Inside GNSS Media & Research

NOV-DEC 2018

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42 InsideGNSS N O V E M B E R / D E C E M B E R 2 0 1 8 www.insidegnss.com As stated in ATT-D §4.1.6 of ICAO SARPs, the MSF com- putation requires an observation period of one year. erefore, a monthly analysis is too short a period for the computation of to be directly compared to the ICAO threshold of 1 × 10 –5 per hour. As a consequence, a monthly analysis can only provide the number of occurred MSF events, while the com- putation of their probability shall be postponed until the end of the year. Table 3 proposes an example of a day-by-day list containing the number of detected MSFs. Within the period of interest, no anomaly was detected. GPS.KPI-3: Probability of Simultaneous Major Failures of 2 or More Satellites e probability of simultaneous major failure (SF) of two or more satellites, , is similar to , with the main difference that the number of involved satellites shall be greater or equal to two. us, the theoretical formulation is with for any received satellite and N prn [.] indicates the num- ber of satellites that satisfy the condition within the square brackets. Like the previous the computation of this metric requires a very long observation period (≥ 1 year), during which the number of simultaneous major failures, namely N SF , shall be counted by taking into account the aforementioned constraints (see the above C-condition). If the N SF parameter is known, it is possible to calculate as follows: BLUEGNSS Type Accuracy Latency Updates Sample Interval Broadcast Orbits ~ 100 cm real time -- daily Sat. clocks ~ 5 ns RMS ~ 2.5 ns Sdev Ultra-Rapid (predicted half ) Orbits ~ 5 cm real time at 03, 09, 15, 21 UTC 15 min Sat. clocks ~ 3 ns RMS ~ 1.5 ns Sdev Ultra-Rapid (observed half ) Orbits ~ 3 cm 3 - 9 hours at 03, 09, 15, 21 UTC 15 min Sat. clocks ~ 150 ps RMS ~ 50 ps Sdev Rapid Orbits ~ 2.5 cm 17 - 41 hours at 17 UTC daily 15 min 5 min Sat. & Stn. clocks ~ 75 ps RMS ~ 25 ps Sdev Final Orbits ~ 2.5 cm 12 - 18 days every Thursday 15 min Sat. & Stn. clocks ~ 75 ps RMS ~ 20 ps Sdev Sat.: 30s Stn.: 5 min TABLE 2 Accuracy of GPS satellite ephemerides and clocks (see IGS products website, Additional Resources) Day of February 1 2 3 4 5 6 7 MSF Number 0 0 0 0 0 0 0 Day of February 8 9 10 11 12 13 14 MSF Number 0 0 0 0 0 0 0 Day of February 15 16 17 18 19 20 21 MSF Number 0 0 0 0 0 0 0 Day of February 22 23 24 25 26 27 28 MSF Number 0 0 0 0 0 0 0 Table 3 List of MSF events (from a BLUEGNSS monthly report) Day of February 1 2 3 4 5 6 7 MSF Number 0 0 0 0 0 0 0 Day of February 8 9 10 11 12 13 14 MSF Number 0 0 0 0 0 0 0 Day of February 15 16 17 18 19 20 21 MSF Number 0 0 0 0 0 0 0 Day of February 22 23 24 25 26 27 28 MSF Number 0 0 0 0 0 0 0 Table 4 List of SF events (from a BLUEGNSS monthly report) where N day is the number of days of the observation period. erefore, a monthly analysis can only provide the number of SF occurring within the period of inter- est. Table 4 provides an example of a day-by- day list containing the number of detect- ed SFs. Within the period of interest, no anomaly was detected. GPS.KPI-4: Positioning Accuracy Positioning accuracy is an indication of the position error dispersion, computed as the difference between the position assessed by a representative GPS receiv- er (i n compl ia nce w it h ICAO SA R Ps APP-B §3.1.3.1.1) and that of a geo-ref- erenced one. Furthermore, the GPS SPS position errors shall not exceed the fol- lowing limits (again from ICAO SARPs, §3.7.3.1.1.1). • Horizontal position error (HPE): 9 m (global average 95% of the time); 17 meters (worst site 95% of the time) • Vertical position error (VPE): 15 m (global average 95% of the time); 37 meters (worst site 95% of the time) erefore, the positioning accuracy (in meters) is typically referred to as the 95-percentile computed as follows. where: • i indicates the i-th epoch related to the i-th position pro-

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