## 4 Results
### Geodetic Data (GNSS and DInSAR) Analysis
#### 4.5.3 Combination of GNSS and DInSAR datasets  
To validate the interferometric results, because no GNSS data were available in the Sarno plain, we used the GNSS data collected at the PACA permanent station, which is located a few kilometres from the study area (Figure 11). The comparison between ENVISAT and GNSS time-series data is displayed in Figure 12. It is worth noting that, to obtain the vertical component identified by the interferometric results, the two acquisition geometries (ascending and descending) were decomposed according to procedures presented in the literature [78, 79]. As a result, both the DInSAR and GNSS detected a ground displacement rate of the same amplitude (0.4-0.5 mm/yr) even if this was at the limit of detectability.  
Figure 9: GNSS daily solutions collected at PACA permanent station located in Palma Campania for the up component. Linear trend, which quantifies the subsidence rate, is plotted (red line). A tentative modelling of the hydrological contribution of the observed ground displacement based on MERRA2 and GLDAS models is plotted as well (**a**). GNSS daily solutions, after correction of the hydrological contribution and final assessment of the subsidence rate. The black brackets mark the time range for which the DInSAR dataset is available for comparison (**b**).  
Figure 10: Mean displacement rate maps for: ERS1/2, (**A**) ascending and (**B**) descending; ENVISAT: (**C**) ascending and (**D**) descending; (**E**) Cosmo-SkyMed images in descending orbit; and (**F**) SENTINEL-1 images in descending orbit (2016–2020 time span). Dashed black lines indicate faults inferred by geomorphological and stratigraphic analysis (see Figure 4).  
Figure 13 shows the time series extracted from the DInSAR images for three selected points, which were deemed more suitable to characterise the kinematics of the study area. The DInSAR data were up components, projected according to the suitable satellite ground tracks. From the comparison between the GNSS and DInSAR time series, we derived a confidence limit for the DInSAR data. For the period when both GNSS and DInSAR data are available, namely, 2003-2010 (Figure 9b), the two techniques provided evidence on the stability of the PACA site located on the Apennine foothill, or a low-rate subsidence, whereas a different behaviour was evident from the DInSAR data for the Sarno plain.  
Figure 11: Mean displacement rate map for ENVISAT images (2003–2010 period): (**a**) ascending and (**b**) descending. Red triangle indicates location of GNSS Palma Campania (PACA).  
Figure 12: Comparison of GNSS and DInSAR time series (vertical component) collected in Palma Campania with the aim of validating the DInSAR data.  
Figure 13: Time series of the DInSAR data (up component) used to characterise the vertical displacements within the Samo basin (**a**–**c**). Refer to Figure 11 to locate the selected points.  
Therefore, after validating the interferometric results in the study area, it was possible to reconstruct the whole time series of displacements, by calculating the vertical components, for the period of 1993-2020 (Figure 13). It is evident that the area was subject to subsidence phenomena, as was well known in the literature [1,21]. Some targets, belonging to specific sectors within the Sarno basin, were selected (SRN1, SRN2, and SRN3 in Figure 11). From the time series, it is possible to observe an almost linear trend (Figure 13). The most relevant displacements were observed in the central-northern sector of the plain (SRN1 and SRN2), where the highest rates of piezometric depressions were recorded during the period of 1992-2003 (Figure 7). The central sector (SRN2) was found to be the most affected by subsidence phenomena, with a cumulative displacement of about -180 mm and a rate of nearly 6 mm/yr (Figure 13b), whereas the area less affected by subsidence, namely the south-eastern sector (SRN3), had subsidence rates of the order of 3 mm/yr (Figure 13c).