If your GPS signal is lost, then NTP Server will continue to serve Stratum1 time for a while. This is called the holdover period and is dependent upon the quality of the oscillator and the quality of the software control algorithms.
The basic NTP Server is provided with a TCXO. If the signal is lost it will drift away from perfect time at the rate of 10 milliseconds during the first day. For better holdover performance you may want to consider an oscillator upgrade. Oscillator drift rates and Stratum 1 holdover periods are listed below:
Oscillator | Drift Rate If No GPS Signal |
TCXO | 10 milliseconds for 1st day |
Premium OCXO | 80 microseconds for 1st day |
Rubidium | 5 microseconds for 1st day |