Investigation into September 2020 GPS SVN 74 Performance Anomaly Todd Walter, Stanford University Zixi Liu, Stanford University Juan Blanch, Stanford University Kristy Pham, William J. Hughes FAA Technical Center John Mick, William J. Hughes FAA Technical Center William Wanner, William J. Hughes FAA Technical Center ABSTRACT On September 20, 2020, GPS Space Vehicle Number (SVN) 74 exhibited unusual behavior that appears to have led to anomalous behavior for some GPS users. This paper investigates the observed signals in order to characterize the broadcast signal behavior over the course of the event. Investigations like this are critical to understanding the potential impact of such events on safety-of-life applications using GPS. In particular, Advanced Receiver Autonomous Integrity Monitoring (ARAIM) has associated requirements to monitor signal behavior and predict future performance levels including accuracy and fault probabilities. The first question that arose from the initial reported behavior was whether or not the event constituted a major service failure as defined in the GPS Standard Positioning Service Performance Standard (GPS SPS PS)[1]. As it turns out, the answer was not so easily determined. GPS has many different methods to indicate an alarm to the user, that if employed will successfully indicate to the user that the satellite should not be included in their position solution computation. Among these methods are an absence of a trackable signal and the use of alternative data patterns in the navigation message. A difficulty with determining whether these indicators were used is that they are not always recorded into data archives and may be confused with network outages or other data loss mechanisms. Although the alarm may result in the loss of data, the absence of data does not guarantee the presence of an alarm. We need to be able to rule out other explanations for data loss in the archival records. We therefore need to obtain and scrutinize the measured carrier to noise ratios and raw navigation data bits in order to firmly establish the presence and timing of these different alarm mechanisms. This paper describes the signal behavior observed indicating different health states and also describes the transition characteristics between each state. We will describe some of the reported receiver behaviors and which mechanisms were used to protect users from the potential use of misleading data. INTRODUCTION On September 20, 2020, two Notice Advisory to Navstar Users (NANUs) were issued describing an outage on SVN 74 (PRN 04) during the first part of that day. These NANUs specified an outage period between (hour:minute:second) 00:31:00 and 05:47:00 GPS time. We later learned of large reported pseudorange errors on this satellite and reports of invalid position estimates via Automatic Dependent Surveillance–Broadcast (ADS–B) data that occurred within this time period. We began investigating this satellite in more detail in order to determine whether the satellite had exhibited faulted behavior or whether users may have reacted badly to unexpected behavior. SVN 74 is the first Block III satellite and therefore may have characteristics not previously seen on other GPS satellites. The question of whether or not the satellite was faulted is an important one. The GPS SPS PS [1] allows for the possibility of excess error on the GPS range measurement, but such behavior is expected to be rare (less often than once per 100,000 satellite operating hours). In fact, the last observed such fault occurred June 17, 2012 [2], so any new fault would be a noteworthy event. However, if it was not a fault, then many receivers may have still had an adverse reaction to what was broadcast, and we may need to take action to ensure that receivers are better prepared to operate under similar events in the future. We also examined the response of the Wide Area Augmentation System (WAAS) [3] and saw that the satellite either set to “Not Monitored” or to “Do Not Use” during the time period of interest. “Not Monitored” is used when the satellite is insufficiently observed and WAAS cannot confidently determine its level of performance. “Do Not Use” is broadcast when the satellite is set to unhealthy or when the satellite appears to be exhibiting a large error.