And they are only about $1500. Chip-scale atomic clock approaches performance of modules

Which applications need a CSAC, with GPS-based clocks so available and prevalent? First, there are applications where the GPS-based timing may not be accurate enough. But there are also many applications where GPS is unavailable, such as underwater exploration, underground drilling, geophysical research, and EMI shielded rooms. There are also in-the-field military situations where GPS and all EM waves are deliberately jammed by patrols, to prevent remote triggering of improvised explosive devices (IEDs), yet there is a need for precise communication-equipment synchronization among combat teams.

Knowing, with some precision, what time it is also helps in acquiring a GPS signal, too. The advantage of a soldier not having to stand out in a field for a minute or so while a GPS receiver acquires a signal and gives coordinates is left as an exercise for the student.

I’ve seen some, up close and personal; they’re pretty cool. In fact, we had a prototype and I took a picture of it with my novelty dime (3″ diameter) to make it look really small.