# Review of Works Combining GNSS and InSAR in Europe
## 5 Discussion  
A substantial number of scientific contributions using remote sensing data derived from GNSS and SAR systems to investigate and analyze ground deformation in European countries, covering a time span of 31 years (from 1999 to 2020), were collected. The spatial distribution of all the scientific contributions, considering scientific articles, book chapters or conference proceedings, mirrors the distribution of the main relevant geohazards affecting each country. Italy and Spain are well represented due to the high number of geohazards affecting their territory, especially considering the subsidence occurring in both Italy [205] and Spain [9] and the landslides affecting Italy [206]. The limited number of scientific contributions combining GNSS and InSAR data to study landslides is surprising considering the relevance of these natural hazards and the major consequences occurring throughout Europe [207]. Nonetheless, the Italian territory is also affected by frequent seismic swarms and strong earthquakes in addition to the presence of volcanoes, and both phenomena are deeply investigated by the joint use of GNSS and SAR data. In the same way, it is surprising how low the number of applications recorded in the Netherlands is considering the importance of subsidence phenomena in that country (e.g., [208]), which exert social and economic consequences and induce damage to structures and infrastructure [209; 210]. Another well-represented nation is Norway, which is strongly affected by landslides, often triggered in quick clay soils [211], and characterized by the presence of permafrost, which can cause four types of landforms [212]: (i) palsa (e.g., [213]),  
Figure 7: Temporal distribution of the scientific contribution recorded in Italy categorized according their field of application. Scientific contributions are missing for 2003, 2004, 2006, 2009, 2016, and 2020. (ii) rock glaciers (e.g., [214; 215]), (iii) ice-cored moraines (e.g., [214; 216]), and (iv) ice-wedge polygons (e.g., [217]). In addition to scientific papers dedicated to the analysis of displacement, general articles employing atmospheric analyses, in which GNSS products are implemented to correct interferograms or remove the geodetic influence on InSAR displacement measurements over wide areas, were collected and analyzed. This approach is even more relevant due to the nature of the wide areas investigated and monitored by InSAR approaches. The combination of DInSAR and GNSS data has not always been categorical, but various trends and approaches utilized over the years can be illustrated. The various works analyzed have shown the potential of combining both data for different purposes. For instance, GNSS data were used to improve the accuracy of the interferograms generated [11], topographically correcting the interferograms and the elevation data to measure ice surface motion maps along the Gjalp Volcano. Most of the works jointly used GNSS and DInSAR data to produce a more precise and reliable analysis of the movements detected by either combining or validating the available data. Indeed, in most of the pioneering works, authors worked to validate DInSAR data, since in most of the cases, the latter were used for the first time and their results needed to be validated. For instance, in [PERSON] et al. [135], in which 3D data derived from TerraSAR-X processing over the La Vallette landslide (southern France) were validated with GNSS data; in other cases, such as in [PERSON] et al. [23] and [PERSON] et al. [146], GNSS data, along with geological and geotechnical data, were used to ground truth C- and X-band imagery. Approximatively half of the scientific works are focused on single case studies or events with a local influence, while 45% involve regional areas.