Interferometric Synthetic Aperture Radar for remote satellite monitoring of bridges
The structural health of critical infrastructure is difficult to assess and monitor with existing methods of evaluation which rely predominantly on visual inspection and/or the installation of sensors to measure the in-situ performance of structures. There are vast numbers of critical structures that need to be monitored and these are often located in diverse geographical locations which are difficult and costly to access. Recent advances in satellite technologies provide the opportunity for global coverage of assets and the measurement of displacement to sub-centimetre accuracy. Such measurements could supplement existing monitoring techniques and provide asset owners with additional insights which could inform operational and maintenance decisions. Most past research within the field of Interferometric Synthetic Aperture Radar (InSAR) monitoring using satellite radar imagery focusses on widespread measurement of land areas, although there have been some case studies using InSAR to assess movements of individual structures such as dams. However, there is limited published research into the use of these techniques for accurately monitoring the displacements of individual civil engineering structures over time and relating these measurements to structural performance. This research focusses on bridges as a specific example of critical infrastructure to establish whether remote satellite monitoring can be used to measure displacements at a resolution that is sufficiently accurate for use in monitoring of performance, and examines the relevance and limitations of satellite monitoring to civil engineering applications in general. In order to assess the millimetre-scale performance of InSAR, an initial evaluation was undertaken in controlled conditions on a purpose-built test bed fitted with satellite reflectors at the National Physical Laboratory in Teddington to validate InSAR displacement measurements against traditional terrestrial in-situ displacement measurements. Subsequently, traditional sensor and surveying measurements of displacements were compared with InSAR displacement measurements at key points of interest on Waterloo Bridge and the Hammersmith Flyover. A further case study on Tadcaster Bridge was undertaken to demonstrate the potential applicability of InSAR displacement measuring techniques for monitoring bridges at risk of scour failure. Scour is the most common form of bridge collapse around the world and to date no cost-effective and widely applicable method for providing advanced warning of impending failure due to scour has been developed. Methodologies for integrating digital, structural and signal processing models for the identification and mapping of InSAR measurement points on bridge structures from SAR imagery were developed, as well as methodologies for combining satellite data with traditional surveying methods. An important outcome of this research was that through comparison of independent measurements, InSAR measurements are of a scale that is applicable to bridge monitoring. Remote sensing can therefore reach global coverage, with unsupervised readings over an interval of days, and as such supplement traditional inspection regimes. However, this outcome must be presented with several limitations. Practical implications of applying InSAR to real bridges are discussed, including imaging effects and the suitability of monitoring different forms of bridge deformation. The key to successful implementation of InSAR monitoring of bridges lies in understanding the limitations and opportunities of InSAR, and making a clear case to satellite data providers on what specifications (resolution, frequency, processing assumptions) would unlock using such datasets for wider use in monitoring of infrastructure. InSAR can provide measurements and useful insights for bridge monitoring but it is limited to specific cases and, at this stage of technological development, it should be considered as a tool for specific bridges and failure mechanisms rather than a full bridge monitoring solution.
Structural Health Monitoring, Bridges, Interferometric Synthetic Aperture Radar, InSAR, Remote Sensing, Bridge Monitoring, Satellite Monitoring
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