Inside GNSS Media & Research

SEP-OCT 2018

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48 Inside GNSS S E P T E M B E R / O C T O B E R 2 0 1 8 all satellites computed since the Initial Services declaration resulted in a mean of 45 minutes, opposed to about 12 hours for GPS (see Additional Resources). Galileo AoD has decreased signifi- cantly in recent years, as illustrated in Figure 7 , with a reduction by a factor of two in the AoD 95% value, from 275 to 140 minutes, before and aer Initial Services. AoD is expected to decrease further with the on-going ground seg- ment and uplink station enhancements. The uplin k a nd ref resh rate of t he broadcast navigation message is opti- mized by the ground segment based on the uplink station availability. e uplink rate can be relaxed, for example, in the case of on-ground maintenance activities. System Availability Figure 8 shows the trend of the Galileo healthy Signal In Space (SIS) availabil- ity calculated as a monthly constellation average. e results provide an average above the 87% minimum annual target set in OS-SDD. As mentioned before, the total number of healthy satellites is expected to increase soon once L10 com- missioning is completed. According to the Galileo OS-SDD, the availability of the OS SIS is defined as the percentage of time a user is able to receive a healthy SIS as a global average, either single frequency (E1, E5a, E5b) or dual frequency (E1-E5a, E1-E5b), from at least one satellite above a minimum elevation angle of five degrees from any point in the coverage area. With the cur- rent available constellation as per Table 1, the expectation for this parameter is 100%. In other words, there is always a Galileo satellite in view broadcasting healthy SIS worldwide. is parameter was devised at the early stages of Galileo constellation deployment, when only a limited number of satellites were avail- FIGURE 8 Constellation Healthy SIS availability 100 80 60 40 20 0 Availability [%] 15 12 9 6 3 0 #operational satellites (black) # healthy satellites (red) Dec-16 Jan-17 Feb-17 Mar-17 Apr-17 May-17 Jun-17 Jul-17 Aug-17 Sep-17 Oct-17 Nov-17 Dec-17 Jan-18 Feb-18 Mar-18 Apr-18 May-18 MEASURING G ALILEO PERFORMANCE FIGURE 6 Cumulative Constellation Age of Data, 01/12/2016 – 31/05/2018 1 0.8 0.6 0.4 0.2 0 0 50 100 150 Age of Data (minutes) 200 250 300 Cumulative Age of Data CDF FIGURE 7 Galileo All Satellites Age of Data (y-axis limit set to 30 hours) 30 25 20 15 10 5 All Satellites A ge of Data (hours) Jul-14 Feb-15 Aug-15 Mar-16 Sep-16 Apr-17 Nov-17

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