As I understand it, they actually run off of a gyro system that is
better then GPS. Also, I have a good friend that captains a navy sub
that told me celestial navigation was required.

In your reading, pick up a red/orange colored paperback called
“Emergency Navigation�. Its available all over and you should be
able to find it at http://www.amazon.com for cheap. You won’t find
much of the good stuff in most books.

As for something else making GPS not work, there are a few things I
have come across and here they are.

On 9/11 GPS was scrabbled in many areas. For anyone in a critical
position in one of the scrabbled areas, OOPS!

In Mexico (and I love this one) I ran across and “electrical
engineer� that had a broken GPS and his spare didn’t work either.
This was a while back but … Turned out to be a bad antenna that I
fixed for him – the engineer☺

That’s the only way gps failures other then lighting that I know of.

As for a spare GPS, the rumor is that if you wrap a handheld in tinfoil
and get hit by lightning, your spare will still work. I don’t know
anyone that has put it to the test but I keep my spare wrapped just the
same.

GPS signals use to be scrabbled all the time. In about 1991 or so the
military started allowing public access of the gps signal. In times of
heightened defense alerts, the gps signal does get less accurate but
then there is also a differential signal that I don’t know much about
but is used to help the gps become more accurate again. All in all,
I’m more worried about lightning, having been through enough of it,
then anything else.