## 2 Data and Methods  
### CYGNSS Data  
The CYGNSS mission of NASA is the most recent Earth-observing GNSS-R constellation. It consists of eight satellites launched onboard a Pegasus XL Rocket on 15 December 2016 (online measurements are available from March 2017 at [[https://podaac-opendap.jpl.nasa.gov](https://podaac-opendap.jpl.nasa.gov)]([https://podaac-opendap.jpl.nasa.gov](https://podaac-opendap.jpl.nasa.gov))). The CYGNSS satellites are located at an equatorial orbit with an inclination of 35\({}^{\circ}\) at about 500 km altitude. The initial research objectives of CYGNSS were the observation of Tropical Cyclones, so the coverage was limited to low and middle latitude ranges (38\({}^{\circ}\)S to 38\({}^{\circ}\)N), and consequently restricting the areas of study for other applications. For instance, the surface scattered signals have provided an unprecedented opportunity to study BSR and dielectric properties [36; 41; 46].  
In CYGNSS, the geometry of observation follows a bistatic configuration [47]. Each CYGNSS satellite is equipped with a GNSS up-looking RHCP antenna and two GNSS down-looking LHCP antennas. The antennas correspond to the bistatic radar receivers kind on the L-band (L1-band frequency at 1.57542 GHz), also called DDM instruments. These instruments are designed to map the scattered signal on the ocean and land surface, which is sampled in time and frequency, thus delivering DDMs at the proximity of the specular point (SP). The DDM instrument computes 4 measurements every second, which are compressed and downloaded to the ground processing facilities [46]. There are three available CYGNSS processing levels delivered freely to the scientific community, ranging from level 0 to 3, and subsequent sub-levels. In this study, we employ level-1 data, which contains geo-located DDMs calibrated into an ideal (analog) power sensor (P\({}_{\rm{DDM}}\)). Additional level-1 parameters required in this study include DDM timestamp (t\({}_{\rm{UTC}}\)), latitude (\(\varphi_{\rm{SP}}\)), longitude (\(\lambda_{\rm{SP}}\)), and incidence angle (\(\theta_{\rm{SP}}\)) of the SP, antenna gain (G\({}_{\rm{r}}\)), GPS effective isotropic radiated power (eirp), and ranges from the receiver (R\({}_{\rm{r}}\)) and transmitter (R\({}_{\rm{t}}\)) antennas.  
The CYGNSS DDM data specifications and processing schemes are given in [48; 49; 50]. The geometric distribution of the observed SPs follows a quasi-random spatiotemporal distribution, due to the changing geometry of GNSS and CYGNSS satellites [41]. This distribution is very different from conventional data delivered by active/passive microwave instruments onboard satellites. Therefore, the challenge includes very different conditions of the first Fresnel ellipses (SP areas), because the distances from the transmitter and the receiver differ considerably from one observation to others. In addition, these areas may vary according to other factors, including the incidence angle and the height of the receiver [47; 51; 52]. The authors in [46] quantified the corresponding mapping resolution near to 150 m depending on the observation geometry. The authors in [53] assessed the spatial resolution of the coherent Fresnel reflection zone to approximately \(0.65\times 0.85\) km\({}^{2}\), also depending on the above factors. The rationale in this manuscript is to derive CYGNSS reflectivity values and combine them with other necessary parameters for SMC estimation. In this case, we employ estimates of VWC and BSR from NASA's SMAP and ICESat-2 missions. Figure 1 shows an example of CYGNSS' SPs distribution from the eight CYGNSS satellites on 1 April 2019, together with ICESat-2 data. ICESat-2 data is introduced in the next section.