To investigate Local Area Augmentation System Ground Facility (LGF) measurement anomalies, a GPS Anomalous Event Monitor (GAEM) is implemented in the prototype LGF at Ohio University. The GAEM is designed to collect approximately 10 seconds of radio frequency data at 5 mega samples per second (Msps) surrounding an anomalous event.
The sampled data are processed in 1-ms blocks. Code phase, carrier frequency and phase, and carrier-to-noise ratio are estimated for selected satellites. The estimation algorithms were implemented and tested in a software-defined GPS receiver and processing results were compared with measurement data from a sequential-processing GPS receiver.
Based on the estimated signal parameters, detection statistics are developed for GPS satellites signal anomalies, including waveform distortions, low signal power, and sudden changes in code and/or carrier measurements. Further, the distributions and the Gaussian overbounds of the detection statistics are derived. Given the probabilities of false alert and missed detection, the detection thresholds and minimum detectable biases are derived for the detection statistics.
Both simulation and measured data are used to verify the derived standard deviations of the detection statistics.