GPS Signal-in-Space Anomalies in the Last Decade Data Mining of 400,000,000 GPS Navigation Messages

For the Global Positioning System (GPS), signal-in-space (SIS) performance is key to the positioning accuracy and the integrity. In practice, SIS anomalies occasionally happens and the consequent user range errors of tens of meters or even more have been observed. In this paper, all potential SIS anomalies in the last decade are screened out by comparing the broadcast ephemerides/clocks with the precise ones. Validated broadcast ephemerides/clocks are generated from more than 400,000,000 broadcast navigation messages logged by all International GNSS Service (IGS) stations during the period 6/1/2000–8/31/2010. Both IGS and National Geospatial-Intelligence Agency (NGA) precise ephemerides/clocks are used as truth references. In addition, the NGA satellite antenna corrections are employed to convert IGS center-of-mass data into antenna-phase-center. The validated broadcast ephemerides/clocks are used to propagate broadcast satellite orbits/clocks at 15-minute intervals that coincide with the precise ones. A potential SIS anomaly is claimed when the navigation message is healthy and in its fit interval but the consequent worst-case SIS range errors (SISRE) exceeds the SIS not-to-exceed tolerance, 4.42 times the user range accuracy (URA) upper bound (UB). Finally, 3275 potential SIS anomalies are screened out. Most anomalies between 2004 and 2009 are confirmed by other literature. Some mysterious anomalies during the first year after SA was turned off are discovered and investigated. Cumulative distribution of anomalous worst-case SISRE shows that approximately 10% anomalies result in worstcase SISRE greater than 10 times URA UB, and approximately 1% anomalies result in worst-case SISRE greater than 100 times URA UB. The total number of potential SIS anomalies per year demonstrates that the SIS performance was improving in the last decade.