Inside GNSS Media & Research

JUL-AUG 2018

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46 Inside GNSS J U L Y / A U G U S T 2 0 1 8 ECCENTRIC SATELLITES clock or on the radio link can be safely neglected. e activity focused then on the estimate of the effects acting directly on the frequency of the on-board clock, namely temperature and magnetic field variations, and on those systematic effects linked to orbit modelling errors. Concerning systematic errors associ- ated to the orbit modelling, a one-year dedicated Satellite Laser Ranging mea- surements was performed during 2016 and 2017 in close cooperation with the International Laser Ranging Service (ILRS). ese allowed disentangling in a good extent systematic errors coming from the orbit and impacting the clock determination from other systematics. A major support for this activity was also received from ESA's Navigation Office at ESOC in Darmstadt, Germany, whose experts generated the accurate clock and orbit products for Galileo satellites 5 and 6 using precise satellite models, which allowed a very accurate model- ling of the non-gravitational orbit per- turbations, notably from solar radiation pressure (SRP). For the other systematic errors, potentially affecting the on-board clocks, conservative upper limits were derived based on the knowledge on the satellite environmental conditions and their known effect on the frequency sen- sitivity of the PHM atomic clocks. Preliminary results presented by SYRTE and ZARM at the ESA' scientif- ic colloquium were very positive. Both consortia did manage to confirm, in an independent way, the gravitational red- shis with accuracies slightly better than the Gravity Probe A reference. During Q1 2018, some of the orbit systematic errors have been further reduced follow- ing refinements of the orbit modelling algorithms performed by ESA's Naviga- tion Support Office. e new estimates are currently under final consolida- tion by the two consortia and it may be anticipated that results will be several times better than the GP-A Reference. Final consolidated results are planned to be submitted in a scientific journal later this year. e success of the Galileo General Relativity tests has triggered a number of additional Fundamental Physics general tests which could be performed exploit- ing the eccentric Galileo satellites 5 and 6 which are currently under assessment by the ESA Galileo Navigation Science Office with the support of ESA's GNSS Science Advisory Committee. ESA also intends to make available the accurate clock and orbits processed for these two eccentric satellites to the public some- time this year for potential exploitation by the scientific community. Conclusion e recovery orbit of Galileo satellites 5 and 6 is compatible with the nominal Galileo constellation and some challeng- es due to the eccentric characteristics of the orbit have been overcome by the modifications in the ground segment. e quality of the navigation signals transmitted by these satellites have been tested and confirmed to be in line with expectations. However, all navigation signals remain flagged in "Test" mode pending the implementation of neces- sary measures to improve robustness of the ground segment for navigation message uplink and the deployment of on-board soware for automated SIS flag setting which are required to include the satellites as part of the Galileo constel- lation for navigation and SAR applica- tions. Although the SAR repeater on-board Galileo satellites 5 and 6 are already part of the SAR Initial Service contributing to the Cospas-Sarsat Program, the setting of SIS flags to "Healthy" would also have positive benefits to the SAR Service since it would allow the signals to be processed by the Galileo receivers installed in the SAR ground segment. is would make the Galileo SAR repeaters available to all MEOLUTs worldwide, without requir- ing those MEOLUTs to retrieve the orbital data from the server made avail- able by GSA for this purpose. The most accurate measurements ever of Einstein's predicted General Relativity gravitational shi were made possible thanks to Galileo, transform- ing an unfortunate difficult situation into an excellent scientific opportunity with further potential exploitation by the scientific community. e Galileo satellites 5 and 6 which were almost considered lost back in the summer of 2014 put under test the inge- nuity of the many engineers involved in the recovery of this mission, thus dem- onstrating the technical excellence and collaborations with experts from various institutions, space agencies and indus- trial partners. Updates on the extraordinary jour- ney of Galileo satellites 5 and 6 can be followed in future editions of Inside GNSS, featuring the final detailed results of Einstein's General eory of Relativi- ty, the GREAT project and their ultimate quest towards the provision of naviga- tion service. Acknowledgements e success of Galileo 5 and 6 mission recovery and exploitation would not have been possible without the excel- lent team effort and technical expertise across European institutions, space agencies and industrial partners. The authors would like to thank: • OHB satellite manufacturer and SSTL payload manufacturer • Airbus Defence and Space (ADS), the Ground Control Segment contractor • Thales Alenia Space, the Ground Mission Segment contractor and system engineering support • SpaceOpal, the Galileo Service Oper- ator • French (CNES), Italian (ASI), Ger- man (DLR) and British (UKSA) space agencies experts for the sup- port on the mission recovery plan • SYRTE, Observatoire de Paris, the GREAT consortium • ZARM, University of Bremen, the GREAT consortium • International Laser Ranging Service (ILRS) • e European Commission (EC) • e European GNSS Agency (GSA) Exploitation and Market Develop- ment Teams • e entire ESA Galileo Project Team, ESA Navigation Science Office and ESA Navigation Support Office

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