An investigation in the use of GPS and INS sensors for structural health monitoring

GPS is used to monitor the structural health conditions of large structures such as high-rise buildings or long flexible bridges. It is well suited for such applications since it provides excellent long term stability for positioning. However, there are a number of problems that occur using such a system, the most significant of which is the requirement to locate the receiver with a clear view of the sky, and in an environment where there is minimal multipath which proves to be very difficult. This paper investigates the performance of a GPS and navigation grade INS integrated system for monitoring the Forth Road Bridge in Scotland. The integrated system is compared to the conventional GPS solution and it is assessed whether there is any performance improvement to be made by the augmentation of such sensors in a structural monitoring environment. The integrated system also provides attitude estimation which is also analysed. In particular it is recognised that the relatively small magnitude dynamics experienced in structural monitoring compared to the magnitude of dynamics experienced in navigation applications has a detrimental effect on the yaw estimation due to weak observability. The paper also develops an implementation of an INS and accelerometer-only solution using a feedback damping loop. Damping loops have been used for decades in the avionics industry to reduce the INS drift with respect to barometric altitude. The INS-only solution is shown to compare to within 2mm of the GPS and INS integrated solution. Furthermore, it is recognised that the small rotational movement of the bridge results in the INS attitude measurements becoming unnecessary if attitude is not required. It is also shown that the accelerometer-only system can work to a similar level of accuracy.