## 1 Introduction  
High mountain areas are unsurprisingly prone to a number of geohazards associated with steep slopes, such as rockfalls, debris flows, landslides, avalanches, and floods. These hazards impact infrastructure and human activity in those areas [1], and their occurrence can have an enormous human and economic cost. For example, in 2010, a rockslide dammed the Hunza River in Pakistan submerging several villages and 22 km of the strategic Karakoram Highway between Pakistan and China [2]. The flood was disastrous for the infrastructure of the mountain valleys and affected millions of people in the irrigated plains of Punjab and Sindh [3]. The economic impact of this disaster was estimated at almost US4 billion [4]. Mountainous terrain can be difficult to access and hazardous to work in, and so remote sensing has a major role to play in the assessment and continued monitoring of the risks posed by geohazards in these settings [5]. The provision of free earth observation (EO) satellite data that is being routinely gathered over the land surface, such as that provided by the Copernicus programme of the European Union ([[https://www.copernicus.eu](https://www.copernicus.eu)]([https://www.copernicus.eu](https://www.copernicus.eu)) (accessedon 28 June 2021)), means that there is a basis for cost-effective monitoring programmes over such regions that could be sustained for the foreseeable future. Furthermore, the development of novel EO data processing techniques that are capable of providing dense sets of measurements without the need for ground surveys means that such monitoring can be undertaken safely and without any of the delays associated with establishing ground-based monitoring networks. An EO technique that is becoming increasingly important in the monitoring of mass movements on slopes is interferometric synthetic aperture radar (InSAR), where its application could improve risk awareness and provide early warning of impending catastrophic slope failures in vast, inaccessible, or otherwise unmonitored regions [6]. However, one of the major disadvantages regarding the application of InSAR surveys to mountainous areas is that the extreme topography can result in differences in illumination and perspective distortions between foreslobes and backslopes, which can subsequently hinder the interpretation. This is a particular problem for oblique-looking synthetic aperture radar (SAR) sensors, where foreshortening, layover, and shadow effects are prevalent in mountainous terrain. This is evident in a study of the 2008 Wenchuan earthquake in China using SAR data, where [PERSON] [7] found that almost 50% of the ground area was obscured by layover and shadow. Similarly, in a landslide monitoring study of a mountainous region of China, [PERSON] et al. [8] found as much as 50% of the area was affected by geometric distortions in data acquired using some SAR sensors. Layover and shadow effects were also responsible for an underestimation of active landslides in an InSAR study of a mountainous area of California and Oregon (USA) by [PERSON] et al. [9]. Further problems that are capable of affecting conventional InSAR surveys of mountain regions include the impersistent scattering characteristics of natural land cover (e.g., vegetation) and seasonal snow cover, which can also limit the density of ground motion measurements that can be obtained due to the decorrelation of the radar signal [10; 11]. The current availability of Copernicus Sentinel-1 C-band SAR data, with high temporal and spatial resolution, and advanced InSAR processing techniques promise to offer an enhanced potential for the routine and ubiquitous remote monitoring of inaccessible areas. Nevertheless, in order to help fully exploit this potential, feasibility assessments are needed to explore the limitations and the enhanced opportunities afforded in terms of obtaining sufficient spatial coverage of ground motion measurements.