Considerations for the Concept of Galileo's Timekeeping System

J.H. Hahn

Abstract:	Satellite-based navigation would be impossible without synchronization of the system-inherent clocks. Obviously one can find high-accurate precision standards (cesium or rubidium standards) onboard the satellites and in the overall ground segment (cesium standards + H-masers). The paper outlines some basic considerations for the conception of the timekeeping system of the Galileo satellite navigation system to be new developed. This European contribution towards a 2”d generation satellite navigation system (GNSS2) is required to be compatible and interoperable with GPS, but independent of it. These features have to be considered in the overall planning just from the beginning. The paper reflects some architecture aspects as the number of Galileo satellites,as well as clock types which are foreseen. One of two implementation strategies on this way has to be selected for the system time: a Master or Composite Clock, the latter having some certain advantages. Due to international recommendations the new Galileo-time has to be in close agreement with UTC what requires a steering procedure. The offset versus UTC has to be broadcast to the users by means of the navigation message. For the system provider this offset is requested too. The objective for Galileo is to implement an at least equivalent or better timekeeping system compared to GPS to allow for a comparison with the present worldwide accepted GPS-standard. Some ideas for collaborations are discussed. It is well understood that the system time development is a great challenge for the European designers. The paper aims to give some insight in the present planning of Galileo to the world-wide concerned timing community and invites inputs to be given to the European system design team.
Published in:	Proceedings of the 31th Annual Precise Time and Time Interval Systems and Applications Meeting December 7 - 9, 1999 Marriott's Laguna Cliffs Resort Dana Point, California
Pages:	91 - 100
Cite this article:	Hahn, J.H., "Considerations for the Concept of Galileo's Timekeeping System," Proceedings of the 31th Annual Precise Time and Time Interval Systems and Applications Meeting, Dana Point, California, December 1999, pp. 91-100.
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