## 2 Data and Methods
### ICESAT and SMAP Data  
In order to retrieve SMC from CYGNSS observables, ancillary or reference BSR and VWC data must be taken into account, so their respective attenuations are compensated in the recorded signal. The reflectivity values that can be estimated by CYGNSS do not account for these contributions, and they must be corrected from VWC. The ICESat-2 and the SMAP missions can provide these parameters, respectively. Then, we can also employ SMAP's SMC estimates to validate the methodology of GNSS-R SMC retrieval presented in this manuscript. Note that SMAP SMC estimates have been computed using SMAP BSR and VWC products and computing CYGNSS SMC estimates with other products than SMAP BSR and VWC might provide discrepancies in the validation. Therefore, we assume that employing SMAP BSR and VWC for CYGNSS SMC validation is the preferred scheme. Notwithstanding, an equivalent estimate of VWC could be derived from other sources, e.g., Landsat [30; 54; 55], making our SMC retrieval completely independent from the SMAP mission, and with possible improvements. Figure 1: The locations of CYGNSS’s SPs from its eight satellites are plotted in green for 1 April 2019. ICESat-2 data from the whole month of April 2019 is plotted in red. Zoom window shows the spatial resolution in more detail. In addition, more accurate SMC products could be obtained with the use of, for example, the high accuracy ICESat-2 BSR estimates. BSR is a key variable for surface scattering models [56; 57; 58; 59]. Generally, BSR is estimated in-situ by more or less sophisticated instruments that allow measuring, at the best, 4 m length 2-dimensonal BSR profiles. These methods are incapable of measuring a large surface extent. In addition, BSR can also be estimated by computing accurate 3-dimensional surface models. These traditional procedures usually measure short local profiles and are not suitable for large areas, as measured by Low Earth Orbit (LEO) GNSS-R satellites, due to incompatible spatial resolution. Besides, vegetated areas, such as agricultural crops, meadows, shrub areas, etc. are not accessible by ground-based techniques. Moreover, in-situ measurements might be unnecessary since GNSS-R deals with L bands, so BSR below 19 cm is imperceptible [35]. In this study, we include a revision of this hypothesis by investigating to what extent BSR must be considered for L band SMC retrieval. Two alternatives are suggested, the SP area and the system frequency band. Since October 2018, the second generation of NASA's ICESat-2 has been operating globally, supplying Earth's surface geophysical and vegetation parameters. ICESat-2 data is acquired by the Advanced Topographic Laser Altimeter System (ATLAS), which operates in the 532 nm band, with a multi-beam micro-pulse laser, also referred to as photon-counting, from which the surface observable range is derived from the travel time of each detected photon. Helped by GNSS, each photon can be accurately located on Earth's surface [39]. In addition to surface height measurements, ICESat-2 supplies BSR and canopy parameters, very valuable data for microwave scattering research [39]. ICESat-2 measurements are processed every 100 m data segments, containing more than 100 signals of photons. Thus, according to this high-density sampling, the fulfillment of the required spatial resolution CYGNSS SMC retrieval might be achieved. ICESat-2 products are available at [[https://nsidc.org/data/atl08](https://nsidc.org/data/atl08)]([https://nsidc.org/data/atl08](https://nsidc.org/data/atl08)). However, due to possible BSR changes throughout the year, we only employ data from April 2019. Figure 2b shows a subset of the ICESat-2 data employed in this study. The SMAP/Sentinel-1 L2 Radiometer/Radar 30-Second Scene 3 km EASE-Grid Soil Moisture (Version 2) product [40] provides SMC estimates for the first 5 cm of surface depth. In addition, as mentioned above, it also provides VWC with a \(\pm\)0.04 volumetric accuracy (VWC \(<\) 5 kg m\({}^{-2}\)).