## 7 Discussion
### Feasibility of Sentinel-1 PS-InSAR for Ground-Motion Detection in Ireland
The frequency distribution of velocities of Sentinel-1-lwithin these sub-areas spans a wider range than that of ENVISAT, and the mean squared error (RMSE) is higher, about 1.1 mm/year for Sentinel-1 and 0.4-0.7 mm/year for ENVISAT. While 99.9% of ENVISAT data lie within the \(\pm\)1.5 mm/year stability threshold, 94% of Sentinel-1 data lie within \(\pm\)2.0 mm/year. Although not affecting the maps presented here (Figure 7, Figure 8, Figure 17, and Figure 18) to any great degree, the latter value may therefore represent a more conservative threshold for'stable' PSs based on Sentinel-1. In addition to an appreciation of the PS noise level, confidence in the interpretation of the computed PS velocities here, and in other studies, arises from a combination of three other factors: (1) a high density and spatial consistency of points indicating movement along the LOS direction for one acquisition track, (2) a presence of a potential movement signal in both ascending and descending tracks, and (3) a land surface and/or sub-surface material conducive to motion. Assessment based on factors such as these is thus partly subjective and partly dependent on the scale of observation. In this study, we present high-confidence examples of ground motion. One of these (Figure 13b) exhibits LOS velocities within the \(\pm\)2.0 mm/year range; in this case our confidence in its validity is based on the detection of similar rates of movements but with opposite signs in both ascending and descending tracks and on the observations of instability at that locality in our visual site inspections. In general for all examples of potential ground motion highlighted in AOI-1, visual site inspections and existing data on subsurface materials provided either direct evidence of past ground motions (e.g., sagged roads or  
Figure 21: Frequency distribution of PS vertical velocities extracted from (**a**) the Burren (AOI-1) and (**b**) the Navan town (AOI-2) for both satellites. The red line is 0 mm/year, the green lines bound the \(\pm\)1.5 mm/year velocity range considered as stable. N is the number of PSs; RMSE is the root mean squared error. The distributions are normalized to the total number of PSs retrieved for each satellite. Directly comparing velocity data obtained from the two satellites is not possible as the LOS angles are different. Therefore, for this comparison velocities calculated along the LOS of each satellite were projected along the vertical direction according to the incidence angle of each PS. collapsed cliffs) or revealed ground conditions that could be plausibly conducive to ground motion (e.g., water seepage or recent construction over peatland). Nonetheless, future work in the application of PS-InSAR in Ireland should involve ground-truth measurements on the ground motion examples presented here and/or in other areas of the island. Future work will consist also in the use of alternative radar wavelengths to better monitor the transportation network. The main limitation of Sentinel-1 in detecting such infrastructure is given by the geometrical characteristics of the roads (width) in respect to the maximum ground resolution achievable with the SAR images. For this reason, X-band images (e.g., from TerraSAR-X or COSMO-SkyMed satellites), which have higher spatial resolution than currently-available C-band images may be useful for more detailed monitoring of ground movements affecting infrastructure (e.g., [65]). X-band data also have the advantage of being more sensitive to subtle deformations thanks to the smaller wavelength. The current results could be improved by better estimating and subtracting the atmospheric noise from the generated displacements. The seasonal changes of the stratified atmosphere, which are correlated with time, are in fact potentially not properly estimated by the conventional high-pass filtering implemented in the standard PS-InSAR algorithm [66].