Using Single Receiver GPS Observations to Analyze the Dynamic Motion of Large Engineering Structures
The objective in monitoring of high-rise engineering structures is to track the variations in the characteristics of structural movement and to detect, locate and assess damage of the structure in an extreme event, such as earthquake, storm or fire. After an extreme event the damage assessment must be carried out as soon as possible because the decision-making time causes considerable financial losses to the owners of commercial structures or the government. Therefore a fast and reliable method of monitoring structural behavior is highly demanded.
Traditionally, damage detection involves analysis of accelerometers data to control the variations in the structure‟s natural frequencies. Apart from that, accelerations measured by accelerometers are double-integrated to obtain deflections which are used for calculating the inter-storey drift ratios and damage locating. However deflections obtained by double-integration are often invalid due to the lack of reliable integration constants and accelerometer-related errors.
Since GPS became available for civilian use, it has been increasingly used for structural monitoring in combination with, or as an alternative to traditional techniques. The large potential of GPS for structural monitoring resulted from its peculiar properties such as high accuracy of positioning, high frequency of observation recording, around-the-clock, all weather and around-the-globe availability, and autonomy in operation. Moreover it does not require inter-visibility between stations. All these advantages make GPS a preferred technique for many structural monitoring applications.
However GPS also has a range of disadvantages requiring the development of special methods and/or the integration with other devices to overcome. The most important GPS limitations are: maximum sampling frequency up to only 100 Hz, the positioning accuracy dependence on the number of visible satellites and their geometry, the influence of multipath, high cost of high precision GPS receivers, and the need for at least two GPS receivers to provide high precision of positioning.
This research is an attempt to obtain structural frequencies and deflections using a stand-alone single frequency GPS receiver. Novel methods based on the use of time series analysis techniques for GPS data processing are developed. As long as GPS carrier phase observations are affected by GPS-related errors with magnitudes much larger than the structural movements of interest, it is impossible to obtain characteristics of structural movement directly. The modeling of some GPS-related errors is performed while others are reduced by differencing and/or high-pass filtering procedures. The data free from GPS related errors is analyzed in the frequency domain to estimate structural frequencies. A new method used to convert processed carrier phase observations to actual structural deflections is also developed in this research. This method uses FFT filtering to focus on the particular component of structural movement and consider geometry of satellite-receiver relative motion.
The methods developed in the thesis are tested using simulated GPS data created specially to model the realistic structural behaviour as a proof of concept. The obtained results indicate that by using the developed methods at least the first three structural frequencies can be estimated with accuracy of Hz and structural deflections can be estimated with the accuracy of 0.02 m.
Currently GPS can be used to monitor structural dynamic characteristics using at least two GPS receivers operating simultaneously. The objective of this thesis is to prove the concept of using just one GPS receiver to monitor structural behaviour using the developed data processing strategies. Any further development in technology can improve the performance of methods developed in this thesis.