Timekeeping for GalileoSat

Jorg H. Hahn, Gianluca Graglia, Guillermo Salgado, Patrizia Tavella and Giovanni Busca

Abstract:	The GalileoSat programme comprises the definition, development and in-orbit validation of the Galileo space- and related ground segment. Galileo is the European contribution towards a 2nd generation satellite navigation system (GNSS2). It is required to be compatible and interoperable with GPS but independent on it. These tasks have to be considered during the present Definition Phase of this new system. The paper addresses some basic considerations for the concept of the GalileoSat timekeeping system. It reflects architecture aspects as the number of GalileoSat satellite and ground station clocks, and the clock types as well as their typical characteristics being specified. The overall time synchronization architecture throughout the Navigation System Control Center (NSCC), Orbitography and Synchronization Stations (OSS) and the satellite clocks is presented. The implementation strategy (master or composite clock) is discussed. Due to international recommendations the new Galileo System Time (GST) has to be in close agreement with the International Atomic Time (TAI) and Universal Coordinated Time (UTC) and this is obtained through a suitable steering procedure to be developed. The offset versus TAI/UTC has to be determined in a defined way (Common-View vs. two-way with GEO-transponder, involvement of one or some European timing labs) and with a certain level of specified uncertainty. This information shall be broadcast to the users by means of the navigation message. In the timing field the establishment of Galileo can be used to provide qualitative improvements. The goal is to implement a cost-effective, but competitive time-keeping system with respect to the present worldwide accepted GPS standard. The paper concludes with the present status of the Definition Phase for the GalileoSat timekeeping system concept.
Published in:	Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000) September 19 - 22, 2000 Salt Palace Convention Center Salt Lake City, UT
Pages:	914 - 922
Cite this article:	Hahn, Jorg H., Graglia, Gianluca, Salgado, Guillermo, Tavella, Patrizia, Busca, Giovanni, "Timekeeping for GalileoSat," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, UT, September 2000, pp. 914-922.
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