Global Positioning System (GPS) receivers are increasingly used for positioning in urban environments and precise timing in critical infrastructures. These scenarios are challenging for GPS receivers because building reflection and obstruction contribute to GPS signal degradation in urban environments, while potential jamming and spoofing attacks disrupt GPS time synchronization in critical infrastructures.We propose using Direct Position Estimation (DPE), augmented with additional navigation information, to enable robust GPS receiver operation in challenging scenarios. Unlike conventional methods, such as scalar and vector tracking, DPE performs Maximum Likelihood Estimation (MLE) of the navigation solution on the raw GPS signal. DPE initializes multiple navigation candidates and searches for the candidate that maximizes the cross-correlation between the expected GPS signal reception and the received GPS signal. The direct search and inherent joint optimization across multiple satellite signals make DPE more robust than scalar and vector tracking. In addition, since the parameter of interest is the navigation solution, DPE provides a natural framework for directly incorporating additional navigation information.The contribution of this thesis is to design and experimentally validate algorithms for deeply integrating additional navigation information into DPE.To improve the robustness of DPE in multipath caused by building reflection, we propose transforming non-line-of-sight (NLOS) GPS signals from being unwanted interferences to useful navigation signals. We include NLOS GPS signals into the expected GPS signal reception as additional line-of-sight (LOS) GPS signals to virtual satellites at mirror-image positions. The satellite mirror-image positions are calculated using information of building reflection surfaces, estimated from available three-dimensional (3D) maps. We conducted experiments in front of the 50~m by 40~m wind tunnel located at NASA's Ames Research Center in Mountain View, California, utilizing the surface of the wind tunnel as a reflector of GPS signals. We demonstrated through our experiment, improved robustness in terms of horizontal positioning accuracy, due to the constructive integration of NLOS GPS signals.In urban environments where GPS sensing is hindered by building obstruction, we propose addressing buildings as additional navigation features instead of undesirable obstacles. We deeply integrate DPE with image map-matching of images captured by an onboard camera against geo-referenced images. The navigation solutions directly estimated from both DPE and image map-matching are fused and used in close-loop GPS signal and camera image tracking. We conducted experiments with joint collections of GPS signals and camera images on our university campus in Urbana, Illinois. We demonstrated, through our experiment, improved robustness in terms of positioning availability, due to the additional vision information.In addition to positioning, GPS receivers are used for time synchronization in critical infrastructures, where they are vulnerable to malicious attacks. For robust GPS time estimation, we propose using the known, static GPS receiver location as prior information. Estimation of the 3D position, clock bias, 3D velocity and clock drift parameters is reduced to estimation of only the clock bias and clock drift parameters. We conducted experiments on the rooftop of our laboratory in Urbana, Illinois, using the collected signals in simulated jamming and spoofing attacks. We demonstrated improved robustness in terms of anti-jamming and anti-spoofing, due to the information redundancy gained from parameter reduction.
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Improving the robustness of GPS direct position estimation
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