| Many applications that utilize the Global Positioning System (GPS) demand highly accurate positioning information. Safety-critical applications such as aircraft navigation require position solutions with not only high accuracy but also with high integrity. Two significant threats to GPS signal quality exist which can make meeting both of these requirements a difficult task.; Satellite signal anomalies, or “evil waveforms,” can result from soft failures of the signal generating hardware onboard the GPS satellite. These subtle anomalies cause distortions of the signal, which if undetected may pose an integrity risk to an aircraft relying on GPS. Signal Quality Monitoring (SQM) is required to reliably detect these anomalies and thereby protect airborne users from this integrity threat. Multipath, or undesired reflected signals from the ground or other obstacles, also distorts the desired GPS signal. In addition to making evil waveforms more difficult to detect, multipath—an ever-present error source—also degrades nominal performance. Multipath mitigation techniques attempt to reduce or eliminate this threat.; This thesis introduces novel signal processing techniques for addressing these twin concerns. First, a comprehensive method for designing a robust signal quality monitor to detect evil waveforms in the presence of multipath is described. This method is used to specify a practical multiple-correlator configuration for the SQM receiver that satisfies the requirements for Category I precision approaches for landing aircraft. Second, a new multipath mitigation approach is introduced that leverages “multipath invariant” properties of the GPS signals. A real-time Tracking Error Compensator (TrEC) algorithm is experimentally shown to provide significant accuracy improvements over existing techniques for low-end (or “narrowband”) receivers. Additionally, it is shown that TrEC may have at least comparable multipath mitigation performance to that of a high-end (or “wideband”) receiver technique. |
