# High Performance Computing in Satellite SAR Interferometry: A Critical Perspective
## 3 Operational SAR Systems and Applications
### InSAR Applications and Products
In general, SAR sensors operating at the C-band (5.3 GHz, 5.7 cm wavelength) bring excellent investigation capability to study deformation phenomena during the spring and summer seasons [134]. Serious drawbacks, however, arise (i) in the presence of vegetated (forest and meadow) zones; (ii) in layover or shading areas that are visible only along with appropriate illumination directions of the scene; and (iii) for landslides that evolve rapidly over time, with rates higher than 15 cm/year. InSAR techniques have been used mainly to analyze slow-moving landslides; however, several works showing long-lasting ground deformation time-series have also been investigated [135; 136; 137; 138; 139]. Global climate change and local ground subsidence can significantly increase the exposure to coastal flooding risk. Human beings also contribute severely to changes in the environment. In this framework, InSAR can monitor changes in highly urbanized zones due, for instance, to groundwater extraction or the deterioration of buildings and private and public facilities [105]. Ground subsidence can also be related to gas, petroleum, open-pit mining, and projects that require the construction of underground facilities pertaining to human activities [140; 141; 142; 143]. InSAR has become an essential instrument for geophysical observation and Earth's surface monitoring, especially in densely populated areas and in regions, such as volcanoes, which are harsh and subject to continuous changes. For volcano observatories, for instance, recent articles have proposed InSAR based monitoring schemes, profiting from the frequent revisit of the last generation missions [144]. As an example of application of the InSAR technique, we show in Figure 4 an InSAR interferogram that reports the ground deformations associated with the Mw 7.4 earthquake that hit a remote mountainous area, far from larger cities, of the Southern Quinghai province (China) on 21 May 2021 at 2:04 local time. The interferogram was produced by processing two different Sentinel-1 SAR images acquired on 20 May 2021 and 26 May 2021 (Path 99, Frame 1295, VV polarization). To give a representative example of ground deformation time-series associated with a volcanic eruption, we show in Figure 5 the results relevant to the Wolf volcano, in Galapagos Islands (Ecuador), which suddenly erupted on 25 May 2015. The results have been obtained by processing COSMO-SkyMed data acquired from April 2014 to August 2015 from both ascending and descending orbits. Vertical and east-west components,  
Figure 4: SAR interferogram shows the ground deformations associated with the Quinghai Mw 7.4 earthquake on 21 May 2021. The interferogram is geocoded and imported into Google Earth. obtained by processing the SAR data through the MT-InSAR SBAS method and subsequent combinations, are shown on the left and right side of Figure 5, respectively. The top panels show the cumulative ground deformation components across the eruptive event in a three-dimensional view (obtained by using Google Earth). The bottom panels show the temporal evolution of the corresponding ground deformations of a point in the area of maximum deformation; the red vertical bars highlight the eruption start time. The deformation occurred mainly after the eruption, without any clear deformation signal before. In addition to ground displacement measurements, MT-InSAR techniques also open up other possible scenarios, in some cases posing questions still to be theoretically addressed. One of these is how to efficiently use multi-polarized SAR data for extracting information on the vegetation coverage, soil moisture or other physical parameters (e.g., [145]). For instance, InSAR coherence maps can be used for Land-Use-Land-Coverage (LULC) analyses as an alternative to the exclusive use of SAR backscattering signatures [146; 147; 148; 149; 150; 151]. New InSAR advances concern the use of multi-polarized InSAR data and products to analyze parameters related to the vegetation coverage and soil moisture content [152; 153].