SAM PULLEN, Stanford University
A: Since at least 1997, the Global Positioning System (GPS) Control Segment (CS) has issued what are known as “Notice Advisor(ies) to NAVSTAR Users” or “NANU(s).”
As originally reported in Section 3.5 of the 2001 version of the GPS SPS Performance Standard (GPS SPS PS), these provide “notification of changes in constellation operational status that affect the service being provided to GPS users, or if the U.S. Government anticipates a problem in supporting performance standards established in this document.”
The most recent 5th Edition of the GPS SPS PS  explains that a key purpose of NANUs is to alert GPS users of foreseeable future GPS satellite outages so that users are not surprised by them and potentially lose continuity. Specifically, Section 2.3.5 of  states the following:
“Scheduled interruptions which are announced at least 48 hours in advance do not contribute to a loss of continuity. Scheduled SPS SIS interruptions are announced by way of the Control Segment issuing a “Notice Advisory to Navstar Users” (NANU). NANUs are similar to the “Notices to Airmen” (NOTAMs) issued regarding scheduled interruptions of ground-based air navigation aids. CS internal procedures are to issue NANUs for scheduled interruptions at least 96 hours in advance.”
“Loss of continuity” (LOC) refers to the unexpected loss GNSS navigation when it was expected to be present (e.g., based on a prediction of which satellites would be usable at a given place and time). A GPS satellite that suddenly becomes unusable during an operation is a possible cause of this. By providing NANUs in advance of known outage periods (“scheduled interruptions”), the GPS CS provides users a means to predict that satellite(s) will be unavailable and thus not experience continuity risk from these events.
By stating that “scheduled interruptions announced at least 48 hours in advance do not contribute to a loss of continuity,” the SPS PS indicates that the GPS SPS is not responsible for any LOC experienced by users because they did not receive or ignored NANUs announcing a future outage that might cause LOC. This makes clear the penalty that SPS and other GPS users may face if they do not access or otherwise account for NANUs.
This penalty has nothing to do with integrity of safety of use, because satellites that are unhealthy due to scheduled outages indicate this by the health messages included in their navigation data fields (e.g., see Section 184.108.40.206.1.3 of the most recent GPS Interface Specification (IS) ). Instead, the disadvantage is potentially experiencing LOC that could have been avoided. The significance of this varies among user classes and will be discussed in the following sections.
Types of NANUs and Means of Distribution
If NANUs were encoded within the navigation data included in GPS satellite signals, there would be little or no reason for users to ignore them. However, this is not the case. Instead, NANUs are provided separately by the GPS CS, and the primary means to access them today is via the Internet, either via e-mail from the GPS CS or via dedicated websites. Because NANUs are not included in the broadcast navigation data, they are not included in the GPS IS . Instead, they are described in Sections 3 and 10 of a separate document, ICD-GPS-240C .
Table 1 (Table 10-1 of ) lists the NANU message types related to scheduled GPS satellite outages. As the first two rows show, scheduled satellite maintenance falls into two categories: “Delta-V maneuvers,” in which small thrusters on a satellite briefly fire to adjust the orbit of the satellite, and “non-Delta-V maintenance,” covering all other activities, most of which concern the atomic frequency standards (“clocks”) on the satellites. NANUs reporting unscheduled outages, when a satellite unexpectedly generates anomalous output and is flagged as unhealthy by the CS in one of several ways (see  and Section A.5 of  for details), occur after the fact and therefore do not help users avoid predictable losses of continuity.
The U.S. Coast Guard Navigation Center (NAVCEN) website is both a designated and convenient interface between the GPS CS and civil users . It maintains a list of active NANUs for the GPS constellation as well as NANU archives going back to 1997. It also has an easy-to-read Constellation Status chart showing the current status and active NANUs (if any) for each satellite in the constellation.
