Inside GNSS Media & Research

JUL-AUG 2018

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44 Inside GNSS J U L Y / A U G U S T 2 0 1 8 ECCENTRIC SATELLITES station, then report the beacon's alert message and position to Cospas-Sarsat Operations. Galileo is one of the main contribu- tors to the global MEOSAR alert service by providing a global space segment and the regional ground segment elements for detection/localization in Europe with three MEOLUT stations deployed in Spain, Norway and Cyprus. e management of the ground segment operations and service provision is carried out from the SAR/Galileo Service Centre located in Toulouse, France in the premises of the French National Space Agency (CNES). e SAR repeater on Galileo 5 (GSAT0201, Cospas-Sarsat designation: 418) was first switched on in December 2014. e SAR In-Orbit Testing (IOT) was performed at switch-on and the testing of the SAR repeater for Cospas-Sarsat (CS) com- missioning was performed in November 2015. Followed by the testing of SAR repeater on Galileo 6 (GSAT0202, Cospas-Sarsat designation: 414) which was switched on in December 2015 and Cospas-Sarsat (CS) commissioning testing completed in March 2016. From August 2015 through February 2016 the Galileo Time and Geodetic Validation Facility (TGVF) located at ESTEC was adapted to allow the processing of Galileo satellites 5 and 6 in eccentric orbits. Orbit and clock predictions in suitable format were generated and made available on a server accessible by the European GNSS Service Centre (GSC) for subsequent dissemi- nation to the SAR community and MEOLUTs. Adaptations of the SAR Ground Segment MEOLUT Tracking Coordination Facility (SGS MTCF) was also completed to allow the ingestion of orbital data retrieved from the GSC. e SAR repeaters on Galileo 5 and 6 were successfully tested, but not used until March 2016 due to the lack of orbital data in the navigation signal-in-space, which is essential for performing the SAR localization function. eir usage com- menced when the orbital data generated by the TGVF was made available via the GSC in March 2016. is allowed MEOLUTs around the world to fetch the orbital data in order to track Galileo satellites, including Galileo 5 and 6. In the period from March 2016 until December 2016, Gali- leo 5 and 6 SAR repeaters have been used, together with other available Galileo SAR repeaters, for system performance valida- tion activities and Key Performance Indicator (KPI) collection. Based on the positive results of the tests, Cospas-Sarsat consid- ers the two repeaters commissioned and available for all users. On December 13, 2016, Cospas-Sarsat declared MEOSAR Early Operational Capability (EOC), based on commissioned MEOSAR repeaters, which included repeaters on Galileo 5 and 6 satellites and since then they are used operationally for MEO- SAR by the French and US Mission Control Centers (FMCC and USMCC). With future deployment of the system, Galileo satellites will be capable of providing the Return Link Service to users with a feedback by sending an acknowledgement message to the users and informing them that the alert has been detected and eventually that rescue operations are under way. Everything is Relative Iterating with several European scientific institutions, it became apparent from the very beginning that the recovery orbits of Galileo satellites 5 and 6, still eccentric, were, in turn, offering a unique opportunity to conduct a test of Einstein's General eory of Relativity by measuring more accurately than ever before the way that gravity affects the passing of time. Indeed, Einstein's eory of General Relativity (GR) predicts that time flows differently for two clocks that have a relative speed and are placed in different gravitational potentials. It should there- fore be possible to test General Relativity by comparing the frequencies of two atomic clocks, in a so-called gravitational redshi test. A gravitational redshi experiment tests the Local Position Invariance (LPI), which is one of the aspects of the Einstein Equivalence Principle which may be tested. As sev- eral alternative theories of gravitation predict violations of this effect – e.g. in attempts to unify GR and quantum theory – experimental constraints are of paramount importance. e recovery orbits of Galileo 5 and 6 and the specifici- ties of the Galileo satellites made these especially suitable for a Gravitational redshi tests ( Figure 9 ), noting that: 1. Galileo 5 and 6 remain in elliptic orbits, with each satellite climbing and falling some 8,500 kilometers twice per day, and providing a periodic modulation of the gravitational redshi at the orbital period (~ 13 hours). 2. Both satellites are equipped on-board with Passive Hydro- gen Maser (PHM) atomic clocks, providing unique stability. 3. Nominal satellite life time aer recovery remained long (~12 years), which allows the possibility to integrate test measurements during a long time. 4. Satellites are permanently monitored with the provision of high accuracy orbits. 5. Laser Retro Reflectors (LRR) are equipped on-board the satellites which allow independent orbit tracking by laser (of high interest to disentangle clock and orbit radial errors). 6. e realization of these tests does not interfere with the potential introduction of Galileo 5 and 6 for navigation service or the nominal Search and Rescue (SAR) service. FIGURE 9 Periodic modulation of the gravitational redshift for 1 day on the eccentric Galileo satellites (source ZARM) observation time (h) clock residuals (ns) 0 2 4 6 8 10 12 14 16 18 20 22 24 400 200 0 –200 –400

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