## 3 Earth Surface Effects on GNSS-R Relevant to Land Applications
### Regional Scale
#### 3.2.1 Africa: Arid Deserts and Congolian Rainforests  
Over bare soil, the scattering is mainly related to surface scattering, being driven by the soil moisture and surface roughness. However, the high spatial variability of the SNR over the Sahara Desert could be explained by both surface (Rocky Mountains, boulder and graves zones, and sand seas) and subsurface scattering contributions as [PERSON] suggested for first time in [11]. Volume scattering could occur under dry conditions allowing the GNSS signals to penetrate the subsurface [40; 41; 42] for rich sand content areas. This could explain the large SNR values over the Sahara despite the low soil moisture (Figure 11c,d). On the other hand, the large waveform spreading (Figure 11e,f) over the Sahara Desert is difficult to be attributed to this geophysical effect because of the low penetration depth around (\(\sim\)2 m) for 0% of volumetric moisture content at L band. The effect of the sand seas (that continuously reshape the surface) on the scattering coefficient, should be considered as a plausible reason, particularly the dunes, which show the characteristics of the sand and the long-term wind [43]. Additionally, almost negligible seasonal variations over the Sahara are observed since SWI (Figure 10g,h) and NDVI are nearly constant during the year in this region (Figure 11i,j).  
Seasonal fluctuations over Africa are found in the latitude ranges from \(\sim\)9\({}^{\circ}\) to \(\sim\)15\({}^{\circ}\), and from \(\sim\)\(-\)20\({}^{\circ}\) to \(\sim\)\(-\)10\({}^{\circ}\), where the geophysical parameters changes are large from Summer to Winter (SWI in Figure 11g,h, and NDVI in Figure 11i,j). Seasonal variations of the Polarimetric Ratio (PR\({}_{\text{Summer}}\sim\)1 dB in Figure 12a vs. PR\({}_{\text{Winter}}\sim\)4 dB in Figure 12b) are anti-correlated to changes in SWI, and NDVI. On the other hand, seasonal variations at H-Pol in the SNR (SNR\({}_{\text{Summer}}\sim\)22 dB in Figure 12c & SNR\({}_{\text{Summer}}\sim\)13 dB in Figure 13c vs. SNR\({}_{\text{Winter}}\sim\)18 dB in Figure 12d & SNR\({}_{\text{Winter}}\sim\)17 dB in Figure 13d), and the trailing edge width (TE\({}_{\text{Summer}}\sim\)460 m in Figure 12e & TE\({}_{\text{Summer}}\sim\)340 m in Figure 13e vs. TE\({}_{\text{Winter}}\sim\)390 m in Figure 12f & TE\({}_{\text{Winter}}\sim\)440 m in Figure 13f) are correlated to changes in SWI and NDVI. This could be explained since the larger scattering coefficient over the soil, associated with larger SWI, increases the SNR and waveforms spreading, while the larger NDVI attenuates H-Pol signals more than V-Pol signals (Figure 5a,b). The spreading of the waveforms in the latitude range from \(\sim\)9\({}^{\circ}\) to \(\sim\)15\({}^{\circ}\) (Figure 11e,f) is similar to that over the Sahara Desert, with very low levels of NDVI. However, as it is shown over the equatorial region, the spreading is significantly reduced because of the effect of the Congolian forests with an AGB \(\sim\)350 ton/ha. This suggests that the canopy height (Figure 7a) plays a more important role than the NDVI in the shape of the reflected waveforms. In fact, the SNR is larger over the Sahara and Kalahari Desert than over Congolian forests despite the larger SWI, because the larger signal attenuation due to the larger AGB (Figure 7a). In regions with lower levels of canopy height, the Polarimetric Ratio is more sensitive to changes in NDVI (Figures 7a, 12a,b, and 13a,b).