## 2 Materials, Methods, and Discussion
### Kinematic and Geometrical Constraints to Monitor Geohazards  
Even if a sufficient number of Sentinel-1 data over Great Britain is already available (see Section 2.1), geometrical distortions represent a spatially-limited constraint (see Section 2.2) and land cover always allows a sufficient density of ISBAS radar targets (see Section 2.3), three systematic conditions can still negatively affect the analysis:  
1. The extension of the investigated phenomena must be over \(\sim\)0.008 km\({}^{2}\) which refers to the size of the smallest possible feature that can be detected by Sentinel-1 data accounting for the spatial resolution of its IW mode. Apart from dolines and sinkholes particular landslide typology (especially falls and topples), UK geohazards have an extension at the kilometric scale.
2. Rather than providing 3-D components, ISBAS measurements are along the LOS direction and, therefore, insensitive to horizontal displacements in the along-track direction [39]. To decompose the InSAR LOS signal into its vertical and horizontal components we, therefore, need sufficient acquisitions of both ascending and descending SAR data [40]. In the absence of both geometries, quantification of the fraction of maximum motions measurable along the satellite LOS can be quantified by means of the R-Index for areas of good visibility or foreshortening. In this regard, most of the UK geohazards do not prevent InSAR analysis by being associated with predominant vertical deformation (see Section 1) and, specifically, this is true for dolines, sinkholes, collapsible ground, compressible ground, and shrink-swell terrains.
3. Finally, a third limitation is represented by the incapability for resolving deformation \(\geq\)\(\lambda\)/4 between two SAR acquisitions [41] that constrains the measurable displacement velocity to \(\leq\)86 cm/year, or 1.4 cm for Sentinel-1 scenes six days apart. Such values are usually large enough to completely assess and monitor strain rates associated with rock and soil viscous, viscoelastic, or creep behaviour induced by human or natural-changes in stress conditions. At the same time, events partially detectable by SAR platforms represent an unlikely occurrence in the UK, temporally in the case of interseismic elastic deformation where an earthquake of \(\geq\)3.7 ML statistically hits every year, and spatially in the case of fall and topple landslides, whose recurrence is mainly limited to Western Scotland [13].  
An example of the S-1 kinematic and geometrical constraints is given in Figure 8 which focuses on a sinkhole of 12 m in diameter which appeared twice (14 August 2015 and 15 September 2015) in Mancunian Way, Manchester and determined the partial collapse of the carriageway within diamicton deposits (Figure 9). The descending interferogram spanning across the date of the event has not been able to detect the phenomenon due to the (1) and (3) conditions, despite its vertical component of movement.  
Figure 9: The sinkhole on Mancunian Way (**a**) Image copyright: MEN SYNDICATION). The 10k Geological map of superficial deposits (**b**) and the wrapped interferogram between 22 July 2015 and 15 August 2015 (**c**). The black square refers to the pixel size and geolocation error is half the size of the pixel. Contains Ordnance Data © Crown Copyright and database rights 2017.