Holdover Mode - Spectracom VersaSync User Manual

3.3.4

Holdover Mode

When input references have been supplying input to VersaSync and input from all the ref-
erences has been lost, VersaSync 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 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the primary references are no longer valid or present.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 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 VersaSync 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 VersaSync 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
CHAPTER •
3 VersaSync User Manual Rev. 6.0
Letter symbol GNSS Constellation
G GPS
R GLONASS
E Galileo
J QZSS
C BeiDou
I IRNSS
The number next to the letter indicates the satellite number. The number below indicates
the signal strength (C/N ).
o
3.3 Managing References
167