## 4 Discussion and Outlook  
### Advantages and Limitations of InSAR  
In this study, our results obtained by using standard repeat-pass InSAR and A-DInSAR (based on PS-InSAR) techniques with Sentinel-1 SAR data and open-source SNAP and StaMPS software packages show its capability in generating DEM and estimating changes in deformation or damages in various geotechnical engineering practices. SAR, as an active radar imaging technique, operates day and night in all weather conditions which has greater advantage over optical remote sensing techniques. InSAR and A-DInSAR techniques have matured and become viable solutions complimenting conventional geodetic, optical, and LiDAR techniques and integrated in multidisciplinary fields. Moreover, the improved temporal and spatial resolutions, wider area coverage, and free access of Sentinel missions allow a weekly repeat observation for ground anomaly detection, and near real-time monitoring of various geohazards.  
Meanwhile, the following limiting factors should be accounted for prior to practice. Geometric and temporal decorrelations are the major limiting factors in the successful implementation of Sentinel-1 SAR interferometry [55]. Geometric decorrelation occurs when the geometries of the master and slave images poorly match each other in association with the changes in a look angle and non-parallel orbits. This also happens when the baseline between the two SAR acquisitions deviates, as a result of the satellite's orbit separation in space, from zero [56]. Non-zero baseline tends to have higher geometric decorrelation. Temporal decorrelation on the other hand occurs as the time separation between SAR observation is large owing to the dynamic surface changes of the Earth. Shorter temporal baseline ensures that these changes can be captured related to the SAR phase, hence, minimizing the temporal decorrelation.  
For deformation detection, mapping, and monitoring at the rest of the test sites (Sites B, C, and D), a shorter perpendicular baseline is favored to reduce the phase signal due to topography. Meanwhile, the temporal baseline depends on the magnitude and type of deformation being investigated. Thanks to the short revisit times and tight orbital navigation of Sentinel-1 constellation (Figure 4 and Table 1), these decorrelation effects were minimized. In addition to these decorrelations, atmospheric perturbations also affect the propagation of the SAR signals on standard repeat-pass InSAR that was used for the Pohang earthquake at Site B.  
The proportion of Sentinel-1's short wavelength to the size of water molecules as it comes closer to the edge of the atmospheric window makes it sensitive to atmospheric effects. The troposphere and ionosphere tend to delay and accelerate, respectively, the electromagnetic wave, which yields incorrect elevation and deformation calculations. Geometric distortions due to layover, foreshortening, and shadowing in mountainous regions could result in miscalculations especially for landslide applications (i.e., in Gyeongju) [29]. Layover occurs when the top of the mountain appears before its bottom in the radar LOS. Foreshortening exists when the slope angle is less than the incidence angle of the observation. Whereas, shadowing happens when the mountain blocks the view of observation.