APPLICATIONS OF PSEUDOLITES IN DEFORMATION MONITORING SYSTEMS

Due to the high precision of the carrier phase observables, the Global Positioning System (GPS) technology has been widely used for measuring crustal motion and ground subsidence, and more recently for monitoring deformation of man-made structures such as bridges, dams, buildings, etc. It is well known that for such GPS-based deformation monitoring systems, the accuracy, availability, reliability and integrity of the positioning solutions is heavily dependent on the number and geometric distribution of satellites being tracked. However, in some situations, such as in urban canyons, dam monitoring in valleys and in deep open-cut mines, the number of visible satellites may not be sufficient to reliably determine precise position. Indoors the task is impossible. Pseudolites, which are ground-based transmitters of GPS-like signals, can significantly enhance the satellite geometry, and even replace the GPS satellite constellation in some situations (such as deformation monitoring indoors). There are three general classes of the potential pseudolite applications in deformation monitoring systems. The first case is GPS augmentation with pseudolite(s), which is suitable for circumstances such as urban canyons, or monitoring in valleys and deep open-cut mines. The second case is indoor applications of pseudolite deformation monitoring systems. Pseudolite arrays can, in principle, replace completely the GPS satellite constellation. This could extend the ‘satellite-based’ deformation monitoring applications into tunnels or underground, where GPS satellite signals cannot be tracked. The last case is an inverted pseudolite-based deformation monitoring system, where a ‘constellation’ of GPS receivers with precisely known ‘orbit’ tracks a mobile pseudolite. The system consists of an array of GPS receivers, the base reference pseudolite and the mobile pseudolite. However, in the case of such pseudolite-only or hybrid pseudolite-GPS deformation monitoring systems, there are some additional issues that need to be addressed. These include the near-far problem, signal interference, multipath, atmospheric delay effects, and locationdependent errors such as receiver and pseudolite location biases. In this paper, the results of a detailed analysis of these factors are presented. Some practical procedures to mitigate or eliminate their influence are suggested. Some experiments have been carried out using NovAtel GPS receivers and IntegriNautics IN200CXL pseudolite instruments. The experimental results indicate that these three classes of potential pseudolite applications for deformation monitoring systems are feasible. Their performance will be demonstrated through some case study examples.