Clock Requirements and Based Navigation Systems Trade-Off for Satellite

E. Detoma, A. Godone, F. Levi, P. Tavella

Abstract:	Satellite-based navigation systems, such as GPS, Glonass, and the planned Galileo European system rely on stable atomic clocks for their operation. Space-borne and ground-based clocks provide the time frame allowing the users to perform passive ranging measurements and quickly locate their position [13]. From the operational point of view, timing stability enters the overall accuracy budget together with orbit estimation and operational constraints. User ranging accuracy is a complex function of many factors, and optimum selection of the clocks requirements is of paramount importance to optimize the overall system. Ideally, user range accuracy is affected by both timing and orbit estimation accuracy. In turn, both timing and orbit estimation are affected by the oscillator intrinsic accuracy and stability. Stability plays a role first in determining the optimum time interval over which the estimation is carried on and, secondly, in determining the maximum length of the time interval over which the computed estimates can be propagated and still be within the navigation system specifications. Operational constraints, such as the maximum uploading rate for civilian systems, and the maximum unattended system operation vs. degraded performance for military systems, interact with purely technical considerations to define the “best” clock for a given application. To all this, we should add the requirements placed on the ground clocks which play a significant role in determining the overall system performance especially in systems (such as GPS or the planned Galileo) relying on the one-way passive ranging technique for orbit and clocks maintenance. In the paper, we will discuss the requirements forced on the clocks by the specific application, as well as the consequent tradeoff dictated by the stability of the orbit and operational constraints. The clock constraints dictated by the simultaneous requirement to produce and disseminate timing information related to UTC (or TAI) will not be discussed, as we will limit our discussion to the aspects related to the navigation services only. In this frame, we would like also to present the rationale for the engineering development of a novel frequency standard (a Rb passive maser) being developed by the IEN and Alenia Spa& under an ASI (Agenzia Spaziale Italiana) contract specifically for possible applications in future navigation systems. The opinions expressed in the papers are of the authors and do not imply necessarily the endorsement of the respective organizations.
Published in:	Proceedings of the 33th Annual Precise Time and Time Interval Systems and Applications Meeting November 27 - 27, 2001 Hyatt Regency Long Beach Long Beach, California
Pages:	385 - 398
Cite this article:	Detoma, E., Godone, A., Levi, F., Tavella, P., "Clock Requirements and Based Navigation Systems Trade-Off for Satellite," Proceedings of the 33th Annual Precise Time and Time Interval Systems and Applications Meeting, Long Beach, California, November 2001, pp. 385-398.
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