Safe satellite navigation with multiple constellations [electronic resource] : global monitoring of GPS and GLONASS signal-in-space anomalies
Keywords: Electrical Engineering

With the modernization of the United States Global Positioning System (GPS), the revitalization of the Russian Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS), and the advent of the Chinese Compass and European Galileo, airborne navigators are eager to use multiple constellations to enhance navigation performance and safety. As these global navigation satellite systems (GNSS) are all based on the principle of multilateration, the quality of satellite ephemeris and clock parameters carried by the signal in space (SIS) plays a vital role in ensuring positioning accuracy and integrity. In practice, GPS and GLONASS SIS anomalies occasionally occur, resulting in user range errors (UREs) of tens of meters or even more, with the potential to expose users to hazardously misleading information. The primary goal of this dissertation is to enable comprehensive, systematic, efficient global monitoring of GPS and GLONASS SIS anomalies using low-cost but imperfect data from a global receiver network. The first part of this dissertation presents methods used to monitor GPS SIS anomalies with emphasis on my innovative data-cleansing algorithm and automated anomaly verification process. The GPS SIS anomalies are identified by a comparison of broadcast ephemerides and clocks with post-processed precise ephemerides and clocks. Then, the identified anomalies are verified using ground observation data. Unfortunately, the broadcast navigation data obtained from a global tracking network, such as the International GNSS Service (IGS), are sometimes corrupted by data-logging errors, which lead to far more false anomalies than true ones. I developed a voting-based algorithm to cleanse the messy navigation data of various errors and inconsistencies. Finally, I identified 1256 GPS SIS anomalies since June 2000, and verified 28 true anomalies since January 2004. The results show that the GPS SIS integrity performance has met the performance standard, and never have two anomalies or more occurred simultaneously over the past eight years. The second part of this dissertation extends SIS anomaly monitoring to GLONASS. Unlike the situation of GPS, there are still no generally accepted precise GLONASS clocks. I addressed this long-standing issue by aligning multiple inconsistent precise clock solutions that have been independently produced by several IGS Analysis Centers. In addition, facing the fact that the GLONASS SIS integrity performance standard has not yet been established, I defined my own anomaly criteria on the basis of the statistics of actual GLONASS SIS UREs. Finally, 192 GLONASS SIS anomalies have been identified since January 2009. Four events of simultaneous multiple anomalies were discovered, including a constellation-wide clock change on 28 October 2009 which impacted all satellites. An analysis of geographic dependency shows that anomalies occur twice as frequently when satellites are not monitored by the GLONASS ground control. The last part of this dissertation thoroughly characterizes nominal GPS and GLONASS SIS errors using three years of data. I developed robust statistical techniques to analyze the bias, distribution, and correlation of ephemeris errors, clock errors, and UREs. The statistics of actual errors show a certain degree of deviation from traditional idealized assumptions. Some of the statistics help define correct criteria for anomaly monitoring. The results also reveal two issues specific to GLONASS SIS: ephemeris error growth with propagation distance and geographic dependency. The methods developed for monitoring GPS and GLONASS SIS anomalies are applicable to other GNSS. The results provided in this dissertation are of great importance for enabling safety-of-life applications based on multiple constellations.