M e r i d i a n I I U s e r M a n u a l
149
"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 Meridian II reports this value as accurately as possible, even during periods of
GPS signal outage where the Meridian II is unable to directly measure the relationship of its timing
outputs to UTC. During these GPS outage periods, assuming that the Meridian II had been synchro-
nized prior to the outage, the Meridian II extrapolates the expected drift of the Meridian II 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 os-
cillator 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 outputs.
If the signal loss condition persists longer, then the final, unsynchronized state will eventually be
reached. If the Meridian II 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.
Once the Meridian II reaches the unsynchronized TFOM state, then the Network Time Protocol
(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 recognize this and cease to use the unsynchronized server.
Summary of Contents for Meridian II
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