There are vast acreage above 5,000 feet in Ca. If it is above sea level, it will effect many ground systems.

Not True. originally, the timing signals were given millisecond delay errors when the military wanted to disable the system. In the late 80's I corrected the incoming signal with one form a local rubidium Oscillator if they disagreed more than 10 picoseconds.

If I told you how the Navy gets its ships there, I would have to kill you!