## 3 Materials and Methods
### InSAR Data
The EGMS, founded by the European Commission (under the European Environment Agency's framework service contract), constitutes the first application of InSAR technology in monitoring ground deformations over an entire continent (Copernicus Participating States and the United Kingdom) based on full-resolution processing of all Sentinel-1 satellite acquisitions [32]. Moreover, a Global Navigation Satellite System (GNSS) high-quality 50 km grid model was developed withinthe EGMS to calibrate the InSAR ground motion products. The EGMS provides data from February 2015 to December 2021 for the three types of products:  
* Basic (L2a): This provides InSAR measurements of ground displacement along the LOS of the Sentinel-1 satellite, covering both ascending and descending geometries;
* Calibrated (L2b): Similar to L2a, this product includes InSAR ground motion measurements that are anchored to a common geodetic reference frame using a large-scale GNSS velocity model;
* Ortho (L3): This offers vertical and east-west ground deformation measurements. These are derived from multiple L2b products with complementary acquisition geometries and are tied to the geodetic reference frame. These products are available through the EGMS explorer [30]. The four partners that compose the OpeRational Ground motion INsar Alliance (ORIGINAL) consortium (e-Geos, TRE Altamira, NORCE, and GAF) operate their own processing algorithms implemented in the different processing chains based on advanced PS and DS InSAR processing techniques. However, end products have the same attributes. The characteristics of the algorithms used by the 4 InSAR Processing Entities (IPEs) are listed in Table 3.  
The EGMS data and processing methodologies, with the exception of SqueeSAR, rely primarily on PSI algorithms. InSAR techniques grounded on DS come into play to supplement PS results, especially in areas where the density of pointwise scatterers falls short of the project's standards [26]. Figure 5 provides a streamlined flowchart of EGMS processing and showcases a visual representation of the three product levels. On the AoI (area of interest) of Yela, the InSAR datasets consist of calibrated Sentinel-1 ascending (A15-001 and A16-103) and descending (D16-081) data, and the horizontal and vertical deformation components are established based on the geodetic reference model data provided by EGMS [30]. Sentinel-1 data provide time series analyses of ground deformation measurements from February 2015 to December 2021, with a temporal resolution of 6 days in both observation geometries until early 2022 and then of 12 days temporal resolution due to a problem with the Sentinel-1b satellite. As of May 2019, Sentinel-1 data can be compared with the available data on injected/withdrawal gas volumes provided by Enagas [18]. The satellite data cover an area larger than that of the UGS facility in Yela. Table 4 shows the main characteristics of the Sentinel-1 datasets that were downloaded and used to analyse ground displacements over the Yela gas storage facility. Figure 5: Schematic representation flowchart of EGMS processing [26]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**IPE** & **Name of the Algorithm** & **Reference** \\ \hline E-Geos & PSP-IFSAR & [33] \\ \hline TRE ALTAMIRA & SqueeSAR\({}^{\circledR}\) & [34] \\ \hline NORCE & GSAR-GTSI & [35] \\ \hline GAF/DLR & PSI performed with Integrated Wide Area Processor (IWAP) & [36] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Main characteristics of the IPEs chain.