Abstract
We assess the feasibility of monitoring the landmass of Great Britain with satellite Synthetic Aperture Radar (SAR) imagery, by analysing ERS-1/2 SAR and ENVISAT IS2 Advanced SAR (ASAR) archive data availability, geometric distortions and land cover control on the success of (non-)interferometric analyses. Our assessment both addresses the scientific and operational question of whether a nationwide SAR-based monitoring of ground motion would succeed in Great Britain, and helps to understand controlling factors and possible solutions to overcome the limitations of undertaking SAR-based imaging of the landmass. This is the first time such a nationwide assessment is performed in preparation for acquisition and processing of SAR data in the United Kingdom, and any other country in the world. Analysis of the ERS-1/2 and ENVISAT archives reveals potential for multi-interferogram SAR Interferometry (InSAR) for the entirety of Britain using ERS-1/2 in descending mode, with 100% standard image frames showing at least 20 archive scenes available. ERS-1/2 ascending and both ENVISAT modes show potential for non-interferometric and single-pair InSAR for the vast majority of Britain, and multi-interferogram only for 13% to 38% of the available standard frames. Based on NEXTMap® Britain Digital Terrain Model (DTM) we simulate SAR layover, foreshortening and shadow to the ERS-1/2 and ENVISAT Lines-Of-Sight (LOS), and quantify changes of SAR distortions with variations in mode, LOS incidence angles and ground track angles, local terrain orientation, and the effect of scale due to the input DTM resolution. The simulation is extended to the ~230,000 km2 landmass, and shows limited control of local topography on the radar terrain visibility. According to the 50 m to 5 m DTM-based simulations, ~1.0–1.4% of Great Britain could potentially be affected by shadow and layover in each mode. Only ~0.02–0.04% overlapping between ascending and descending mode distortions is found, this indicating the negligible proportion of the landmass that cannot be monitored using either imaging mode. We calibrate the CORINE Land Cover 2006 (CLC2006) using Persistent Scatterer (PS) datasets available for London, Stoke-On-Trent, Newcastle and Bristol, to quantify land cover control on the PS distribution and characterise the CLC2006 classes in terms of the potential PS density they could provide. Despite predominance of rural land cover types, we predict potential for over 12.8 M monitoring targets for each acquisition mode using a set of image frames covering the entire landmass. We validate our assessment by processing with the Interferometric Point Target Analysis (IPTA) 55 ERS-1/2 SAR scenes depicting South Wales between 1992 and 1999. Although absolute differences between predicted and observed target density are revealed, relative densities and rankings among the various CLC2006 classes are found constant across the calibration and validation datasets. Rescaled predictions for Britain show potential for a total of 2.5 M monitoring targets across the landmass. We examine the use of the topographic and land cover feasibility maps for landslide studies in relation to the British Geological Survey's National Landslide Database and DiGMapGB mass movement layer. Building upon recent literature, we finally discuss future perspectives relating to the replication of our feasibility assessment to account for higher resolution SAR imagery, new Earth explorers (e.g., Sentinel-1) and improved processing techniques, showing potential to generate invaluable sources of information on land motions and geohazards in Great Britain.
Highlights
The possibility of mapping historical and recent land motions over wide areas of the Earth's surface with satellite differential Synthetic Aperture Radar Interferometry (InSAR) (e.g., Rosen et al, 2000) is increasingly stimulating interest within the geohazard community, especially with respect to operational monitoring of hazards threatening population and infrastructure, both in and outside Europe (Bally, 2012)
The feasibility of SAR imaging data to study Britain has been analysed in relation to the availability of archived stacks of imagery suitable forinterferometric applications, terrain visibility to the radar sensor modes, and the likelihood of the various land cover types to provide good radar reflectors to monitor with Persistent Scatterer Interferometry (PSI) methods
We showed that topography exerts limited control on the visibility of the British landmass, with only ~1.0–1.4% of the total territory potentially affected by radar shadow and layover to the ERS-1/2 and ENVISAT modes in each acquisition mode according to the 50 m to 5 m resolution Digital Terrain Model (DTM)-based simulations
Summary
The possibility of mapping historical and recent land motions over wide areas of the Earth's surface with satellite differential Synthetic Aperture Radar Interferometry (InSAR) (e.g., Rosen et al, 2000) is increasingly stimulating interest within the geohazard community, especially with respect to operational monitoring of hazards threatening population and infrastructure, both in and outside Europe (Bally, 2012). Our assessment combines the simulation of the effects of topography on the visibility of target areas to the satellite SAR acquisition geometry, with the quantification of land cover control on the success of PSI processing of archive SAR data via prediction of radar target densities These effects are summarized into a series of feasibility maps, which help to understand where SAR-based applications are likely to succeed and where not. We discuss the potential of improved processing techniques which can provide denser and often more complete spatial coverage of SAR-derived results even in rural areas, where achieving C- or X-band InSAR results can be challenging This feasibility study both addresses the scientific and operational question of whether nationwide SAR-based monitoring of ground motion would succeed in Great Britain, and helps to understand controlling factors and possible solutions to overcome the limitations of undertaking such monitoring. Analysis of the feasibility is focussed on ERS-1/2 and ENVISAT data archives, to assess how these would perform to create a baseline nationwide InSAR dataset as a background land motion product in view of higher spatial and temporal resolution SAR sensors onboard forthcoming and existing Earth explorers
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