Precise Positioning Service Performance Standard - GPS.gov

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February 2007 GPS PPS PS 2.3.4 PPS SIS Integrity The PPS SIS integrity is defined to be the trust which can be placed in the correctness of the information provided by the PPS SIS. PPS SIS integrity includes the ability of the PPS SIS to provide timely alerts to receivers when the PPS SIS should not be used for positioning or timing. For this PPS PS, there are four components of integrity as follows: Probability of a Major Service Failure. The probability of a major service failure for the PPS SIS is defined to be the probability that the PPS SIS's instantaneous URE exceeds the SIS URE not-to-exceed (NTE) tolerance (i.e., misleading signal-in-space information [MSI]) without a timely alert being issued (i.e., unalerted MSI [UMSI]). Alerts generically include both alarms and warnings. Time to Alert. The time to alert (TTA) for the PPS SIS is defined to be the time from the onset of MSI until an alert (alarm or warning) indication arrives at the receiver's antenna. Real-time alert information broadcast as part of the NAV message data is defined to arrive at the receiver’s antenna at the end of the NAV message subframe which contains that particular piece of real-time alert information. SIS URE NTE Tolerance. The PPS SIS URE NTE tolerance for a healthy PPS SIS is defined to be ±4.42 times the upper bound on the URA value corresponding to the URA index "N" currently broadcast by the satellite for the authorized user. The SIS URE NTE tolerance for a marginal PPS SIS is not defined in this PPS PS. There is no SIS URE NTE tolerance for an unhealthy PPS SIS. Alarm Indications. An otherwise healthy PPS SIS or marginal PPS SIS becomes unhealthy when it is the subject of a PPS SIS alarm indication. For the PPS SIS alarm indications defined below, there is no PPS SIS URE NTE tolerance. The PPS SIS alarm indications are defined to include the following: (1) The PPS SIS ceases transmission (e.g., no power or negligible power) or otherwise becomes unusable. (2) The elimination of the standard P(Y)-code (and the corresponding elimination of the standard C/A-code). (3) The substitution of non-standard P(Y)-code for the standard P(Y)-code (and the corresponding substitution of non-standard C/A-code for the standard C/A-code). (4) The substitution of PRN P(Y)-code number 37 for the standard P(Y)-code (and the corresponding substitution of PRN C/A-code number 37 for the standard C/Acode). (5) The failure of parity on 5 successive words of NAV data (3 seconds). (6) The broadcast IODE does not match the 8 LSBs of the broadcast Index of Data Clock (IODC) (see ICD-GPS-224, ICD-GPS-225, or IS-GPS-200). (7) The transmitted bits in subframe 1, 2, or 3 are all set to 0's or all set to 1's. (8) Default NAV data is being transmitted in subframes 1, 2, or 3 (see ICD-GPS-224, ICD-GPS-225, or IS-GPS-200). 1 st Edition Page 12
February 2007 GPS PPS PS (9) The 8-bit preamble does not equal 10001011 2 , decimal 139, or hexadecimal 8B. Warning Indications. An otherwise healthy PPS SIS becomes marginal or unhealthy when it is the subject of a PPS SIS warning indication. PPS SIS warnings are typically provided in advance of the onset of MSI (i.e., preemptive warnings). The PPS SIS warning indications are defined in paragraph 2.3.2 above, plus: (1) An appropriately inflated URA index "N" value (appropriately inflated to cover the expected large PPS SIS URE). Notes: 1. A PPS SIS alarm indication exists when the satellite is not usable because it is not transmitting the standard P(Y)-code (and standard C/A-code) modulation on the L-band carrier signals. As indicated above, specific PPS SIS alarm indications include the following: (a) when the L-band carrier signals have no modulation (i.e., unmodulated carrier signals), (b) when the L-band carrier signals are modulated by nonstandard P(Y)-code (and nonstandard C/A-code), and (c) when the L-band carrier signals are modulated by P(Y)-code number 37 (and C/A-code number 37). These PPS SIS alarm indications are specifically called out above because of their relatively high probability of occurrence. 2. The PPS SIS alarm indications related to the NAV message data are considered “weak” indications since PPS receivers do not necessarily continuously read each satellite’s NAV message data either by design or by circumstance (e.g., radio-frequency interference [RFI] can prevent reading NAV message data). As a rule of thumb, these weak PPS SIS alarm indications are assumed to have a five minute lag time before PPS receivers take notice of them for alerting purposes. 3. See Appendix A, Section A.5 for additional background information on integrity. 2.3.5 PPS SIS Continuity The PPS SIS continuity for an available and healthy PPS SIS is the probability that the PPS SIS will continue to be available and healthy without unscheduled interruption over a specified time interval. Scheduled interruptions which are announced at least 48 hours in advance do not contribute to a loss of continuity. Scheduled PPS 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. OCS internal procedures are to issue NANUs for scheduled interruptions at least 96 hours in advance. 2.3.6 PPS SIS UTC(USNO) Accuracy The PPS UTC(USNO) accuracy for a healthy PPS SIS is defined to be the 95% error in the parameters (ref. 20.3.3.5.2.4 of IS-GPS-200) contained in that PPS SIS which relate GPS time to UTC(USNO). 1 st Edition Page 13
Page 1 and 2: GLOBAL POSITIONING SYSTEM PRECISE P
Page 3 and 4: ASSISTANT SECRETARY OF DEFENSE 6000
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February 2007 GPS PPS PS Percentage
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February 2007 GPS PPS PS and VDOP v
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February 2007 GPS PPS PS Satellite
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February 2007 GPS PPS PS - F - FOC
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February 2007 GPS PPS PS - R - R R9
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Precise Positioning Service Performance Standard - GPS.gov

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