Safran VersaSync User Manual page 235

What is the recommended setting for the Holdover Timeout period?
The factory default Holdover period is 2 hours (7200 seconds). The value can be
increased to up to 5 years. During this time period, VersaSync will be useable by
its NTP clients (or other consumers) after GNSS reception has been lost.
The length of time is really based on the type of oscillator installed in a unit, and
what the typical accuracy requirements are for the NTP clients. The longer it can
run in Holdover mode before it expires, the longer it can continue being a central
time source for all of its clients. But the longer VersaSync runs in Holdover, the lar-
ger the offset to true UTC time will become, because the undisciplined oscillator
will drift over time:
The better the type of oscillator installed, the more stable it is while in Holdover
and therefore, the less its time will drift away from true UTC time. This results in
more accurate timing, over extended durations upon the loss of GPS input. For
instance, a Rubidium oscillator will maintain significantly better time over a
longer Holdover duration than a TCXO oscillator (TCXOs are considerably less
stable than a Rb oscillator).
Oscillator Phase Drift
The chart below provides typical stability performance for the oscillator types
that can be found in VersaSync units. These numbers are based on the oscillator
being locked to a reference for two weeks, but then loses GPS reception for an
extended period of time, while the ambient temperature remains stable.
This data can help you determine how long of a Holdover period can be tol-
erated, based on how much time drift may occur after GPS input is lost. The lar-
ger the time error that can be tolerated by VersaSync clients, based on the
oscillator installed, the larger the Holdover timeout period can be set to.
Table 3-3:
- 4 hours
- 24 hours
CHAPTER •
3 VersaSync User Manual Rev. 12
Note: Changes made to the Holdover Timeout always take effect
immediately. If VersaSync is in Holdover and the Holdover timeout is
changed to a value that is less than the current time period that Ver-
saSync has been in Holdover Mode, the unit will immediately declare
loss of synchronization.
Estimated Phase Drifts
1PPS Phase Drift in Holdover
(no reference available)
OCXO
OCXO
(high performance)
3 µs 2.8 µs
40 µs 30 µs
3.3 Managing References
CSAC
1µs
7 µs
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