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Large engineering structures such as railway and highway earthworks, bridges, pipelines and dams may need to be monitored for a number of reasons. These include general performance monitoring and providing a warning of incipient or actual failure (e.g. a landslip). New infrastructure construction projects, particularly large basements and tunnels in urban areas, may require extensive monitoring systems to enable the resulting ground displacements to be measured and compensated for where necessary. The cost of such monitoring, especially over large geographical areas which may be remote or inaccessible, is significant. More efficient monitoring and early warning systems have the potential to save large sums of money, and even human life. One of the most effective ways of assessing the performance of infrastructure is to measure surface variation (displacement) and relate instability or loss of performance to the rate of change of this variation. A number of technologies are currently used for surface variation measurement; these include extensometers, D-GPS systems, prism monitoring, reflectorless laser systems, photogrammetry, and interferometric linear ground based synthetic aperture radar. All of these systems have advantages and limitations. Many are expensive, some only operate over limited distances, others require installations to monitor particular locations (such as reflectors), and some will not operate in the dark or in poor weather.The use of satellite imagery offers the potential for cost-effective measurement of surface variations. Spaceborne Interferometric Synthetic Aperture Radars (InSAR) make use of orbiting satellites to image a given area. Images from successive passes of the satellite can be used to calculate ground displacements. The primary drawback with spaceborne InSAR surface change detectors is that they were developed for global, rather than local, area monitoring purposes and have a long satellite revisit time. Another potential problem is that using only one or two satellites, an area of interest could be in an electromagnetic shadow (i.e., the satellite cannot illuminate the area due to an obstacle blocking the satellite signal). This can occur especially in urban areas or hilly terrain.Recent advances have enabled the development of a subclass of InSAR using ground surface mounted receivers, the Passive Interferometric Space-Surface Bistatic Synthetic Aperture Radar (PInSS-BSAR). The PInSS-BSAR topology has a stationary receiver fixed on the ground, with the imaging antennae pointed towards the area of interest. A satellite moving relative to the surface generates an electromagnetic ranging signal illuminating the observation area. The signal is reflected by the earth's surface, and received by the radar antennae. By using two antennae, one fixed above the other, it will be possible to calculate the change in displacement in the vertical direction. PInSS-BSAR is best utilised using non-cooperative transmitters, i.e. satellites being used for other purposes. Global Navigation Satellite Systems, such as GPS and Galileo provide large numbers of non-geostationary, simultaneously operating satellites above the horizon, which illuminate a particular region at different angles. At any time, the satellites should cover the entire surface of the planet without any points in electromagnetic shadow. The range of such as system is expected to be kilometres, and its ability to monitor continuously will provide effective early warning of excessive displacements.The proposed research seeks to develop a cost-effective monitoring system using PInSS-BSAR to measure surface variations, with specific application to linear infrastructure such as roads and railways, and their associated embankment and cutting slopes. The prototype device will be verified against existing conventional surface displacement instrumentation already installed to monitor two large failing infrastructure slopes.
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