## 2 Study Area and Data
### InSAR for Remote Ground Motion Monitoring  
InSAR is a geodetic method of measuring changes in the Earth's surface height. It is based upon interferometry, where the interference of electromagnetic waves from two SAR acquisitions is utilised to derive precise changes in elevation [27]. Deformation studies have been revolutionised by differential InSAR (DInSAR) since the first detection of differential motion [28]. However, it is widely acknowledged that phase decorrelation (or incoherence) is a significant limitation of InSAR, which severely affects the spatial distribution and density of measurements outside of urbanised and arid terrain with sparse vegetation cover [29]. This effect is most prevalent in higher frequency radar bands, such as C-band, where incoherence is pervasive over agricultural fields, forests, semi-natural areas, and wetlands [30]. The poor density of InSAR measurements in rural areas in some extents can be mitigated through the deployment of artificial corner reflectors, installed in situ [31], although in steep, inhabspitable terrain this can be dangerous and laborious over vast areas and further requires routine maintenance to ensure that their integrity is guaranteed over the entire monitoring period, including during times of snow cover. Logistically and administratively, it can slow the start of a monitoring programme and, as it is not possible to install them retrospectively, it excludes the possibility of exploiting historical observations contained within archived datasets [10]. The interaction of radar (particularly C-band) with snow is complex [32] and can result in significant interferometric decorrelation. Therefore, it is important to consider how this may affect the monitoring capabilities in high mountain areas and if any mitigating action can be taken. In permafrost areas in the high Arctic, where the ground is covered in snow for long periods, InSAR can prove almost impossible to use throughout an entire year due to prolonged incoherence and, to mitigate against this, artificial corner reflectors usually need to be installed in the field [33]. Lake Sarez falls in the 3000-4000 m elevation zone in the Himalayas where snow generally covers less than 40% of the land area throughout the year; there is a large inter-annual variability, with up to 60% cover in February to March, falling to almost nothing from April to August [34]. Nonetheless, these conditions could still significantly restrict the capabilities of InSAR techniques that require persistently high coherence throughout the entire period of observation. The challenge of obtaining a dense set of ground motion measurements has been demonstrated through previous InSAR surveys of the Lake Sarez area [35; 36], which have been attempted using the persistent scatterers InSAR (PSInSAR) [37] and Small Baseline Subset (SBAS) [38] algorithms. With coverage limited due to the effects associated with the extreme topography, maximising the density of measurements is especially important for capturing both large- and small-scale mass movements to enable a comprehensive assessment of hazards in the Lake Sarez area. In addition to considering multiple SAR imaging geometries, this may also be achieved using advanced InSAR techniques, such as the Intermittent Small Baseline Subset (ISBAS) algorithm [39; 40]. The ISBAS algorithm has been demonstrated to greatly increase the density and spatial distribution of measurements at millimetre-level accuracy over bare soils and vegetated surfaces [41], which it achieves through a modification of the SBAS algorithm to consider the intermittent nature of coherence over dynamic land cover types. A number of comparative studies of PSInSAR survey point densities over different types of land cover classes have been attempted over the UK [42] and Nepal [43].