Inside GNSS Media & Research

NOV-DEC 2018

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40 InsideGNSS N O V E M B E R / D E C E M B E R 2 0 1 8 www.insidegnss.com repor ts on GPS, GPS/EGNOS, a nd Galileo performance for the following purposes. From ICAO Doc-9849 7.8.3.1 – GNSS performance assessment may be used by States in order to: a) periodically verify the performance of signals that have already been declared operational (such as GPS L1 and GLONASS L1); and/or b) collect statistical data on technical elements to support decisions for operational approvals in a particular airspace based on new GNSS signals and/or constellations (such as GPS L5, GLONASS L3, BDS and Galileo). Specifically, the declared objective of monthly reports is a periodic compli- ance analysis of the following services: • GPS SPS (Standard Positioning Ser- vice) • GPS/EGNOS APV-I and LPV-200 • Galileo OS (Open Service) to ICAO requirements (noted earlier and in the Additional Resources) within the BLUE-MED FAB. Conversely, daily reports aim at pro- viding a local characterization of the aforementioned positioning services on an hour-by-hour basis. erefore, their results are more detailed and very use- ful for the investigation of any detected anomaly. According to ICAO, t wo dif fer- ent sets of key performance indicators (KPIs) are used to characterize the ser- vice provided by GNSS core constella- tions in stand-alone mode (e.g., GPS- only and Galileo-only) or augmented by SBAS systems (e.g., GPS/EGNOS). e following subsections provide an over- view of these KPIs, including a descrip- tion of the RFI monitoring implemented by the BLUEGNSS sentinels. Performance Parameters of Core Positioning Systems (GPS and Galileo) Per ICAO, a list of core constellation performance parameters suitable for periodic verification is reported in Table 1 . Such parameters are focused on GPS, but they can be also extended to other constellations, such as Galileo, even if the related SARPs are not available yet. GPS.KPI-1: Range Domain Accuracy e range domain accuracy is an indica- tion of the SiS user range error (URE) dispersion that describes the statisti- cal uncertainty of the modeled pseu- dorange due to errors in the broad- cast orbit and clock information (O. Montenbruck et alia). It is well known that broadcast ephemerides are used by GNSS receivers to assess the posi- tion of GPS satellites. Such estimations are typically affected by errors of some meters (see Table 2) and could degrade the receiver position computation. Fig- ure 10 shows the difference between the estimated and actual satellite position. It is worth noting that the main con- tributions of the URE metric are ephem- eris and clock errors, because antenna variations and signal imperfections are usually at the centimeter or milli- meter level (see Additional Resources). erefore, the instantaneous URE for a user at the location u can be computed as shown in Eq. (1), where s and b are respectively the actual satellite position and clock bias, sʹ is the satellite position derived from broadcast ephemerides, bʹ is the broadcast satellite clock bias, · is the vector dot product, and ||·|| is the norm operator. Figure 11 shows an example of a range domain accuracy plot in which the mean and worst URE are always below the ICAO threshold. GPS.KPI-2: Probability of Major Service Failure From ICAO SARPs §3.7.3.1.4 – Probability of major service failure. e probability that the user range error (URE) of any satellite will exceed 4.42 times the upper bound on the user range accuracy (URA) broadcast by that satellite without an alert received at the user receiver antenna within 10 seconds shall not exceed 1×10-5 per hour. Att. D §4.1.6 – A major service failure is defined to be a condition over a time interval during which a healthy GPS sat- BLUEGNSS FIGURE 9 Basic architecture of BLUEGNSS system Daily NANU DATA REPOSITORY EDAS DATA REPOSITORY EUREF DATA REPOSITORY IGS DATA REPOSITORY NAGU DATA REPOSITORY Daily Data Processing & Statistic Production Daily Browser BLUEMED PORTAL (statistical report) CMF PORTAL (sentinel network monitoring) CMF REPOSITORY GNOME + EDAS + IGS + NANU + NAGU Data CMF (Ciampino CRAV) 15sec + Daily 15sec + Daily 15sec + Daily 15sec + Daily Send (15sec) -> UDP unicast extended heartbeat Sync (Daily) -> Statistical data repository synchronization Daily Daily GNOME Sentinel Italy (LIML) LOCAL REPOSITORY GNOME Sentinel Greece (LGKO) LOCAL REPOSITORY GNOME Sentinel Cyprus (LCLK) LOCAL REPOSITORY GNOME Sentinel Malta (LMML) LOCAL REPOSITORY Daily FIGURE 10 Difference between the actual and estimated satellite positions. It is worth noting that the radial error component (i.e., range error) is generally lower with respect to the along-track and cross-track errors, but its effect on the PVT solution accuracy is more relevant. Rx Estimated Sat. Position Actual Sat. Position Rng Error Eph. Error

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