Inside GNSS Media & Research

SEP-OCT 2018

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Page 50 of 67 S E P T E M B E R / O C T O B E R 2 0 1 8 Inside GNSS 51 Galileo constellation. Most of the satel- lites are currently operating with Pas- sive Hydrogen Masers (PHM) as prime clock. With 26 spacecra in orbit, and considering the quadruple redundancy implemented on each satellite, the total number of Galileo atomic clocks deliv- ered to space since 2011 amounts to 104. A limited number of units have shown failures, however this is mitigated by the quadruple redundancy in place and by the recent refurbishment of some clock components performed by the satellite manufacturer, already implemented in L9 and L10 satellites. Figure 14 illustrates a recent Allan deviation characterization for the Gali- leo satellite clocks, showing an overall excellent frequency stability and com- pliance with requirements. In terms of broadcast clock prediction accuracy, the Galileo constellation shows similar or better performance than GPS and other GNSS in the nominal range of the navigation message Age of Data (<3 hours), as reported in several studies (P. F. Madrid Navarro et alia; G. Galluzzo et alia). In-Field Galileo Performance Evaluation In addition to the system performance at the Signal In Space interface and associ- ated figures of merit, ESA is also analyz- ing the attainable end-user performance through field tests conducted in different environments, from open sky to urban canyons, as well as with a variety of hardware setups. Position, Velocity, and Time (PVT), User Equivalent Ranging Error (UERE), and Time To First Fix (TTFF) are some of the metrics evaluated for static, pedes- trian, and vehicular users, using both professional and mass-market receivers ( Figure 15 ) (E. Breeuwer et alia). Monthly horizontal and vertical positioning accuracy are monitored continuously for fixed user positions at the Galileo Experimental Sensor Sta- tions comprising the TGVF worldwide network ( Figures 16 and 17 ). Table 3 provides a summary of the detailed positioning accuracy figures obtained at the GESS sites with Galileo- only measurements for the full month of June 2018, using F/NAV E1-E5a dual frequency service, and limited to PDOP≤6 intervals. A simple least square PVT algorithm is used to generate these solutions. e cumulative errors for all the stations in the horizontal and verti- cal domain were 2.65 meters and 4.42 meters (95%), respectively. An example of a vehicular test with Galileo-enabled smartphones is shown in Figure 18 . e positioning error was calculated against a precise reference generated with a professional inertial FIGURE 13 On-board master clock operational history GSAT0213 GSAT0212 GSAT0213 GSAT0207 GSAT0211 GSAT0210 GSAT0209 GSAT0208 GSAT0206 GSAT0205 GSAT0204 GSAT0203 GSAT0202 GSAT0201 GSAT0104 GSAT0103 GSAT0102 GSAT0101 Dec RAFS-A RAFS-B PHM-A PHM-B Feb Apr Jun Aug Oct Dec Feb Apr Jun 2016/2018 FIGURE 14 Allan Deviation of Galileo clocks (April 2018) 10 –12 10 –13 10 –14 10 –1 5 10 1 10 2 10 3 Sampling time [s] 10 4 10 5 10 6 Allan deviation FIGURE 15 Galileo Field Testing at ESA-ESTEC

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