The disciplined mode is the default mode. It eliminates long-term frequency drift, also called aging. As long as there is a valid satellite signal, the local rubidium oscillator is continu-ously adjusted to minimize the deviation from the satellite-derived reference signal.

The hold-over mode is entered either automati-cally, if the satellite contact is lost, or manually. The automatic adjustment is replaced by the normal aging characteristics of the rubidium oscillator.

The manual hold-over mode is mainly intended for the hopefully rare occasions when the satel-lite contact is sporadic, causing unacceptable mode switching interference.