# Review of Works Combining GNSS and InSAR in Europe
## 1 Introduction
GNSS data play a crucial role in different phases of the InSAR processes since changes in atmospheric refraction can result in a misinterpretation of InSAR results [10]. Specifically, such phases include SAR satellite acquisition, InSAR processing and InSAR post-processing. The GNSS pointwise data can be used for InSAR calibration; Reference Point(s) identification, to which the displacement measured by Persistent Scatterers (PS) is referred; tropospheric and/or ionospheric delay correction [10] or unwrapping the interferometric phase [11] during MTInSAR processing; post-calibration and deformation velocity correction assessed by MTInSAR approaches [12; 13; 14]; GNSS deformation measurements comparison with the MTInSAR ground deformation [15]; or MTInSAR product validation by comparison with information derivable by the GNSS network [16]. These are the only possibilities available when attempting to combine the SAR and GNSS datasets taking into consideration the singular analysis of ground deformation. Indeed, many authors have published interesting works about the combination of InSAR and GNSS data for different purposes, such as (i) atmospheric correction (e.g., [17; 18]); (ii) subsidence or uplift analysis (e.g., [9; 19]); (iii) tectonic and seismic investigation e.g., [20; 21]); (iv) landslide back-analysis (e.g., [22; 23]); (v) volcanic event studies e.g., [24; 25]); (vi) infrastructure investigation (e.g., [26; 27]); (vii) glacial analysis and recognition (e.g., [28; 29]); and (viii) other more general applications (e.g., [12; 30]). GNSS data and InSAR are also combined to investigate the atmospheric contribution to InSAR processing with consequences to the velocity and cumulative displacement in the ground deformation analysis [31]. Atmospheric investigation is a complex task due to its heterogeneity. It can be simplified by separately considering the troposphere, approximatively from 0 to 40 km above the Earth's surface, and the ionosphere, approximatively from 50 to 1000 km. In addition, the troposphere influences the wet lower portion (approximately 0-11 km) and the dry upper layer (approximately 11 km above the Earth) differently, which can be more difficult and simpler to model, respectively [32]. Considering the wide areas covered by the SAR images,e.g., 250 km with Sentinel-1 [33], the atmospheric effects combined with the reported high precision of the available InSAR approaches could be challenging, especially if the available datasets of SAR images are limited [34; 35]. This review focuses on the collection and analysis of scientific contributions that combined GNSS/GPS data and InSAR approaches, i.e., both DInSAR and MTInSAR techniques, for pursuing different perspectives, such as ground deformation recognition and analysis or atmospheric contribution assessment over the entire European continent, excluding Turkey and Russia. In addition, a detailed investigation on Italy, the country most represented by scientific contributions combining GNSS and InSAR data, was conducted. The state-of-the-art of the various applications of these two techniques can improve the knowledge of the scientific community and help in the further development of new approaches or additional applications in different fields. In this review, the DInSAR and MTInSAR approaches were both considered to cover a wider range of field applications, to both rapid and slow phenomena, but analyzing the scientific contribution separately. In contrast, GNSS, GPS, cGPS (continuous GPS), and DGPS (Differential GPS) are considered unique approaches for avoiding discrimination with low statistical soundness considering that the basic approach is the same.