# Remote Sensing of Snow Cover Using Spaceborne SAR: A Review
## 4 Critical Auxiliary Data Necessary to Support Detecting SCE from SAR Data
### Digital Elevation Model, Influence of Topography on SAR-Based Snow Detection  
The influence of the topography on SAR-based snow cover detection is evident, as snow cover distribution is directly linked to elevation, aspect (facing direction of topography), and slope of the study region. Moreover, local topography affects the LIA, which largely affects the quality of SAR image pre-processing and final results. Elevation plays a significant role in snow cover distribution. Algorithms like the wet snow-dependent dry snow detection relies on the spatial relationship between wet and dry snow cover extent and elevation (see Section 3.3.3). [PERSON] (2016) analyzed the seasonal difference of backscattering coefficients in various elevation zones and found larger differences in higher altitudes. [PERSON] et al. (2013) also found the importance of elevation is crucial for total snow cover mapping. Aspect and slope are also critical for not only how snow is distributed but also how SAR can sense the snow. [PERSON] et al. (2017) compared the snow line altitude for several regions and discovered that glaciers facing south have higher snowline altitudes, which is caused by more intense solar radiation on south facing slopes. [PERSON] et al. (2019) identified that the seasonal difference between H and \(\alpha\) is more pronounced for front-slope (the slope facing the sensor). Topography not only affects snow distribution, it also affects the SAR signal viability. Based on the DEM a shadow and layover map of SAR can be generated. Steeper topography or regions closer to the nadir of the sensor would naturally lead to more SAR shadow and layover regions [PERSON] et al. (2019),  
Figure 10: External/Ground truth data used for validating SAR-based snow cover products. * Advanced visible and near infrared radiometer type 2 (AVNIR-2) sensor onboard on Advanced land observation satellite (ALOS) satellite; moderate resolution imaging spectroradiometer (MODIS) sensor onboard on Terra and Aqua satellites; ** Landsat-series missions compose different optical sensors. **  
where useful ground information is blocked or degraded. Hence, almost all studies calculated the mask during the geocoding step and later re-used it to mask out the results, as illustrated in Figure 6. However, these terrain-induced radiometric effects and the local illuminated brightness should be mitigated by adding a terrain correction based on DEM as well as the acquisition geometry of SAR [178]. Moreover, for the backscattering-based approach, the influence of LIA on the SAR signal was already identified in [PERSON]'s first research [126], and confirmed by many subsequent studies [103; 121; 141; 86; 179]. The backscattering of wet snow is more susceptible to the variability of LIA than dry snow because the backscatter of the latter is mainly reflected from the snow/ground interface, as modeled by [PERSON] and [PERSON] [131]. [PERSON] et al. [140] found the backscattering from snow can decrease even more for a very high LIA, which reduces the difference between wet and dry snow. This angle-dependency influences the accuracy of backscattering-based thresholding approaches, especially for wide-swath SAR images such as ASAR and Radarsat [96; 154]. It was also suggested that the LIA difference between the reference and the observed image should not exceed \(10^{\circ}\)[175]. The range direction resolution would also decrease dramatically in low LIA, and the signal-to-noise ratio for low reflective surfaces decreases in high LIA [140]. Therefore, areas with extreme LIA values should be masked out by setting LIA limitations [126; 127; 134; 140]. The influences of LIA on both InSAR-based and PolSAR-based approaches are also significant. He at al. [149] found the coherence value increases from \(0^{\circ}\) to \(30^{\circ}\) LIA and decreases from \(30^{\circ}\) to \(90^{\circ}\) LIA. [PERSON] et al. [35] found that when the LIA is less than \(35^{\circ}\), the dominant scattering mechanism changes from volume to single-bounce scattering. [PERSON] et al.