# Review of Works Combining GNSS and InSAR in Europe
## 3 Temporal and Spatial Distribution of Scientific Production in Europe
### Temporal Distribution  
The first scientific contribution outlining the combined use of GNSS and InSAR data dates back to 1999 [42]. This article focused on a model used to estimate the geometry and slip distribution of the fault plane originating from the 26 September 1997, Colfiorito earthquakes (central Italy) by combining SAR interferometry and GPS measurements. In the following years, since 2000, the scientific community maintained a greater interest in data integration, also involving other European countries, such as Iceland, Spain, and Greece. Since 2009, excluding 2008, with a high number of 8 scientific contributions, a slight increment of scientific contributions was identifiable until 2012, with more than 10 articles, books or conference proceedings. The maximum number of scientific contributions was reached in 2015, with 29 contributions from 14 different countries. Next, in 2018 and 2012, a total of 20 and 17 works were published, respectively. It is worth noting that Italian case studies were published annually, ranging from the first application in 1999 to 2019, only excluding 2009, 2016, and 2020 (Figure 2).  
Figure 1: Spatial distribution of the scientific contribution of Europe.  
The analysis of the temporal evolution in relation to the field of application of scientific contributions (Figure 3) revealed that early studies were focused on the analysis of tectonic-induced deformations. The rise of an anticline beneath an urban area in Catania in 2001 [87] and the modeling of the dislocation of the edifices involved in the 1997 Umbria-Marche seismic sequence [88] were analyzed by integrating GNSS and InSAR data. In 2001, the first atmospheric investigation application was presented, showing primary results regarding the calibration of atmospheric effects on SAR interferograms by GPS and local atmosphere models [38]. In 2002, the joint application of GPS and DInSAR for volcano and seismic monitoring was carried out in different cases in Spain for the 1992-2002 period [55]. In 2005, the first application of the combined use of GNSS and SAR data for recognizing subsidence was published. In particular, five different measurement techniques, among which are the DGPS and MTInSAR approaches, were exploited for mapping and monitoring land subsidence in the Venice lagoon [43]. A year later, the first application to landslide studies was published dealing with the Triesenberg-Triesen landslide in Leichteinstein, where GPS monitoring data were compared with 3D models generated using an MTInSAR approach on ERS datasets [89]. Among the less represented fields of application, in 2002, the first application to glacial research was proposed, where an interferogram was generated to produce an ice surface motion map of the Gjlap volcano in Vatnajokull, Iceland [29]. Regarding infrastructure and uplift case studies, the first key scientific contribution focused on the monitoring of the urban area of Bratislava, Slovakia [26] and on the investigation of an area close to the geothermal site of Landau, Germany [19; 63].  
In recent years, the combined use of GNSS and SAR data has been adopted for large-scale analyses. Indeed, ground deformation maps have been produced at national scale in the United Kingdom [90] and at continental scale covering all of Europe [91]. In this case, GNSS data were implemented to identify and filter out possible residual atmospheric artefacts that may affect the quality of the employed MTInSAR data.  
Figure 2: Temporal evolution of the scientific contributions combining GNSS and InSAR data in Europe per country.