSS-R technique. GNSS satellites transmit signals to the Earth's surface. The signal is reflected by the Earth's surface and captured by GNSS-R receivers onboard low Earthorbiting satellites. The specific locations of the specular points depend on the geometric positions of the transmitting and receiving satellites.



## *2.2. SMAP Data*

The SMAP mission, launched by NASA in January 2015, has been designed to collect L-band signals which are sensitive to land surface soil moisture (~5 cm of depth). The SMAP satellite is equipped with active radar and passive radiometer sensors. However, the radar failed to work in orbit after two months. Hence, the satellite currently relies only on the L-band radiometer to retrieve land surface information, such as soil moisture and vegetation information, by measuring brightness temperature. Since the signals received by SMAP satellite and GNSS signals are both L-band with similar frequencies and sensitivity to the land surface, SMAP data can be employed for comparison analysis with CYGNSS data. The SMAP satellite observes land area between 85◦S and 85◦N, with a revisit frequency of 2–3 days [28].

The data product utilized in this study is the SMAP Enhanced L3 Radiometer Global Daily 9 km EASE-Grid Soil Moisture, available at https://nsidc.org/data/SPL3SMP\_E/ versions/4 (accessed on 16 October 2021) [34–36]. The enhanced SMAP product is an interpolated and gridded result of SMAP Radiometer measurements, which are posted to the 9 km Equal-Area Scalable Earth Grid, Version 2.0 (EASE-Grid 2.0). The primary data are summarized in Table 2. It is worth noting that two datasets from SMAP, soil moisture and vegetation opacity, were employed in this study. Soil moisture is used to verify the accuracy of the inversion results, while vegetation opacity information is fed as auxiliary input data to the model for flood inversion.

**Table 2.** Main parameters of SMAP mission data.

