## 7 Discussion  
### Feasibility of Sentinel-1 PS-InSAR for Ground-Motion Detection in Ireland  
Studies of using PS-InSAR and Sentinel-1 imagery for detecting ground instability in temperate oceanic climate zones (e.g., [61]) have so far been rarely documented in peer-reviewed scientific literature. This paper presents a new case study of using PS-InSAR and Sentinel-1 data in such climate zones, and the first in Ireland. Recent Sentinel-1-based InSAR studies in such zones (e.g., [62; 63]) have employed an implementation [9] of the Short Baseline Subset (SBAS) methodology [2]. The SBAS technique uses a lower-resolution approach that can help in defining large-scale deformations, but it can consequently lose the capability of detecting small-scale movements. For the same data, the full-resolution approach of PS-InSAR has the main advantage of being able to detect smaller-scale movements. As highlighted by our study with Sentinel-1, this capacity can be critical where the dominant scales of motion are small (down to 100 m\({}^{2}\) or less), such as in tectonically-quiescent regions like Ireland and elsewhere. Figure 19: Displacement time series extracted for the points selected with the white circles in AOI-2. (**1**) south-west section of the mine site; (**2**) main mine site; (**3**) Tailing Storage Facility (TSF) site. See Figure 17 for point locations. Although previous attempts to apply multi-temporal InSAR to Ireland have been based on PS-InSAR but with ERS-1/2 and ENVISAT data [19; 20; 21], these studies were restricted to urban areas. Here we highlight that the coverage obtained with Sentinel-1 is many times greater in terms of PS number, density and coverage across different land-cover types than is achievable with ENVISAT (e.g., Figure 7 vs. Figure 8). With respect to Sentinel-1 the more limited results obtained with ENVISAT can be mainly related to four factors: (1) the sparser and more uneven temporal distribution of the ENVISAT images; (2) the longer revisit time (35 days); (3) the slightly lower ground resolution achievable (~20 m by 4 m in range and azimuth, respectively); and (4) the larger orbital tube (i.e., the range of the satellite's geometrical baselines; in the order of \(\pm\)1 km for ENVISAT and of \(\pm\)150 m for Sentinel-1; see Supplementary Materials). The main advantage of Sentinel-1 is, indeed, given by both its higher frequency of image acquisition (reduced temporal decorrelation) and its smaller orbital tube (reduced geometric decorrelation), which result in enhanced coherence of the backscattered radar signal (i.e., radar signal changes at each ground pixel in successive images are minimized). This is shown for our study in Figure 20, where the Sentinel-1 PS coherence is distributed toward higher values, especially in the rocky landscape of the Burren. Moreover, the higher image acquisition frequency of Sentinel-1 permits the detection of short-term displacement variations that could be useful to better understand the factors and mechanisms influencing the occurrence of ground instabilities. Without independent validation (via, e.g., precise levelling campaigns), some caution should be exercised regarding the accuracy of the values of ground motion presented here. Elsewhere, comparison of ground displacement velocities derived from PS-InSAR on ERS-1/2-ENVISAT and Sentinel-1 imagery with ground displacement velocities measured by precise levelling [58; 64] indicates that differences are within 1.5 mm/year and 1.0 mm/year, respectively. An evaluation of the noise levels in two apparently stable sub-areas in AOI-1 and AOI-2 (Figure 21) shows that the Sentinel-1 data  
Figure 20: Frequency distribution of PS coherence extracted from (**a**) the Burren (AOI-1) and (**b**) the Navan town (AOI-2) for both satellites. N is the number of PSs. The distributions are normalized to the total number of PSs retrieved for each satellite. are slightly noisier than that of ENVISAT.