Inside GNSS Media & Research

NOV-DEC 2018

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44 InsideGNSS N O V E M B E R / D E C E M B E R 2 0 1 8 www.insidegnss.com GPS.KPI-6: Service Availability e GPS horizontal or vertical positioning service, within the generic interval T = [τ, τ + Δ], is available (or equivalently the availability flag = true) only if the 95% horizontal or vertical position error (respectively HPE 95% and VPE 95% ), computed within the T-interval, is respectively lower than 17 meters or 37 meters. us, both the horizontal and vertical service availabilities (respectively indicated with the acronyms: HSA % or VSA % ) are defined as the percentage of true availability flags calculated within a daily interval of 24 hours. In compliance with ICAO, this metric should satisfy the following targets. • HSA % ≥ 99% horizontal service availability, average location (17 meters, 95% threshold). • VSA % ≥ 99% vertical service availability, average location (37 meters, 95% threshold). • HSA % ≥ 90% horizontal service availability, worst-case loca- tion (17 meters, 95% threshold). • VSA % ≥ 90% vertical service availability, worst-case location (37 meters, 95% threshold). Figure 14 provides an example of HSA and VSA plots achieved by setting the Δ-parameter equal to zero. In this way, the sliding window is collapsed to a sample and, consequently, the availability flag is computed for each HPE and VPE sample. Both plots show that the service availability is always > 99%. GPS.KPI-7: HDOP/VDOP Dilution of precision (DOP) is a scaling factor, that binds the user-satellite geometry to the position accuracy, see Eq. 11. where σ rng and σ xPE are respectively the range and position error standard deviations. Specifically, xPE stands for horizontal and vertical position errors (respectively HPE and VPE) and con- sequently xDOP indicates respectively the horizontal (H) and vertical (V) DOPs. Roughly speaking, DOP indicates the quality of user- satellite geometry. erefore, according to Eq. (11), low DOPs indicate favorable geometries and, consequently, low position errors, and vice versa implies worse accuracies. More details on this metric are available in Additional Resources (E. D. Kaplan and C. J. Hegarty; P. Misra et alia). Figure 15 and Figure 16, respectively, propose some examples of monthly and daily xDOP plots. GPS.KPI-8: Number of Used and Tracked Satellites e number of used and tracked satellites is another useful metric for investigating the causes of poor position accuracies. Indeed the case of only a few tracked satellites is an indication of an anomaly that affects the reception of positioning signals. is is caused for example by possible interfering signals, mask- ing effects, hardware damages, etc. At the same time, the num- ber of satellites used allows us to shi the attention to the PVT solution and the motivations for possible satellite exclusions. BLUEGNSS FIGURE 14 Service availability plots (from a BLUEGNSS monthly report) January 2018 HSA % 0 2 (a) HSA % vs. ICAO thresholds (b) VSA % vs. ICAO thresholds 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 110 100 90 80 70 60 50 110 100 90 80 70 60 50 January 2018 VSA % 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 FIGURE 15 xDOP vs day-of-month (from a BLUEGNSS monthly report) February 2018 (a) HDOP 95% plot HDOP 95% 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 16 14 12 10 8 6 4 2 0 February 2018 (b) VDOP 95% plot VDOP 95% 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 16 14 12 10 8 6 4 2 0 FIGURE 16 HDOP and VDOP plots vs time (from an Italian BLUEGNSS daily report) Time (h) HDOP and VDOP 0 2 4 6 8 10 12 14 16 18 20 22 24 10 8 6 4 2 0

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