# Review of Works Combining GNSS and InSAR in Europe
## 5 Discussion
Combining the information regarding the year of publication of the scientific contributions collected and the investigated SAR satellite, it is possible to state that the later publications aimed to investigate the evolution of a phenomenon as long as possible. Obviously, this cannot be affirmed for publications involving atmospheric interactions and corrections or for those investigating the co-seismic earthquake effects of DInSAR. The incremental increase in the number of publications reported demonstrates that the remote sensing techniques investigated in this review, i.e., geodetic and InSAR approaches, were being readily employed by the scientific community as a result of their benefits and the consequences for stakeholders and end-users as environmental managers. In recent years, this interest began to be traduced in the publication of national and regional GMS (Ground Motion Services) due to the free availability of InSAR data and, in some cases, the already GNSS-calibrated model. In fact, a higher number of applications, and thus a greater number of papers, was collected and recorded in those countries in which a Ground Motion Service (GMS) already exists or is nearly ready for presentation. In 2002, Italian law allocated funds for a Not-ordinary Plan of Environmental Remote Sensing aiming at monitoring considered at risk hydro-geomorphological areas, providing nationwide ground deformation maps calibrated by GNSS data. ERS and ENVISAT interferometric products, processed by PSInSAR and PSP-DINSFAR approaches, were published in the first phase of the project (2008-2009) and then updated in a second phase (2010-2011), while COSMO-SkyMed data were implemented in the same project in the third and final phases (2013-2015) [218; 219]. Next, the first nation that made available updated GMS data over the entire territory was Norway in 2018; this publication was provided by the WebGIS portal Radasat-2 data (2010-2018) and Sentinel-1 (2015-2019), not calibrated by GNSS information [220]. In 2019, a year later, BodenBewegungsdienst Deutschland presented the Germany GMS, making the Sentinel-1 PS data available, GNSS-calibrated, processed thanks to the PSI approach [221] by the DLR German Aerospace Center research institute. In addition, Danish GMS and Dutch GMS are under development and will be published soon. More locally, Tuscany (central Italy) and Veneto Region (North-East Italy) have produced regional GMSs that are providing Sentinel-1 MTInSAR data free of charge processed by the SqueezeSAR [196] algorithm. These three regions are the object of continuous monitoring, which is systematically updated and interpreted every 12 days. In addition, the application of a data-mining algorithms also highlight the PS with relevant trend changes [222; 223]. Taking into account the usefulness and versatility demonstrated by this review regarding the combination of GNSS and InSAR data in ground deformation analysis, in the future, PS data will become available, and the national and regional GMS should be GNSS-calibrated. Currently, this procedure is sometimes conducted by researchers as a further step for specific investigations with good results [20; 100]. Consequently, the promising results obtained by the scientific community and the free availability of data, which permit drastic cost reductions, have drawn increasing interest from the administrative managing office for the mapping and monitoring of the ground deformation issue [223; 224; 225]. In addition, since 2016, Europe has been working on an initiative pushing to implement the available data provided by MTInSAR products at continental scale in a project named European Ground Motion Service (EGMS) [226].