Clock Requirements and Trade-Off for Satellite-Based Navigation Systems

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.