EndRun Sonoma N12, User Manual

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S o n o m a U s e r M a n u a l 105
"Smarter Timing Solutions"
Appendix
A
Time Figure of Merit (TFOM)
This appendix describes the Time Figure of Merit number. The TFOM number indicates the level of
accuracy that should be included in the interpretation of the time-of-day and ranges from 3 to 9:
3 time error is < 100 nanoseconds
4 time error is < 1 microseconds
5 time error is < 10 microseconds
6 time error is < 100 microseconds
7 time error is < 1 milliseconds
8 time error is < 10 milliseconds
9 time error is > 10 ms, unsynchronized state if never locked to GPS
In all cases, the Sonoma reports this value as accurately as possible, even during periods of GPS
signal outage where the Sonoma is unable to directly measure the relationship of its timing outputs
to UTC. During these GPS outage periods, assuming that the Sonoma had been synchronized prior
to the outage, the Sonoma extrapolates the expected drift of the Sonoma timing signals based on
its knowledge of the characteristics of the system oscillator - either the Temperature Compensated
Crystal Oscillator (TCXO), Oven Controlled Crystal Oscillator (OCXO) or Rubidium oscillator.
The extrapolated TFOM is based on a conservative estimate of the performance of the oscillator and
should be considered ‘worst case’ for a typical benign ambient temperature environment.
Due to this extrapolation behavior, after initial synchronization, brief periods without GPS satellite
visibility will not induce an immediate alarm condition. (Removal of the antenna to simulate this will
induce an immediate alarm, however.) If the condition persists for long enough periods, you should
see the TFOM character change to indicate a gradually deteriorating accuracy of the timing out-
puts. If the signal loss condition persists longer, then the final, unsynchronized state will eventually
be reached. If the Sonoma is unable to re-synchronize within one hour after reaching this state, the
Alarm LED will light and the
faultstat
command will show a No Signal Time-Out fault.
If the GPS Subsystem reaches the unsynchronized TFOM state, the NTP daemon will report that it is
running at stratum 16 and the leap indicator bits will be set to the fault state. NTP clients will recog-
nize this and cease to use the unsynchronized server.