Inside GNSS Media & Research

JUL-AUG 2018

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42 Inside GNSS J U L Y / A U G U S T 2 0 1 8 ECCENTRIC SATELLITES uplink by the Mission Up-Link stations (ULS). e Galileo Time and Geodetic Validation Facility (TGVF) – tasked with advanced Orbit Determination and Time Synchronisation (ODTS) processing and independent system performance monitoring, supported by a worldwide network of sensor sta- tions – also required adaptations to allow the processing of Galileo satellites 5 and 6 in eccentric orbits to generate orbital data for the Search and Rescue (SAR) community. Based on the positive outcomes of this assessment, the EC authorized the initiation of the required modifications. Following the seamless upgrades of the Galileo core infra- structure from June to July 2016, Galileo 5 and 6 have been injected into the ground segment navigation processing and started the broadcast of navigation messages for testing pur- poses on August 5, 2016 (ref: Notice Advisory to Galileo Users - NAGU 2016029 and 2016030) with one S-band uplink per orbit (~14 hours). e almanacs for Galileo 5 and 6 are not broadcast since the orbital parameters, in particular semi-major axis and eccentricity, do not fit in the range of values foreseen for this field in the Open Service Signal-In-Space Interface Control Document (OS SIS ICD) given the eccentric nature of the sat- ellites. e almanac data is a reduced-precision subset of the clock and ephemeris parameters of the active satellites in orbit. e Galileo almanac orbital parameters consist of, semi-major axis, eccentricity, inclination, longitude of the ascending node, argu- ment of perigee and mean anomaly. e main hurdle in using Galileo 5 and 6 operationally was that their corrected orbits still fall outside of the "almanacs" broadcast within navigation messages to locate satellites. However, the satellite's signal could still be received in open sky search. With the deployment of C-band dissemination capability in November 2016, the performance did significantly improve due to more frequent navigation messages uplinks from ground, resulting in a reduction of the Signal In Space Ranging Error (SISE). Figure 6 illustrates the instantaneous F/NAV SISE global average during the period January - March 2017. e effects of SISE with and without Galileo 5 (E18) and Galileo 6 (E14) were clearly shown before and aer the Galileo ground segment optimization on March 18, 2017. e opti- mization led to the reduc- tion in the Age of Data (AoD) – elapsed t i me since the generation of a navigation message data set by the ground seg- ment and its final recep- tion at user level – which resulted in improvements in the ranging accuracy. ere is a particular sen- sitivity of Galileo satel- lites 5 and 6 SISE to the Age of Data, to a much larger extent than the sat- ellites in nominal orbits, as shown in Figure 7 . I n nom i na l op er a- t iona l cond it ions , t he Galileo ground segment is able to limit the maxi- FIGURE 6 F/NAV SISE during January - March 2017 doy 2017 FNAV SISE as observed @any age SISE at GA (m) 1 11 21 31 41 51 61 71 81 10 8 6 4 2 0 doy 2017 SISE at GA (m) 1 11 21 31 41 51 61 71 81 10 8 6 4 2 0 FIGURE 5 Galileo-only position fix using (GSAT0101, 0102, 0103 and 0201)

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