# Towards a Digital Twin Prototype of Alpine Glaciers: Proposal for a Possible Theoretical Framework
## 2 Glaciers Monitoring
### The Role of Remote Sensing and Its Contribution to Glacier Inventories
Sentinel-1 (S1) and Sentinel-2 (S2) constellations now ensure acquisitions with a revisit time of, respectively, 10 and 5 days and a spatial resolution of 10/20 m for the majority of S2/S1 bands. Some recent works provided systematic applications aimed at exploiting the large volume of data provided by Copernicus Sentinel missions for detecting glacier outlines, including debris-covered glaciers [30]. In particular [31; 32] provide an effective overview of snow cover mapping potentialities from satellite, with special concerns about Sentinel-2 and Landsat missions. Specifically, [31] validated snow cover mapping from the Copernicus Land Monitoring Service for High-Resolution Snow and Ice Monitoring (HRSI) with reference to test plots showing different aspects, canopy cover, and solar irradiance conditions, located on the Pyrenees (Spain) and on the Sierra Nevada (USA). HRSI relies on a combination of Sentinel-2 and Landsat-7/8 satellite scenes, LiDAR-based, and in situ datasets. An accuracy of 25-30% RMSE was found. Moreover, it was observed that highly accurate LiDAR-derived tree-cover density maps did not improve the subcanopy FSC retrievals. In [32], the Theia Snow collection, an alternative to the Copernicus HRSI, was similarly validated, and the accuracy of snow-covered area detection related to snow depth. It was found that the proposed algorithm was able detect the snow-covered map with an accuracy higher than 90% and increasing as snow depth decreases. The adoption of optical imagery in glacier monitoring implies some constraints, mainly related to cloud coverage and presence of rocky debris. Specifically, the presence of cloud cover strongly limits a continued and proper identification of glacier outlines that should be identified as close as possible to the end of the ablation period, i.e., the summer season. Since clouds are very common in the Alps during summer, this is a significantly limiting aspect that optical data suffer from [33]. Additionally, the presence of rocky debris that can partially, or completely, cover the ice, is a complex issue in glacier mapping, being that spectra of covering rocks are identical to those of the surrounding rocks. As a result of these limitations, many studies propose the adoption of the synthetic aperture radar (SAR) to map glaciers [34]. Interferometric SAR (InSAR) and polarimetric SAR (PolSAR) approaches can, in fact, be used to both overcome the problem of clouds and to separate the identification of debris coverage from ice [35]. It is worth rememberingthat SAR does not exploit sun radiation, but it is the sensor itself in charge of providing the lighting signal (in the microwave range) and recording the correspondent echo. Thanks to its relatively long wavelength, the SAR signal can pass through clouds and can operate during both day and night time. Additionally, SAR waves can penetrate into the snowpack, providing useful information such as snow/ice grain size and Snow Water Equivalent, SWE (liquid/frozen water content) [36]. The SAR backscatter coefficient, from the signal amplitude, provides information about the properties of ice, such as roughness and dielectric value that are significantly different from the ones optical data can generate. Additionally, different polarizations (horizontal and vertical) enable the detection of other features of glaciers such as shape, state of ice and prevailing development of volume. Phase information can, also, be exploited and coherence- or interferometry-based approaches applied to describe the deformation and stability of surfaces. This permits, for example, the detection and quantification of glacier velocity [36; 37]. Further to glaciers outline identification, other base requirements to glacier inventories is the information related to glacier topography: elevation, slope, aspect and thickness. DSMs from different sources can support this type of analysis, providing more or less accurate description of glacier geometry.