Ron Natalie wrote:
The regulations just say that if you are going to use GPS that your alternate requirements
have to be met without using GPS. They're figuring satellite geometry failure (GPS system)
not the reliablity of the box installed.

Which is more likely though? I guess, by induction or extrapolation or
whatever, that the FAA figures ATC can handle one guy having a bad day
at a time (if his GPS/NAV box fails) and not half the airplanes all at
once (widespread GPS jamming or satellite system failure).