Table 2 shows a partial version of this chart (covering the A- and B-plane satellites) on May 1, 2021. The highlighted row for SVN 71 (PRN 26) in orbit slot B5 has an active NANU of type “FCSTDV,” which reports that this satellite is predicted to be unavailable at a specific date and time interval (Julian Day of the Year (JDAY) 126, corresponding to May 6, 2021, between 0240 and 1440 Zulu time, referring to UTC). This NANU is numbered “2021024,” referring to the 24th NANU issued in 2021. The NANU type indicates that SVN 71 will be maneuvered (changing its orbit) during this planned maintenance.
Accessing the full NANU text on the NAVCEN website shows that this NANU was issued at 1453 UTC on April 30, or approximately 5.5 days (132 hours) before the predicted start of the outage. This lead time greatly exceeds the 48 hours of advance warning stated in the SPS PS as sufficient to avoid loss of continuity. Even 48 hours of warning are sufficient to allow users to check for NANU updates once or twice per day rather than needing to monitor them continuously.
However, as reported by the Applied Research Laboratories, University of Texas at Austin (ARL:UT) , the minimum lead time of 48 hours for scheduled maintenance events was occasionally violated in the years between 2014 and 2017. As a result, the reporting standard in Table 3.6-3 (Section 3.6.3) of the latest SPS PS is for NANUs to be issued at least 48 hours in advance for 95% of the events. The reason that occasional delayed warnings are acceptable is that they only protect against loss of continuity as opposed to loss of integrity. In addition, late changes to maintenance plans due to other, higher-priority activities (such as an unexpected outage of another satellite) need to be accounted for.
Other types of NANU message types are described in Section 10.1 of , including reports of unscheduled (“surprise”) outages after they begin. These NANUs confirm to users that a satellite appearing to be unhealthy or unusable has been detected by the GPS CS and is being assessed (message type “UNUSUFN”). Once the outage is resolved and the satellite returned to healthy operations, another NANU stating the outage period serves as a record of the event (message type “UNUSABLE”). The table of notification times in Section 10.2 of  gives nominal and maximum delay times of 15 and 60 minutes, respectively, after the start of an unexpected outage.
NANU Usefulness in Avoiding Loss of Continuity
This brings us to the question of the quantitative value of scheduled NANUs in helping users avoid loss of continuity that should have been predictable. The answer depends on the frequency of scheduled outages for which NANUs give advance warning. Table 3, which is Table A.7-1 of the most recent SPS PS, provides conservative quantities for these events for satellites occupying the 24 primary or “Baseline” slots in the GPS constellation . The earlier rows of this table refer to the operational satellite lifetime prior to experiencing an End of Life (EOL) failure.
From Table 3, the average number of short-term scheduled (STS) outages that NANUs should warn of is about 2.0 per year (per satellite), which is approximately the same as the average number of short-term unscheduled (STU) outages that cannot be predicted. This suggests that following and using NANUs allows one to avoid potential loss of continuity from approximately half of satellite outages. If each of the 24 Baseline satellites has 2 scheduled and 2 unscheduled outages over a typical year, or 48 of each type, the rate of occurrence of either type of outage is about 48 / 8766 = 5.5 × 10-3 per hour.
For a given user in a specific location and time, what matters for computing the probability of LOC is the number of “critical satellites” among those visible to him or her. A critical satellite is one whose sudden loss would, by itself (with no other changes), cause LOC . GPS users with good sky visibility can typically view 8 to 12 GPS satellites at a time, but in most cases, none of these satellites are critical because the loss of any one of them does not degrade navigation performance significantly.
Occasionally, one of these satellites might be critical. Conservatively assuming one critical satellite as a basis for analysis, the probability rate of either a scheduled or unscheduled outage on that one critical satellite is about 2.0/ 8766 = 2.3 × 10-4 per hour. Again, since even one critical satellite is uncommon, the actual continuity risk due to satellite outages of either type is much smaller. However, for weaker satellite geometries containing 4 to 7 visible satellites (e.g., in urban canyons and other regions of obstructed sky visibility), multiple critical satellites are possible.
