## 4 Data and Methods
PSs identification: the point target candidates are identified by considering the Amplitude Dispersion Index (ADI), which is defined as the ratio between the standard deviation and the mean of the amplitude values of a pixel [1]. Pixels with low ADI in all acquisitions are selected as PSs. 5. First inversion: in this step, the phase components related to the topographic residuals and to the displacement velocity are calculated by using a linear velocity model and are then removed from the interferograms. The coherence (i.e., the measure of decorrelation due to temporal and geometric degradation [51]) and elevation of the image pixels are also calculated. 6. Second inversion: the atmospheric phase components are estimated using the products of the linear model calculated in the previous step and are subtracted from the interferograms by using high-pass (365 days) and low-pass (1200 m) filters. The PSs with coherence values lower than 0.75 are discarded. Considering such parameters, the precision of the measured velocities was calculated to be in the 0.06-0.39 mm/year range. The precision is calculated with the following formula (after [52]): \[\sigma=\frac{\lambda}{4\pi}\ \sqrt{\frac{1-\gamma^{2}}{2\gamma^{2}}}\] in which \(\sigma\) is the calculated precision, \(\lambda\) is the signal wavelength, and \(\gamma\) is the average pixel coherence. Velocities shown in time-series plots below are reported to one significant digit, which thus represents a conservative estimate of the velocity precision. 7. Geocoding: the final interferometric products are geocoded into the WGS 84 UTM zone 29N projected coordinate system and exported as shape or raster files for the post-processing analyses. Given its size and the data volume, AOI-1 was divided in sub-areas of around 10 km \(\times\) 10 km, each of which was processed independently. A \(\sim\)10% overlap of these sub-areas was maintained to check the quality and consistency of the results. This approach facilitated a more refined InSAR processing and produced more manageable results for the post-processing analyses. The SAR images are acquired from an inclined view geometry, with a Line of Sight (LOS) angle of \(\sim\)33\({}^{\circ}\) (\(\theta\)) from vertical in the case of Sentinel-1 and with incidence angles at the topographic surface ranging from 32\({}^{\circ}\) to 35\({}^{\circ}\). Each displacement measurement therefore includes both vertical (up-down) and horizontal (east-west) components. To extract the up-down and east-west components, it is necessary to combine the information provided by both the ascending and descending orbits [53]. The displacement component along the north-south direction cannot be properly measured because the  
Figure 6: Examples of connection graph between the master (red dot) and slave (black dots) images generated with the PS-InSAR approach. The graphs are obtained from (**a**) the descending Sentinel-1A/B dataset, track 96, and (**b**) the descending ENVISAT dataset, track 352, both acquired over AOI-1. SAR satellites have near-polar orbits and are not sensitive to movements parallel to them. Two further sets of images acquired in ascending orbit (AOI-1: 88 images, track 74, 20 April 2015-05 March 2018; AOI-2: 121 images, track 103, 22 April 2015-24 January 2018) were therefore also processed to assess if an observed movement was dominantly horizontal or vertical. Two stacks of descending ENVISAT images were also processed over both AOIs, 28 images (track 352) acquired from 07 February 2003-08 October 2010 for AOI-1, and 37 images (track 37) from 07 November 2002-08 July 2010 for AOI-2. Both image sets have VV polarization, LOS of \(\sim\)23\({}^{\circ}\), revisit time of 35 days and ground resolution of \(\sim\)20 m \(\times\) 4 m, respectively, in range and azimuth directions. The obtained results are used for comparison with those of Sentinel-1 in terms of number of detected points, point density and noise level. Lists of all Sentinel-1 and ENVISAT images used in this study are available in the Supplementary Materials.