Global Positioning System satellite Signal Quality Monitoring (SQM) is required to ensure the integrity of the received signal for aviation safety-critical systems. Failure mitigation is not addressed since failure detection ensures system integrity. The GPS Anomalous Event Monitor (GAEM) is introduced, consisting of a GPS receiver serving as an anomaly sensor, and the Software Defined Radio, allowing for a thorough analysis of signal malfunction modes through advanced signal processing techniques. Algorithms to monitor the GPS signal by the anomaly sensor are developed and in case of possible signal inconsistencies the signal is analyzed by the Software Defined Radio.
For the purpose of quality monitoring it is essential to understand the impact of the radio frequency front-end on the received signal, and implicitly onto the signal parameter estimation process; otherwise a signal inconsistency may be flagged which is induced by the monitoring system. Thus, radio frequency front-end induced errors are examined and the statistics for signal parameter estimators are derived.
As the statistics of an anomalous signal are unknown, a non-parametric, non-homoscedastic (uncommon variance of sample space) statistical test is developed. Berry-Esseen bounds are introduced to quantify convergence and to establish confidence levels. The algorithm is applied to the detection of signal anomalies, with emphasis on interference detection.
The algorithms to detect GPS signal anomalies are verified with experimental data. The performance of the interference detection algorithms is demonstrated through data collection in a shielded measurement chamber. Actual GPS signals in combination with interference sources such as narrowband, wideband and pulsed interference were broadcast in the chamber. Subsequently, case studies from continuous GPS monitoring are included and observed anomalies are discussed. The performance demonstration of the GPS anomalous event monitor is concluded with a field experiment to investigate the effects of aircraft overflights on GPS signal distortions.