## 4 Results  
Analysis of the ERS (1992-2006), Envisat (2003-2010) and TerraSAR-X (2011-2014) velocities shows that the city of Brussels is, overall, characterized by a regional uplift, with a diminishing trend through time (Figure 5A-C). The Sentinel 1 dataset (2014-2017) indicates that the uplift has faded and the area that was previously uplifting is now characterized by negative velocities (Figure 5D). The C-band missions (ERS, Envisat, Sentinel 1) acquired data at a similar ground resolution, resulting in PS point densities of the same order of size (circa 200 PS/km\({}^{2}\) in the study area, see Table 2). A steady increase between the successive missions may be attributed to improved geometric specifications. Owing to its higher spatial resolution, the point density achieved by TerraSAR-X is an order of magnitude larger (circa 2700 PS/km\({}^{2}\) for the study area). In the urbanized centre of Brussels, point densities are highest, exceeding 800 PS/km\({}^{2}\) and 6000 PS/km\({}^{2}\) for the C-band satellites and TerraSAR-X, respectively. The Soignes Forest located to the SSE of the Brussels city centre (Figure 5A) and the Laeken Park in the northern part of the city centre do not contain PS points. Vegetation is responsible for the loss of coherence between the acquisitions and thus the capability to detect PS. For ERS (Figure 5A), the general observed trend of uplift is most pronounced in the city centre of Brussels, along a SW-NE oriented axis (average velocity of 2.7 mm/year). The affected area roughly corresponds to the Senne river valley inside the historic heart of Brussels. For Envisat (Figure 5B), the main uplift area has shifted along the Senne river valley towards the northern suburbs of Brussels (average velocity of 2.3 mm/year). In the SW of the city, the ground movements are reduced to the range of stable values. A notable subsidence feature is observed in Forest/Uccle districts, at the SW border of the Brussels Region: here, an area of circa 1.5 km\({}^{2}\) is characterized by substantial negative velocities (average velocity of \(-\)3.1 mm/year). For TerraSAR-X (Figure 5C), the data show a more diffuse pattern in the city centre, with most of the velocities close to stable values. An uplift is still visible westwards of Brussels (average velocity of 1.8 mm/year), but the eastern part of the study area is characterized by stable velocity values. Distinct, local subsidence features are observed in some areas characterized by erratic negative velocities. Lineament \"La\" corresponds to the railway tracks at Schaerbeek station. Lineaments \"Lb\" and \"Lc\" correspond to the NNW-SSE oriented Maelbeek and NNE-SSW oriented Woluwe river valleys, respectively. During the Sentinel 1 time frame (Figure 5D), the general trend of ground movements appears to have stabilized. Within a zone (which is SW-NE oriented, 200-800 m across, and corresponds to the Senne river valley), the direction of movement reversed from uplift to slight subsidence (average velocity of \(-\)1.3 mm/year). Lineament \"La\" is still visible as a distinct subsiding feature. However, as the range of the velocities is quite small and the observation period is short in comparison with ERS and Envisat the Sentinel 1, the results look noisy. In the future, after one or two years of new Sentinel 1 acquisition, a new processing operation will be conducted in order to verify the current observations. From all datasets, histograms were plotted to analyze the frequency distribution of the average annual velocities (Figure 6A-D). Values in the range of \(-0.49\) to \(0.5\) mm/year can be considered as stable during the observation period, relative to the reference point. Although the majority of PS points in Brussels appear more or less stable, the ERS and Envisat histograms show a clear asymmetry towards positive values, confirming the general trend of uplift in the area. Figure 5: Colour classification based on the average annual velocities of PS points for ERS (**A**); Envisat (**B**); TerraSAR-X (**C**); and Sentinel 1 (**D**).