# Potential and Limitations of the New European Ground Motion Service in Landslides at a Local Scale
## 2 Materials and Methods
### Sentinel-1 A-DInSAR by Means of European Ground Motion Service  
The new EGMS service processes all Sentinel-1 data over most of Europe and it is implemented by the European Space Agency (ESA) [49]. The service provides, for registered users, visualization, data exploration, and preliminary analysis of the Sentinel-1 A-DInSAR products from 2015 to 2021, based on the SAR data provided by Sentinel-1A/B satellite constellations. This service identifies and processes sets of measurement points (MP): Persistent Scatterers [25] and Distributed Scatterers [26; 50] by using all acquisitions, both ascending and descending orbits. According to [32], three EGMS products are available: level 2a, level 2b, and level 3. Level 2a corresponds to basic products of deformation velocity and deformation time series measured in both ascending and descending orbits and in the Line of Sight (LOS) direction at full Sentinel-1 resolution. The time series are associated with each MP. The velocity and displacement deformation are relative, being referred to as a stable virtual reference point chosen during the processing of the SAR Sentinel-1 image stacks on a frame-by-frame basis. Level 2b includes basic products calibrated by using a Global Navigation Satellite System (GNSS) reference network at full Sentinel-1 resolution. After the calibration procedure, LOS measurements are no longer relative to a local reference point and they are absolute values. Level 3 (ortho) corresponds to East-West component (horizontal motion) and vertical component (vertical ground motion) derived from level 2b products at 100 \(\times\) 100 m pixel (Copernicus DEM resolution). The estimation process of horizontal and vertical motion consists of the decomposition of the Calibrated (L2b) product to provide two discrete geospatial layers containing purely vertical and purely East-West displacements resampled to a 100 m grid. The horizontal and vertical MP are defined from all ascending and descending MP within each cell. Then, they are separately averaged, with the resulting time series referenced to the center of the cell. All three product levels have an acceleration field. This attribute consists of a single given value (mm/yr\({}^{2}\)) for each detected point. However, we have observed that for the four datasets, the values of acceleration are not significant and are redundant for this study. Since there is only one value for each MP, this value cannot be correlated with the acceleration and recuperation intervals observed and studied in Section 3.4. All these data have been managed with a Geographical Information System (GIS) environment. The theoretical algorithm basis can be found in the technical documentation on the EGMS website [51]. Four different processing chains contribute to the generation of the EGMS products: E-GEOS, TRE ALTAMIRA, NORCE, and GAF/DLR [50; 51; 52]. The main processing steps of these chains are as follows: (1) reference image selection, co-registration, and interferogram generation; (2) selection of MP candidates; (3) multi-interferogram phase unwrapping; (4) estimation of the atmospheric phase screen; (5) generation of displacement time series for each MP; (6) geocoding. Co-registration involves resampling all secondary images onto the grid of the primary image to ensure pixel-to-pixel alignment. To avoid phase artifacts between bursts, an azimuth co-registration accuracy better than 0.001 pixels is necessary [51; 53]. Once all wrapped differential interferograms are ready, the EGMS processing chains identify a set of MP. The selection of MP candidates is based on amplitude and phase estimators. For amplitude, low values of the amplitude dispersion index (DA) help identify coherent scatterers within the SAR scene. The pixels with DA values lower than 0.25 are considered good MP [51]. For phase, the spatial coherence map of each differential interferogram is computed. Coherence maps are then averaged and those pixels exceeding a certain threshold (typically above 0.7) are selected as MP [26; 27; 50].