Since the probability of LOC due to GPS satellite outages only doubles if NANUs are ignored, many GPS users do so and thereby avoid the cost and complexity of accessing NANUs separately from receiving GPS signals. This is particularly true for safety-of-life-critical systems such as GBAS, where requiring the GBAS ground facility to access the Internet could expose it to security threats. These systems instead accept the possibility of LOC due to scheduled outages and include it in their continuity risk budgets, showing that their overall continuity risk requirements (e.g., an overall LOC probability no greater than 8 × 10-6 per 15 seconds for CAT I aircraft precision approaches) are met despite this.
NANU Usefulness in Avoiding Loss of Availability
While incorporating NANUs may be difficult for real-time GPS users, they are vital to offline GPS planning services that continually simulate GPS constellation performance in order to predict near-future outages in specific regions and times and relay that information to users. Aviation service providers, for example, provide Notices to Airmen (NOTAMs) indicating when and where hazards may exist or specific levels of service may be unavailable. Incorporating NANUs in these predictions is straightforward and logical, even though scheduled outages are uncommon.
One GPS-related availability prediction service oriented toward aviation but available to all is the RAIM prediction tool included within the FAA’s Service Availability Prediction Tool (SAPT) website . It provides predictions of potential outages for aviation users of GPS RAIM (in accordance with the rules for Terminal and En Route Area Navigation, or RNAV, in FAA AC 90-100A) due to satellite geometry as and other causes (e.g., planned RF interference tests by the Department of Defense). This tool provides graphical outputs along with .kml data files designed to interface with user flight planning software.
NANU-like Alerts from other GNSS Constellations
The need for NANU-like alerts of degraded GNSS constellation performance has been recognized by other GNSS service providers. Galileo, for example, issues “Notice Advisories to Galileo Users” (NAGUs), as called for by Section 220.127.116.11.1 of the most recent Galileo Open Service—Service Definition Document (OS SDD) . Section 3.6.1 of the OS SDD states that planned and general NAGUs for events affecting the OS Signal-in-Space (SIS) should be issued at least 24 hours before service is affected. Appendix E of the OS SDD defines the types and formats of NAGUs in detail.
The Russian GLONASS constellation also provides its own NAGUs, or “Notice Advisories to GLONASS Users,” from the “Information and Analysis Center for Positioning, Navigation and Timing (PNT IAC of the Central Research Institute of Machine Building) and the SPOCD of the Russian Ministry of Defense” at least 48 hours before “scheduled interruptions,” following the GPS precedent. This information comes from Section 3.6.2 of the GLONASS Open Service Performance Standard , which has a table that mirrors the one in Section 3.6.3 of the GPS SPS PS .
The Chinese Beidou constellation also provides “user notices” as an alerting mechanism. Section 8.3 of the Beidou Open Service Performance Standard  provides standards for the timing of advance notice of scheduled outages. The table in this section appears to be identical to the one in Section 3.6.1 of the Galileo OS SDD, meaning that advance notice of planned outages should be provided at least 24 hours in advance. Appendix C of  provides templates of the two types of user notices issued by Beidou.
Correction: GPS Expanded Slot Behavior
Brent Renfro of ARL:UT noted that the last sentence of the section labeled “GPS Continuity and Availability Standards” on page 23 of the January/February issue of Inside GNSS was misleading as written. This sentence read: “The notes under Table 3.7-1 [of the GPS SPS PS] clarify that, while the 24 baseline slots should be occupied by at least one satellite, the six expandable slots of these 24 shown in Table 3.2-2 are not always occupied by two satellites, and if they have two satellites, losing availability from one of them does not constitute loss of availability from that slot.” The precise definition of expandable slot availability is given in Sections A.7.2.4 and A.7.2.5 of the GPS SPS PS . An expandable slot is considered to be available when one of the three following conditions is met:
• The expandable slot is in its Baseline configuration, with a single trackable and healthy satellite filling the nominal (non-expanded, or “collapsed”) slot defined in Table 3.2-1 of .
• The expandable slot is in its expanded configuration, with the pair of locations defined in Table 3.2-2 of  for that slot both being occupied by trackable and healthy satellites.
