# Review of Works Combining GNSS and InSAR in Europe
## 5 Discussion
Temporally, more recent papers involve more large areas due to various factors: (i) the possibility to have more and improved algorithms for processing large SAR datasets; (ii) the technological progress of the last few years, allowing us to have available advanced computational resources, e.g. cloud computing by virtual machine system and parallel or distributed computing architectures used to process larger dataset over wide areas; (iii) the willingness of different radar satellite constellations, also in different bands, i.e., X-band, C-band and L-band; and (iv) the ease of obtaining free SAR images at a medium resolution, such as the Sentinel-1 constellation, by the Copernicus Access Hub of the ESA (European Space Agency), ensuring systematic worldwide covering acquisition since April 2014, or the availability of limited imagery datasets at a high resolution, i.e., X-band, by means of dedicated scientific projects in open calls. Only a few articles, 10 of the 174 examined, have as a main aim the analysis of more case studies in the same nation, within very wide areas, or throughout entire countries. In detail, four articles are noteworthy for the extension and the relevance of the work. The first two focused on the ground deformation occurring in Netherlands [176; 178], while the remaining focus on the generation of atmosphere Precipitable Water Vapor (PWV) maps over large areas in Portugal [169] and the feasibility assessment of a national InSAR ground deformation map over the United Kingdom [90], which both took place in 2017. In 2020, the first continental-scale processing of Sentinel-1 imagery was described in [PERSON] et al. [91], which provided a calibrated ground deformation map of Europe with the velocity of deformation and cumulative displacement corrected by the GNSS network data. The authorship, taking into account the affiliation of the first author of the scientific contributions on ground deformation or atmosphere contribution analyses over European countries, demonstrates that approximately 80% of the total refers to the same nations. For the remaining approximately 20%, 10 authors are referred to Countries outside of Europa (e.g., 4 from USA, 5 from China, and 1 from Canada), while the other main authorships are of Italian (8), Portugal (6), and French (5) researchers. This can be justified by considering (i) collaborations or (ii) projects from different universities, institutions, or entities and (iii) the PhD abroad periods or (iv) research abroad time of researchers. Italy, with 9 contributions authored by non-Italian investigators, is the most investigated nation, not only by Italian researchers but also those from European (2) and international (5) investigators. Next, Portugal is unique in that it was only analyzed by Portugal researchers that also investigate Romania (6 contributions to 7 are from Portugal researchers), followed by Spain, with 5 contributions from European researchers. The correlation between the temporal evaluation of European and Italian studies is interesting. For the first few years, this relation is easily comprehensible considering that the scientific contribution collected for the first three years, i.e., 1999-2000 and 2001, involved the Italian territory. It is interesting to note, comparing Figures 2 and 7, that the trend is very similar, with an incremental increase that progressed until the peak contribution in 2015, which was then followed by a reduction. The high number of scientific contributions recorded over the Italian territory severely influences the trend of the temporal distribution that occurred over Europe. In contrast, the studies published in 2020 do not follow this role; in fact, the last analyzed year exhibits a relevant number of scientific contributions of 2019 even if no publications were published involving Italy. A similar correlation can also be identified when considering the European and Italian temporal distributions and the associated field of application.