PCI GPS

What if PCI GPS Loses GPS Signal with PCI Unit
If the PCI GPS unit is started in GPS mode with the satellites in view and then the unit loses synchronization, the unit will continue to operate (and count time) in a flywheel state, based on where synchronization is lost. Once the satellites return to view, the PCI GPS unit will resynchronize to GPS. The PCI GPS cards are also capable of synchronization to an external time code or 1PPS (provided the card is in the correct decoding mode). In this case, as well as in the GPS mode, if the input signal is lost, the PCI GPS unit will continue to count time based on the card's internal oscillator until the input signal is returned. Once in time code or 1PPS decoding mode, the unit will not synchronize to GPS unless the mode is changed.

If you are running a PCI GPS card and lose GPS lock, the PCI board automatically "fly-wheels" causing it to maintain time based off the on-board 10MHz oscillator. This applies to Time Code Decoding Mode as well as GPS Mode.

Symmetricom's PCI GPS Cards
Symmetricom's PCI GPS card is in the bc635/637PCI receiver module, which provides precision time and frequency reference to the host computer and peripheral data acquisition systems. Time is acquired from either the GPS satellites using a supplied antenna/receiver (bc637PCI only) or from time code signals, typically IRIG B.