GPS Carrier Phase Tracking in Difficult Environments Using Vector Tracking For Precise Positioning and Vehicle Attitude Estimation
Type of DegreePhD Dissertation
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In this dissertation, two approaches are developed to improve the carrier phase tracking performance of a software-defined GPS receiver. The first approach combines a non-coherent vector tracking architecture with a local phase locked loop to improve carrier phase tracking performance of a stand-alone (i.e. not base station) GPS receiver. The Vector Frequency Locked Loop (VFLL) aided Phase Locked Loop (PLL) receiver is shown to provide a more robust carrier phase tracking performance at low carrier-to-noise density (C/N$_0$) ratios. The VFLL aided PLL is able to maintain phase lock of signal with 2 to 3 dB lower C/N$_0$ ratio. The more significant improvement is that, while the scalar tracking receiver quickly lost phase lock completely at low C/N$_0$ ratios, the VFLL aided PLL only slipped cycles. The second approach is designed to include measurements from a local base station. The Real-Time Kinematic (RTK) vector phase locked loop receiver is derived. The RTK VPLL receiver is a true carrier phase tracking receiver in that the navigation filter is updated using correlator outputs, and there is no local loop filter for each tracking channel. The tracking loop is closed by predicting the received carrier phase using the navigation solution. Base station measurements are combined with relative position vector estimates from the navigation filter and fixed carrier ambiguities to close the tracking loops. In simulation, the RTK VPLL maintains phase lock at C/N$_0$ ratios 4 to 8 dB lower than a traditional scalar tracking receiver. During experimental testing, the RTK VPLL receiver maintains the navigation solution and carrier phase tracking throughout tests in moderate and heavy foliage. There are times during the experimental test that the RTK VPLL receiver slipped cycles of the carrier, and the navigation solution degrades as a result. The environment prevents the standard receiver from reporting a high precision solution for even longer periods. Finally, the benefits of the RTK VPLL receiver design are investigated in a multi-antenna configuration. Data from multiple GPS antennas mounted on a rigid body may be used to estimate the attitude of the platform. The solution for finding the three Euler angles (i.e. roll, pitch, and yaw) of the platform given two relative position vectors is provided. Two studies are performed to identify possible improvements to the software receiver design developed in this dissertation. A modified two antenna RTK VPLL algorithm was developed and tested in simulation. It is shown that the two antenna algorithm does not significantly improve the carrier phase tracking performance at low C/N$_0$ ratios. An RTK carrier ambiguity estimation procedure is developed using the multiple antenna configuration and the known antenna separation distance. The new baseline constrained ambiguity estimation algorithm significantly reduces time to fix and provide a method for rejecting incorrect fixes.