Symmetricom 1100, 1000 manual Normal Tracking, GPS Holdover

Chapter 1 Overview of the TimeProvider

BesTime

Two weighting algorithms - tracking and holdover - determine which of the inputs are used as the ensembled reference. The tracking algorithm is used during normal operation. If the GPS becomes disqualified, BesTime uses the holdover weighting algorithm to determine what the GPS input would have been had it not been lost. You can use the RTRV-BESTIME-STATS command to view the current weighting for each input.

The BesTime control loops are designed so that the short-term (under 10 minutes) stability is controlled by the LO, medium-term (10 minutes to several hours) stability is controlled by the ensembled reference, and long-term stability and accuracy is determined by the GPS control input.

The control loops can use either measurement data or prediction data. The inputs are measured for phase, frequency, and drift transients. When the input to either loop is determined to be valid, then the measured data is used to generate the correction signal to the local oscillator. If BesTime detects a transient on the input, it switches to the prediction data before the transient can start to affect the correction signal.

Normal Tracking

During normal GPS tracking, the GPS input provides the source of long-term stability for all inputs to the ensembling algorithm. The ensembling improves the medium-term stability of the output signals.

GPS Holdover

If GPS signals are lost, then all inputs to the ensembling algorithm have lost their source of long-term traceability. BesTime substitutes the inputs with calibration predictions with respect to GPS. Each input is weighted according to the estimated prediction error over 24 hours.

44 TimeProvider User’s Guide 097-58001-02 Revision G – April 2008