PN - Thanks. I believe you have corroborated what I was trying to say, without saying it.
HMS Cornwalls' GPS has had plenty of press, in any case!
I honestly know bugger all about GPS in practice, but I do know a thing or two about mil radio systems in general.
Q. Why would an exoatmospheric nuclear detonation cause global system outage? Surely only those birds within range would be toast, those on the dark side of the "extra sunshine" could survive, and (less any EMP effects) continue? As I said, I know bugger all about GPS, are there not enough birds?