# Remote Sensing of Environmental Changes in Cold Regions: Methods, Achievements and Challenges
## 2 Principles and Methods
### Remote Sensing of Ice  
#### 2.1.1 Glacier Mass and Movement  
Glicers are slow moving masses of ice formed over time by the accumulation and compaction of snow, holding 75% of Earth's freshwater [(49)] and 10% of the global land area, including most of Greenland and Antarctica [(50)]. Glacier mass balance is highly sensitive to climate change and controls a glacier's long-term behavior and evolution. A glacier flows under its own weight due to the pull of gravity, and thus transports ice mass to lower altitudes. Remote sensing is the only practical approach for inferring glacier mass and movement over large regions.  
Glacier mass is commonly estimated through independent or combined gravimetry and altimetry measurements [(51)]. Satellite and aircraft-based gravimetry measurements have been widely used in glacier mass change assessments [(52)]. Glacier mass can also be indirectly estimated through glacier area and thickness measurements. Glacier area change events, such as ice calving, can be precisely detected using satellite images acquired over different times [(53)]. Glacier thickness change caused by ice melting or accumulation can be measured via geodetic approaches, including point measurements from altimetry and digital elevation models (DEMs) derived from photogrammetry or interferometric SAR (InSAR) techniques (Supplementary Table S2). Satellite laser altimeter measurements can achieve decimeter to centimeter accuracy levels; for example, the Geoscience Laser Altimeter System (GLAS) onboard the Ice, Cloud, and land Elevation Satellite (ICESat) provided decimeter-accuracy elevation data with a 70-m ground footprint over the global ice sheets [(54)]. The ICESat-2 satellite was launched in late 2018 and has a significantly improved laser system providing observations with enhanced spatial resolution, temporal sampling, and measurement accuracy [(55)]. Alternatively, satellite photogrammetry using optical-IR (e.g., ASTER, IKONOS) and SAR/InSAR (e.g., ERS-1/2 and ENVISAT) measurements have been successful in providing glacier raster DEMs [(56; 57)]. In particular, the SAR InterferometerRadar Altimeter (SIRAL) onboard Cryosat-2 is capable of measuring changes in the thickness of both sea ice and land ice under three different measurement modes (low resolution, SAR and InSAR) [58].  
Glacier movement can be detected from repeat-pass satellite images using feature tracking and Differential InSAR (DInSAR) techniques. The feature tracking method identifies and matches ice surface features from satellite images and calculates the moving distance of the features over different acquisition times. The methods have been applied to both optical-IR and SAR image series, including Landsat Operational Land Imager (OLI) [59], Moderate Resolution Imaging Spectroradiometer (MODIS) [60], and ERS-1/2 SAR [61] sensors. DInSAR uses repeat-pass SAR imagery to calculate glacier motion velocity after removing the topography signals from the sensor interferograms using an external DEM or a combination of interferograms [62; 63]. The accuracy of the feature-tracking method can be within the sub-pixel level, while that of DInSAR is up to half of the radar wavelength.