|Title:||Study of GPS multipath effects for structural deformation monitoring|
|Subject:||Hong Kong Polytechnic University -- Dissertations.|
Global Positioning System.
Electronics in navigation.
|Department:||Department of Land Surveying and Geo-Informatics|
|Pages:||xviii, 189 p. : ill. ; 31 cm.|
|Abstract:||GPS signal multipath effects are one of the most important error sources in precise GPS positioning and navigation. Although various methods have been proposed to reduce the effects, the existing methods are not always as effective as desired. This thesis aims to develop further the methods for mitigating the multipath effects. A Vondrak filter is proposed for smoothing out the multipath effects in precise GPS applications such as structural vibration monitoring. The technique has a good signal resolution at the signal truncation frequency band, i.e. at the upper or lower limit of a frequency band. The proposed filter is compared with two commonly used filters, i.e. the wavelet and adaptive FIR filters, for such applications. Results from the study reveal that the performances of the Vondrak and wavelet filters are similar and superior to the adaptive FIR filter. Due to the good filtering properties of Vondrak and wavelet filters, new filtering methods (i.e. cross-validation Vondrak filter (CVVF) and cross-validation wavelet filter (CVWF)), based on the Vondrak or wavelet filter and the technique of cross-validation, are developed for separating noise from the signals in GPS coordinate series. Test results show that both the proposed CVVF and CVWF methods are effective signal decomposers but the former is superior to the latter. In investigating the variations in GPS multipath day-to-day repeatability, we propose to integrate the CVVF method, the existing stochastic SIGMA-A model and the aspect repeat time adjustment (ARTA) method to maximize GPS accuracy improvements. Test results show that the correlation of multipath signals decreases with the increase of the time interval between the current date and the date when the multipath model was established. The shorter the period of multipath signal, the weaker the correlation. A sidereal filtering method is also developed based on GPS single difference observations for mitigating the effects of GPS signal multipath and diffraction. Test examples show that the new filtering method can reduce the GPS signal multipath and diffraction effects more effectively, and improve the accuracy by about 50-80%. The method is also advantageous in that it can be implemented in real-time applications such as deformation monitoring. Finally, the thesis investigates the multipath mitigation using modernized GNSS signals due to the fact that the additional redundancy gives better averaging effects in the adjustment model. A GNSS data simulator is used to generate multipath contaminated GPS, GLONASS and Galileo data. Results show that an accuracy improvement of 63% on average can be obtained by using the future GPS/GLONASS/Galileo multiple-frequency data when compared to the current GPS single-frequency data.|
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