• The expandable slot is in a “non-standard” configuration, with two trackable and healthy satellites being present but not in the locations defined in Table 3.2-2 of , and the performance of these two satellites exceeds that of a single satellite in the Baseline configuration for that slot.
Note that this definition does not include the case implied by the last component of the sentence cited above, in which a single trackable and healthy satellite in one of the two expandable slots is present but is not moved to the Baseline location for that slot.
This article describes one key purpose of GPS NANUs and similar warning messages provided by other constellations: allowing users to avoid unnecessary loss of continuity and improving near-future predictions of GPS availability by accounting for scheduled GPS satellite outages. Since NANUs must be accessed separately from the information contained in GPS satellite signals, they require extra steps to integrate into GPS receivers. Thus, most users who lack convenient Internet accessibility ignore them and accept scheduled outages as contributors to continuity and availability loss. However, user support systems with Internet connectivity, such as providers of service availability predictions and Assisted GPS (AGPS) base stations supporting cellular communications, can and should incorporate NANUs in their operations.
(1) Global Positioning System Standard Positioning Service Performance Standard, Washington, DC, U.S. Dept. of Defense, 3rd Ed (superseded), Oct. 2001. www.gps.gov/technical/ps/2001-SPS-performance-standard.pdf
(2) Global Positioning System Standard Positioning Service Performance Standard, Washington, DC, U.S. Dept. of Defense, 5th Ed, April 2020. www.gps.gov/technical/ps/2020-SPS-performance-standard.pdf
(3) Interface Specification Document: NAVSTAR GPS Space Segment/Navigation User Segment Interfaces, El Segundo, CA, GPS Enterprise Space & Missile Systems Center (SMC), IS-GPS-200L, October 2020. www.gps.gov/technical/icwg/IS-GPS-200L.pdf
(4) Interface Control Document: NAVSTAR GPS Control to User Support Community Interfaces, El Segundo, CA, GPS Enterprise Space & Missile Systems Center (SMC), ICD-GPS-240C, May 2019. gps.gov/technical/icwg/ICD-GPS-240C.pdf
(5) U.S. Coast Guard Navigation Center (NAVCEN) website, U.S. Dept. of Homeland Security, navcen.uscg.gov/. GPS Constellation Status is at navcen.uscg.gov/?Do=constellationStatus. NANU information is at navcen.uscg.gov/?pageName=gpsAlmanacs
(6) C. Shively, “Satellite Criticality Concepts for Unavailability and Unreliability of GNSS Satellite Navigation,” Navigation, Vol. 40, No. 4, Winter 1993-94, pp. 429-450. ion.org/publications/abstract.cfm?articleID=100164
(7) “Getting Started with RAIM SAPT,” U.S. Federal Aviation Administration, https://sapt.faa.gov/raim-start.php. Also see “RAIM Prediction Tool” at the SAPT home page, https://sapt.faa.gov/default.php
(8) Galileo Open Service–Service Definition Document, European Global Navigation Satellite Systems Agency (GSA), Version 1.1, May 2019. gsc-europa.eu/sites/default/files/sites/all/files/Galileo-OS-SDD_v1.1.pdf. Active Galileo NAGUs are at gsc-europa.eu/system-status/user-notifications, and current constellation status (with relevant active NAGUs) is summarized at gsc-europa.eu/system-service-status/constellation-information
(9) GLONASS Open Service Performance Standard (OS PS), Russian Defense Ministry and Roscosmos State Corp., Edition 2.2, June 2020. glonass-iac.ru/en/GLONASS/documents.php
(10) BeiDou Navigation Satellite System Open Service Performance Standard, China Satellite Navigation Office, Version 2.0, December 2018. beidou.gov.cn /xt/gfxz/201812/P020181227529449178798.pdf
(11) B. Renfro, N. Boeker, A. Terry, An Analysis of GPS SPS Performance for 2014, TR-SGL-17-02, gps.gov/systems/gps/performance/. Also at same URL: Performance for 2016, TR-SGL-17-06 and 2017, TR-SGL-18-02, add second author M. Stein.