[michaelt]

Although clock holdover might seem like a pointless feature - GPS outages are very unusual* - what holdover is mostly for is giving you time to respond if someone clumsily knocks your antenna over, or crushes your coax cable, or it gets damage that makes it fill with water, or something like that.

A lot of the demand for high-precision clocks is for cell phone base stations, where there's no guarantee there'll be someone on hand to make repairs promptly.

* They happen occasionally, of course.

[nullc]

In downtown mountain view I saw brief GPS outages a couple times a week, presumably due to people driving around with GPS jammers (e.g. to knock out GPS tracking of their own cars).

[fanf2]

Various militaries do regular GPS jamming exercises

https://www.ofcom.org.uk/spectrum/information/gps-jamming-ex...

https://www.navcen.uscg.gov/?pageName=gpsServiceInterruption...

[sophacles]

GPS spoofing is also a thing. Here's a paper where they are able to spoof time while letting the receiver think it hasn't moved (much, within error bars): https://ieeexplore.ieee.org/document/6451170

That paper is in context of power grid equipment, but the GPS attack generalizes.