## 3 Materials and Methods  
Four datasets of SAR images were acquired and processed (Table 2, Figures 3 and 4), spanning a total period of 25 years. The ERS, Envisat and Sentinel-1 missions acquired data in C-band (frequency 5.7 GHz, wavelength 5.6 cm) at a ground resolution of circa 5-20 m; the TerraSAR-X data were acquired in X-band (frequency 9.6 GHz, wavelength 3.1 cm) at a ground resolution of c. 3 m (Stripmap mode). An external digital elevation model (DEM) data from the Shuttle Radar Topography Mission (SRTM 3-arc second) with 90 m horizontal resolution was used to remove the topographic component of the interferometric phase. A common reference point (Figure 1) was defined 15 km NW of Brussels (4.54\({}^{\circ}\) E 50.932\({}^{\circ}\) N). The ERS1/2 scenes of descending track 423 were obtained in raw L0 format from ESA. An initial selection comprised all scenes providing full coverage of Belgium, with restrictions on the baseline difference (maximum of 1200 m relative to an arbitrarily chosen master acquisition) and Doppler shift (maximum of 1200 Hz) to ensure good coherence. During various pre-processing steps, some scenes were dropped due to failure of focusing or coregistration. The final, fully processed dataset includes 78 acquisitions between 1992 and 2006. All images were coregistered to a single master image, and selected by minimizing spatial and temporal baselines. The conversion from raw L0 to SLC (Single Look Complex) format was done using ROI_PAC software [25]. The SLC files were oversampled with a factor of two and interferograms were produced using the Doris InSAR processor [26] and recomputed ERS-1/2 ODR orbits [27]. The interferograms were then imported in StaMPS (Stanford Method for Persistent Scatterers) for Persistent Scatterer processing [5]. The default parameters were used, except a slight relaxation of the thresholds for amplitude dispersion for selecting PS candidates (0.42 instead of 0.4) and standard deviation for phase noise weeding (1.2 instead of default 1), in order to improve the number of PS points. After unwrapping, spatially-correlated errors due to atmosphere, DEM and orbit error were iteratively estimated and removed. A dataset comprising 73 ENVISAT scenes of descending track 423 in raw L0 format was obtained from ESA. All scenes had baselines of less than 1000 m with respect to the chosen master (Table 2) and were successfully processed following the same procedures as for ERS, except for the use of DORIS Precise Orbit State Vectors and no changes made to the default parameter settings in StaMPS. A dataset of 74 TerraSAR-X and TanDEM-X Stripmap (SM) descending scenes in SLC format was obtained from German Aerospace Centre (DLR). Creation of interferograms and PS processing were carried out using DORIS and StaMPS. Some parameter settings in StaMPS (size of spatial filters and windows) were adjusted to fit the higher spatial resolution of TerraSAR-X. This dataset is characterized by a high spatial resolution (3 m) and short baselines (maximum of 348 m). The final dataset consisted of data acquired by the Sentinel-1A and 1B missions and accessed through the Copernicus Open Access Hub. The geometric specifications of this satellite constellation (i.e., short spatial and temporal baselines [28]) promise improved data quality (higher coherence) and enhanced performance of PS-InSAR techniques, but the processing of TOPSAR data is more challenging. The study area is covered by 64 descending scenes acquired in Interferometric Wide Swath Mode (IWSM), with a maximum baseline of 119 m. The complete PS-InSAR data processing chain was