Spectracom NetClock 9489 - Holdover Mode

For daily ephemeris data:
http://[IP address of your unit]/files/pub/gps/data/daily/[YYYY]/[ZZZ]/15n/spec[ZZZ]0.15n.Z
For almanac data:
http://[IP address of your unit]/files/pub/gps/data/almanac/[YYYY]/[ZZZ]/[ZZZ].alm
Where: YYYY: Year (Example: "2017"), and ZZZ: Day of year (Example: "050" for 19-Febru
ary)
3.4 Holdover Mode
When input references have been supplying input to NetClock and input from all the references
has been lost, NetClock will not immediately declare loss of time synchronization, but first will
go into Holdover mode. While the unit is in Holdover mode, the time outputs are derived from
the internal 10MHz oscillator incrementing the System Time, but the oscillator is not dis
ciplined/steered by the external reference e.g., GNSS.
Because of the stability of the internal oscillator, accurate time can still be derived even after all
the primary references are no longer valid or present. The more stable the oscillator is without
an external reference, the longer this holdover period can be and have it still maintain very
accurate outputs. The benefit of Holdover is that time synchronization and the availability of the
time outputs is not immediately lost when input references are no longer available.
While NetClock is in Holdover, the only difference is the Holdover and associated Minor
alarm are asserted. There are no changes to NTP or any of the other outputs, i.e. while in Hol
dover mode, NTP inside NetClock continues to be at the same Stratum level it was at before
going into Holdover mode (such as Stratum1 when synced to GPS). Should the Holdover
period expire, however, or the unit is rebooted, the NTP Stratum will go to 16, preventing any
clients from being able to sync with NetClock until GPS or another reference has been restored.
How long will the unit remain in Holdover mode?
NetClock will remain in Holdover mode until either:
a.
Any enabled and valid input reference becomes available again: If one or more ref
erences return and are declared valid before the Holdover period has expired (even
momentarily, i.e. for at least one second), NetClock exits the Holdover mode and returns
to its fully synchronized state.
b.
The Holdover Timeout period expires. In this case, NetClock will declare loss of syn
chronization.
Note that Holdover mode does not persist through reboots or power cycles. If a reboot or
power cycle occurs while NetClock is in Holdover mode, it will power-up and remain in a “not
synchronized” state until at least one valid Time and 1PPS input reference becomes available
again. While in this state, NTP will be Stratum15 and outputs will not be usable. If the input ref
erences are restored and then lost or declared not valid again, NetClock will then go back into
Holdover mode.
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CHAPTER 3 • NetClock User Reference Guide Rev. 16
3.4 Holdover Mode