Anywhere that GPS is used in a timing-critical application, they'll be using a timing receiver (like http://www.trimble.com/timing/thunderbolt-e.aspx) to handle their time reference; those use the GPS signal (when they have it) to discipline a (ovenized) quartz crystal, so that they have good performance during "holdover" (loss of GPS) especially for short-term outages. In its default configuration, the Thunderbolt-E linked above, for example, will be within +- 8us after being in holdover for 24 hours.