EndRun Tempus LX GPS, User Manual

Page: 89 / 116

T e m p u s L X G P S U s e r M a n u a l 77
"Smarter Timing Solutions"
Appendix
E
Time Figure-of-Merit (TFOM)
This appendix describes the Time Figure of Merit (TFOM) number. The Tempus LX displays this
number on the front panel via the Receiver Status display (see Chapter 5). The TFOM is also printed
out in the time-of-day fields printed by the Tempus LX
gpsstat
and
gntpstat
commands (see
Chapter 6). The TFOM number indicates the level of accuracy that should be included in the inter-
pretation of the time-of-day and ranges from 4 to 9:
4 time error is < 1 us
5 time error is < 10 us
6 time error is < 100 us
7 time error is < 1 ms
8 time error is < 10 ms
9 time error is > 10 ms, unsynchronized state if never locked to GPS
In all cases, the Tempus LX reports this value as accurately as possible, even during periods of
GPS signal outage where the Tempus LX is unable to directly measure the relationship of its tim-
ing outputs to UTC. During these GPS outage periods, assuming that the Tempus LX had been
synchronized prior to the outage, the Tempus LX extrapolates the expected drift of the Tempus LX
timing signals based on its knowledge of the characteristics of the internal 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 outputs.
If the signal loss condition persists longer, then the final, unsynchronized state will eventually be
reached. If the Tempus LX is unable to achieve re-synchronization within one hour after reaching
this state, the red LED will illuminate. The fault status field returned in either of the
gpsstat
or
gntpstat
commands will have the appropriate bit set to indicate a loss-of-signal time-out condition.
If the GPS subsystem reaches the unsynchronized TFOM state, the NTP daemon will cease to use the
timing information returned by the GPS subsystem in its polling event timestamps. At this point, the
NTP daemon will report in its replies to network NTP clients that it is running at stratum 16 and the
leap indicator bits will be set to the fault state. NTP clients will recognize that and cease to use the
unsynchronized server.