R T M 3 2 0 5 U s e r M a n u a l
107
"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 RTM3205 reports this value as accurately as possible, even during periods of GPS
signal outage where the RTM3205 is unable to directly measure the relationship of its timing outputs
to UTC. During these GPS outage periods, assuming that the RTM3205 had been synchronized prior
to the outage, the RTM3205 extrapolates the expected drift of the RTM3205 timing signals based
on its knowledge of the characteristics of the system oscillator - Oven Controlled Crystal Oscillator
(OCXO). 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 RTM3205 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 RTM3205 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 RTM3205
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