An operating condition of a clock which is no longer locked to an input reference. In holdover, the clock maintains the same frequency and phase from when it was last locked to a reference.  

Holdover begins when the clock is disconnected from the input reference. On Sentinel, that means when the GPS or the external 1PPS input has been removed. 

Holdover terminates when the clock locks to a new reference input. On Sentinel, that means when the GPS or the external 1PPS input has been restored, and the Sentinel has re-locked to the input.

A mode of Sentinel in which the internal battery is used to keep the Rubidium oscillator powered and in holdover mode when the main power supply is removed. In this mode, the rest of the instrument is powered down and the battery only provides power to the Rubidium. Transport mode is only required if a time / phase measurement is to be performed at a remote site where GPS is not available.

On Sentinel, transport mode is only available if the battery option is installed. Transport mode can be enabled when switching off the instrument. 

After being disciplined, the Sentinel Rubidium will typically maintain accuracy to within 100ns for 12 hours after entering into holdover state. 

Note that the actual duration depends on the status of the Sentinel Rubidium before it is put in holdover mode.  Additionally, environmental conditions such as temperature changes and magnetic field change may also impact the performance obtained.