In reply to KevinD:
> I thought most of those exercises used jamming and buggered it up all round.
Think you're right actually. Presumably it's pretty localised as shipping as heavily (over) reilant on GPS.
> GPS did have, at least, two variants for many years. Civvie which had some inaccuracy built in and the military variant which didnt and was more tightly restricted. I am not sure how the two were managed separated technically though.
I think the military one is encrypted and uses multiple frequencies making it harder to jam.