Inside GNSS Media & Research

NOV-DEC 2018

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www.insidegnss.com N O V E M B E R / D E C E M B E R 2 0 1 8 InsideGNSS 41 ellite's ranging signal error (excluding atmospheric and receiver errors) exceeds the range error limit of 4.42 times the upper bound on the user range accuracy (URA) broadcast by a satellite for lon- ger than the allowable time to alert (10 seconds). e probability of 1×10-5 in Chapter 3, 3.7.3.1.4 corresponds to a maximum of 3 major service failures for the entire constellation per year assum- ing a maximum constellation of 32 satellites. From the definition reported in the ICAO SARPs ATT-D §4.1.6, the probability of major service failure (MSF), can be formulated as shown in Eq. (2). where for any received satellites belonging to GPS con- stellation and one year of observation (ICAO Doc-9849). e alert flag is provided by the GPS NAV message in the HOW word (see GPS Interface Control Document in Additional Resources). In compliance with the previous defini- tions, the computation of this metric requires an observation period of one year (leap year equals 366 days), during which the number of MSF, namely N MSF , shall be counted by taking into account all aforementioned constraints (see the above C-condition). When the N MSF parameter is available, it is possible to calcu- late by using Eq. (3) (which is a possible practical implementation of Eq. (2)). TABLE 1 List of GPS performance parameters with the related definitions and targets (ICAO) ID Parameters Definitions Targets Notes GPS. KPI-1 Range domain accuracy Measurement of range error due to satellite clock and ephemeris deviations URE % Threshold Healthy satellites and URA = 0 GA URE 99.94% 30 meters WS URE 99.79% 30 meters GPS. KPI-2 Probability of major service failure (MSF) Probability that over a specific time interval, a healthy-satellite ranging signal error (excluding atmospheric and receiver errors) exceeds the broadcast range error limit by a given factor Pr(MSF) ≤ 1×10 −5 per hour, with: - 1 year of observation - healthy satellites - no alert flag raised within an interval of 10 sec Probability of 1×10 -5 corresponds to a max of 3 MSFs for the entire constellation per year assuming a constellation of 32 satellites, see ICAO SARPs GPS. KPI-3 Probability of simultaneous major failures of 2 or more satellites Probability of simultaneous major failures of 2 or more satellites Pr(SF) ≤ 1×10 −9 per hour, same conditions of MSF Simultaneous MSF of 2 or more satellites GPS. KPI-4 Positioning accuracy 95th-percentile of position error, intended as the difference between estimated receiver positions and the reference one, calculated over any interval of 24hrs, for any point within the service volume Global Average Worst Site -- HPE 9 meters 17 meters VPE 15 meters 37 meters GPS. KPI-5 Continuity Probability that healthy SIS per satellite will continue to be healthy without unscheduled interruptions over a specified time interval C br ≤ 2×10 −4 /h Measured in terms of continuity breaks, C br , and valid only for unscheduled interruptions GPS. KPI-6 Service availbility Percentage of time over any 24h interval that the predicted 95% position accuracy is less than a specified value within the service volume Global Average Worst Site HPE-95% < 17 meters VPE-95% < 37 meters HSA % ≥ 99% ≥ 90% VSA % ≥ 99% ≥ 90% GPS. KPI-7 HDOP/VDOP DOP is a scaling factor that binds the user- satellite geometry to the position accuracy: σ PE = σ rng · PDOP N.A. For local performance analysis GPS. KPI-8 Number of used and tracked satellites Number of satellites used within PVT solution vs number of tracked satellites N.A. For local performance analysis FIGURE 11 Range domain accuracy vs day-of-month (from a BLUEGNSS monthly report) December 2017 User range error (m) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 40 35 30 25 20 15 10 5 0

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