Redundant Atomic Frequency Standard Time Keeping System With Seamless AFS Switchover

Victor S. Reinhardt, Charles B. Sheckells

Abstract:	This paper describes a redundant time keeping system (TKS) that provides an adjustable 10.23 MHz frequency output phase-locked to one of four 13.4 or 10 MHz atomic frequency standards (AFS) (5 MHz AFS with x2). Phase and frequency adjustment (I ps, 0.02 uHz resolution) are accomplished without disturbing the input AFS. The TKS has a virtually perfect frequency step characteristic when a frequency change is executed. Data are presented showing that the deviation from a perfect frequency step is a transient of peak value less than 1.50 ps which decays in a fraction of a second. The TKS also has the ability to monitor and output the phase of a hot spare AFS relative to the primary AFS with a 1.3 ps resolution. By utilizing this phase information, the TKS can perform the seamless switchover of a digitally phase-locked VCXO from the primary AFS to the hot spare in case of primary AFS failure or problems. Seamless switchover is defined as the maintenance of both output phase and frequency continuity while and after the TKS switches the VCXO phase tracking from the primary AFS to the hot spare. Experimental data show that seamless switchover is accomplished with only a 50 ps phase transient decaying in a fraction of a second and with no observable change in frequency. Seamless switchover has applications both in space and ground time distribution systems where continuous availability and Predictability after an AFS failure is desirable. For a GPS satellite, seamless switchover can eliminate a l-2 week down time that occurs when an AFS failure is diagnosed and the satellite is prepared with a new AFS. The paper also describes a Kalman filter simulation that models the behavior of a GPS satellite phase prediction parameter during seamless and non-seamless switchover. For the non-seamless case, the simulation shows that the Kalman filter takes at least 6 hours for the filter to settle (with the orbital parameters fixed). For the seamless switchover, no detectable performance change is observed during or after switchover.
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:	101 - 110
Cite this article:	Reinhardt, Victor S., Sheckells, Charles B., "Redundant Atomic Frequency Standard Time Keeping System With Seamless AFS Switchover," Proceedings of the 31th Annual Precise Time and Time Interval Systems and Applications Meeting, Dana Point, California, December 1999, pp. 101-110.
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