# Remote Sensing of Snow Cover Using Spaceborne SAR: A Review
## 2 Characteristics of Snow and SAR  
### SAR Sensor Characteristics  
Radar remote sensing is an active sensing approach, which transmits electromagnetic (EM) radiation with wavelengths between 0.3 to 0.01 m and senses their echoes from the Earth surface [62; 63]. The spatial resolution of the acquired imagery is approximately equal to one-half the length of the actual antenna and is independent from platform altitude [64]. Comparing to other contemporary active spaceborne sensors such as scatterometers, SAR is therefore able to monitor with higher spatial resolution and thus preserves more ground surface details. Due to its active and relatively long wavelength characteristics, SAR does not rely on solar illumination and can therefore operate both day and night. SAR can also penetrate clouds, enabling measurements of the surface under all weather and illumination conditions. These abilities are particularly valuable when it comes to snow cover monitoring, as snow-covered alpine regions are often covered by clouds and high latitudes regions are affected by polar darkness during winter [65]. Furthermore, longer wavelengths of SAR can penetrate into the snowpack [66], potentially providing information about snowpack conditions such as snow grain size and SWE (liquid/frozen water content), and can even penetrate the frozen layer on the top surface of snow [67]. Owing to the unique sensing characteristics of SAR, the snow information recorded in SAR imagery is fundamentally different when compared to optical/multispectral imagery. The former records surface characteristics related to the roughness and dielectric properties; the latter records the reflection/absorption of the incoming solar irradiation at the top layer of the surface [68]. As snow, ice, and clouds are characterized by comparatively similar reflection properties in the visible and--depending on the cloud phase--the near to medium infrared part of the spectrum, confusions can occur when attempting to classify snow cover and discriminate it from ice or clouds [69; 70]. SAR sensors can overcome this ambiguity as they measure surface properties in a different realm in terms of a backscatter coefficient, which is a function of the wavelength of the SAR signal, and the roughness and dielectric properties of the surface rather than the reflection properties. An additional advantage of SAR sensors is derived from the phase information they record. Different polarizations (horizontal, vertical) allow for the detection of additional physical characteristics of the surface, such as shape, material, or angle of an observed target. Furthermore, based on the phase information recorded by SAR, coherence and interferometry can be generated [71; 72; 73], which can indicate the deformation and stability of ground features. Such information can be analyzed to detect and quantify, e.g., glacier velocity or other moving targets [74; 75; 76]. However, SAR also has some practical drawbacks for cryospheric application. The first is the comparatively low temporal resolution (i.e., revisit days, often longer than five days) when compared to operational optical/multispectral missions due to the trade-offs of orbit design including spatial resolution, tilt angle, swath width [77; 78; 79]. Based on the cryosphere report given by the Global Climate Observing System (GCOS) [80], the minimum requirement in terms of temporal resolution for spaceborne snowmelt area products for subsequent research of hydrology and climate is one to five days. Current optical/multispectral missions are designed with shorter, even daily revisiting time (e.g., Advanced Very High Resolution Radiometer (AVHRR), Moderate Resolution Imaging Spectroradiometer (MODIS), Sentinel-3A/B).