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Holdover

VelaSync is equipped with a high-quality oscillator which is capable of providing good time, even while not actively receiving time updates via GPS.

TimeKeeper utilizes this feature and hence will remain in holdover for 2 hours (7200 seconds) by default, i.e. continues to use the GPS timing source, even when it reports no GPS signal.

Once the 2 hours expire without the GPS time becoming available again, TimeKeeper will begin comparing all available time sources to determine if the holdover time is out of range with other time sources by using the Sourcecheck algorithm (even if Sourcecheck is disabled). For more information, see Validating a Timing Source.

If Sourcecheck determines that a different time source previously configured by you (e.g., a PTP server) provides a better quality time than the oscillator, TimeKeeper will automatically switch over to that time source until the GPS source becomes available again. This concept is referred to as Failover; for more information, see Failover.)

The 2-hour holdover time can be changed:

  1. Navigate to Configuration > TimeKeeper configuration: Sources, and open the TimeKeeper GPS pull-down menu.
  2. Enter a number in seconds into the Holdover Limit field.
  3. Configuring the Holdover Limit

    Note: If left blank, the Holdover Limit defaults to 7200 seconds.

About Disciplining

The oscillator under normal operating conditions provides TimeKeeper with the time signal and the 1PPS signal to be distributed over the network. The GPS source only steers the oscillator, in order to counteract the unavoidable (albeit miniscule) drift of OCXO or even Rubidium oscillators. This process is called Disciplining.

In its standard configuration, TimeKeeper uses the time and 1PPS signal provided by the oscillator, steered by the GPS time, since this is by far the most accurate time source commercially available. If the GPS signal is temporarily lost, however, the oscillator will freerun for the duration of the holdover time, before TimeKeeper selects the next-best timing source e.g., a PTP- or NTP-server until the GPS source becomes available again to discipline the oscillator which will then again take over the role as primary timing source.

This standard configuration can be changed: see Configuring Timing Sources to learn more.