## 3 Earth Surface Effects on GNSS-R Relevant to Land Applications
### Regional Scale
#### 3.2.3 North America and Greenland: Lakes Region and Ice Cap  
GNSS-R applications can be extended to the study of snow and ice [44; 45; 46]. The signal wavelength (\(\lambda\)) at L2 is \(\lambda\) \(\sim\)24 cm, which is much longer than that radiometers operating at different frequency bands such as AMSR-E and AMSR-2. The wavelength is larger than the grain size and even the snowpack thickness. As a consequence, the medium can be assumed to be homogeneous so that the scattering is determined by the local density fluctuations. At this frequency, ice has a very low absorption coefficient and the refractive index is small. The penetration depth and the scattering coefficient depend on frequency and snow properties. In the case of dry snow, the penetration depth is expected to be around hundreds of meters and therefore the GNSS-R technique is potentially optimum for subsurface studies [47]. On the other hand, the penetration depth of GNSS signals over sea ice is \(<\)1 m [48].  
Extended regions of Antarctica and Greenland are affected by melting effects (e.g., Figure 17). A recent study [49] additionally shows areas alternatively frozen and thawed in Greenland's inner ice cap (magenta line in Figure 17). Greenland is a focus region of this study (Figure 18). Firstly, it is noted that over bare soil the scattering is significantly determined by soil moisture and surface roughness. However, over dry ice, volume scattering [50], and scattering at different layers [44] could occur allowing the GNSS signals to penetrate the subsurface. The larger waveform spreading over Greenland (Figure 18e,f) could be attributed to this geophysical reason. The effect of subsurface scattering is to increase the waveforms' peak power (Figure 18c,d) and the signals' spreading [50], in agreement with Figure 3e,f. At the same time, in the frame of this work, the large SMAP's antenna gain \(\sim\)36 dB partially compensates the signal attenuation. Therefore, the sensitivity to penetrating signals as seen by SMAP is large as compared to satellites with smaller antennas. Figure 18 shows high values of SNR \(\sim\)(18, 22) dB, PR \(\sim\)(3, 5) dB, and trailing edge width \(\sim\)(500, 600) m over areas in the inner ice cap. However, the waveforms parameters (SNR, PR, and trailing edge) take alternatively low and high values over these areas. On the other hand, the parameters levels are qualitatively similar to those over outer Greenland (see melting effects in Figure 17). Additionally, the width of the waveforms' trailing edge over Greenland increases from Summer (Figures 10 and 18e) to Winter (Figures 10 and 18f) when the weather is colder and the amount of dry ice increases, and therefore the penetration depth would be larger. This waveform data is consistent with the waveforms spreading being due to subsurface scattering effects.  
Over North America, it is found that during Summer there are regions with high SNR values corresponding to areas with lakes and high SWI (Figure 18g). During Winter, these power peaks are lower because of the effect of snow over the surface that reduces the scattering coefficient.