## 4 Discussion  
PSIG and GEP approaches have been applied in the surroundings of Penas Cape for the first time to increase the number of PS in the study area and to facilitate the analysis and interpretation of the detected deformation areas. The results are discussed below in the following order: (i) PSIG and GEP results at a regional and local scale to detect and monitor coastal instabilities in cliff coast; (ii) observed relationships between coastal instabilities and coastal retreat in the study area; (iii) future research directions. The LOS mean deformation velocity maps obtained by the two approaches (Figures 4 and 5) allowed us to observe similar VLOS rates at a regional scale. PSIG has measured VLOS rates from \(-\)17.1 to 37.4 mm year\({}^{-1}\) and GEP has measured VLOS rates from \(-\)23.0 to 38.3 mm year\({}^{-1}\). Nevertheless, they present important differences in the interpretation of the obtained datasets at regional and local scales. Some of these differences are observed in the squared areas (Figure 6a,b) in Figure 4. These differences in A-DInSAR patterns are mainly that the number of positive PS on the GEP VLOS map (36.4% of positive PS with VLOS > 2.5 mm year\({}^{-1}\)) are greater than PSIG VLOS map (1.4% of positive PS of PSIG). The different results obtained by both approaches could be explained by the following factors; (1) the pixel resolution of software (14 \(\times\) 4 m for the PSIG product versus 90 \(\times\) 90 m for the GEP product), (2) the degree of selection and manipulation of the input parameters during the processing, (3) the technique implemented in each approach: thus, PSIG software is based on the PSI technique [2; 3; 56], while the GEP service has implemented the P-SBAS technique [1; 62], (4) the variable amount, in each processing, of the uncorrected atmospheric component and (5) the location of the reference points. Both approaches used have allowed the detection of different coastal instabilities at a regional scale (Figure 5). Firstly, in Figure 5a, the deformations (with VLOS rates between \(-\)12.4 and 0.6 mm year\({}^{-1}\)) are located in an electrical sub-station, in which ground movements have been related with slope instabilities that have developed on (i) destabilized slope and ii) anthropic fill settlements [63]. Secondly, different ground movements were detected in the Musel Port (Figure 5b), with VLOS rates between \(-\)8.6 and 6.3 mm year\({}^{-1}\). These movements are produced by wave processes, tides, and the migration of sandy sediments. Additionally, being an area reclaimed from the sea, the deformations may be due to ground settlement taking place in the filler materials [64]. Positive accumulated ground motion trends can also be distinguished due to: i) accumulation of materials (anthropic activities) and (ii) different affections on the breakwater wall that produce a relative movement towards the satellite. Finally, GEP has allowed deformations to the West of Penas Cape to be detected and identified (Figure 5c), with VLOS from \(-9.1\) to \(8.1\) mm year\({}^{-1}\). The affected materials are the bedrock and overlying rasa deposits. The authors of [45] indicate that the West of Penas Cape shows the greatest coastal retreat in the area, with \(13.38\) m between 2006 and 2017. We suppose that these deformations, from \(-2.5\) to \(-9.1\) mm year\({}^{-1}\), are due to coastal instabilities, which may be directly related to erosion of the cliff foot and undercutting sea action. Other deformations detected reflect the accumulation of anthropic materials in open pits, works, and quarries, with VLOS greater than \(5\) mm year\({}^{-1}\). In addition, some detected PS are associated with artifacts produced by uncorrected atmospheric components or processing errors. These processing errors, observed in some time series where there are jumps and aliasing effects, may be due to phase unwrapping. Regarding local analysis, A-DInSAR processing has allowed us to distinguish local ground motions in all the selected areas.