When a GPS disciplined time server or server appliance encounters a condition where the GPS signal becomes unavailable or unreliable a local oscillator is used to keep time. The local oscillator is generally a TCXO, OCXO, or Rubidium standard. Such systems are able to deliver fairly accurate and precise time in the event of a short outage like this. A quality server with a local Rubidium oscillator should be able to keep accurate time within 10 μsec in the event of a five day outage.

GPS signals can become unavailable for numbers reasons that are far more mundane than being jammed such as someone disconnecting the antenna, lightning strike, a rat chewing through the cable, meth heads stealing the copper cable and/or antenna, etc.