The Use of Pseudolites to Augment GPS Data for Bridge Deflection Measurements

The use of accelerometers and GPS, either separately or as a combined approach is becoming a reliable and useful tool for real-time bridge deflection monitoring.

The use of such sensors could well make such structures safer, as the engineers have a better grasp as to the structure’s health. Typically a bridge may be designed to last 120 years, but this is under pre-determined loading conditions, including wind and traffic loading. If these conditions do not actually exist then the bridge may not last as long as expected.

Research into this field has been underway at the University of Nottingham for nearly 10 years. The results show that a combined system of dual frequency GPS receivers and triaxial accelerometers overcomes the drawbacks of both individual systems. RTK GPS for such monitoring is prone to error sources such as tropospheric delay, multipath and cycle slips. Adaptive Filtering (AF) is being investigated as a tool that can be used to combine the measurements from triaxial accelerometers and dual frequency GPS receivers. As well as to detect and correct the GPS data for tropospheric delay, multipath and cycle slips both at reference and rover stations, the accelerometer data increases the 10 Hz GPS data to a combined data rate of several hundred Hz. The GPS data also compliments the accelerometer data by correcting its drift characteristics over time.

However, GPS is also restrained by the satellite constellation geometry. As a consequence, the vertical component of 3D coordinates is less accurate than that of plan. Furthermore, in the UK or other mid- or high latitude areas, the North-South accuracy is worse than that of the East-West. Due to the line-of-sight restraints in the bridge site, measurable satellites are always limited and it is difficult to maintain tracks to enough satellites for highly accurate kinematic positioning even with the redundant measurements from a triaxial accelerometer.

One method to overcome the satellite constellation limitation is to augment the GPS data with that of pseudolites. Pseudolites transmit GPS-like signals, and are ideal for a static structure with restrict movements such as a bridge.

Research is underway at the University of Nottingham, investigating the use of pseudolites for such an application. The initial research focuses on the improvement of the positioning accuracy of the 3D coordinates. As an example, real bridge deformation monitoring with GPS, triaxial accelerometers and a total station is introduced in this paper. DOP values are simulated with actual GPS satellite geometry viewed at a suspension footbridge over the River Trent in Nottingham. Potential locations of pseudolites are recommended according to the accuracy requirements of deformation monitoring and topography at the bridge site. Fundamental theory and formulas are developed and used to estimate the possible positional accuracy from an integrated GPS and pseudolite system. The results of these trials illustrate that it is indeed possible to increase the accuracy of the resulting data using pseudolites in both plan and height components to millimetre level.

The work is part of a large project funded by the UK´s Engineering and Physical Sciences Council (EPSRC), in collaboration with Cranfield University. The positional data produced from the trials by the University of Nottingham is then used by Cranfield University in order to create and update their Finite Element (FE) models of such structures. In effect, a real-time self-correcting FE model is created.