# Review of Works Combining GNSS and InSAR in Europe
## 1 Introduction  
The Global Navigation Satellite System (GNSS) and Synthetic Aperture Radar Interferometry (InSAR) can be used for various applications owing to their principal functions. GPS enables the collection of three-dimensional (3D) deformation information, considering the coordinates of surveyed points. Since the 1970s, InSAR [1] has permitted the investigation of the phase difference (interferogram) between two images acquired over the same area at different times. The GNSS term refers to any satellite constellation that provides information globally about the positioning, timing, and navigation data of elements to a GNSS receiver that then commutes this information to a precise location. The GNSS family includes several regional satellite systems, e.g., GPS (Global Positioning System), Galileo, and GLONASS (GLObal'naya Navigatsionnaya Sputnikovaya System). GNSS performances are (i) accurate in the determination of real position, velocity and time of travel between the transmitter and receiver; (ii) intact, allowing the insertion of an alarm in case of anomaly positioning; (iii) able to work continuously without interruptions; and (iv) able to fulfil all the criteria of accuracy, integrity, and continuity. This system provides three-dimensional along the East-West, North-South, and up-down direction for ongoing deformation through the precise determination of the receiver position with millimetric accuracy in order to assess the main components. GNSS is very useful for investigating regionally low spatial frequencies, e.g., tectonic or geodynamic deformation fields, but local patterns or movements cannot be appreciated due to the large distance between adjacent GNSS stations. Themain limitation of the measurement approach of GNSS is a lack of data in some areas due to the absence of GNSS stations since this technique is station-dependent [2]; however, the European continent is sufficiently covered by more than 300 distributed multi-GNSS reference stations [3]. The InSAR approach is a consolidated remote sensing technique employed for the detection and monitoring of ground deformation that has become widely used over the last decade. The first application dates back to the late 1980s [4] and was based on the DInSAR (Differential InSAR) approach that enabled the collection of cumulative deformation derived from the phase difference of two subsequent SAR acquisitions captured during two consecutive satellite passages over the same area. The transition from a single interferogram (DInSAR) to a multi-interferogram approach permitted a multi-temporal analysis (MTInSAR, Multi Temporal InSAR) [5], which sensibly and rapidly improved the investigation of long-term deformation events. MTInSAR approaches are indeed applicable to investigate the temporal evolution of ground displacement over time by exploiting coherent (i.e., electromagnetically stable) backscattered points corresponding to reflected elements on the ground. The spread and rapid development of different processing algorithms has permitted wide application to various geohazard fields [6], such as subsidence, landslide, tectonic and volcanic phenomena [7; 8]. These InSAR techniques also have certain drawbacks strictly related to the (i) geometrical effects between the LOS (Line of Sight) and the slope; (ii) atmospheric contribution and aliasing phase; and (iii) snow, vegetation, or variable land cover of a target area. Nonetheless, these techniques enable the (i) investigation of wide areas with a high measurement precision; (ii) containing of costs with respect to the benefits [9]; (iii) temporal repetition (up to 6 days over the same area with the Sentinel-1 constellation); (iv) all-weather and day/night operation; (v) investigation of remote and inaccessible areas; and (vi) the possibility to back-analyze a certain phenomenon back to the '90s by using archived satellite data, e.g., ERS1/2 data.