Holdover Mode - Spectracom VelaSync 1232 User Manual

3.3 Managing References
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-
February)
3.3.4

Holdover Mode

When input references have been supplying input to VelaSync and input from all the ref-
erences has been lost, VelaSync will not immediately declare loss of time syn-
chronization, 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 Sys-
tem Time, but the oscillator is not disciplined/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after all the primary references are no longer valid or present.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 oscil-
lator 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 syn-
chronization and the availability of the time outputs is not immediately lost when input
references are no longer available.
While VelaSync 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 Holdover mode, NTP inside VelaSync 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 VelaSync until GPS or another
reference has been restored.
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