I certainly wouldn't trust GPS alone, but then I'm paranoid enough to require belt, braces and duct tape if keeping the trousers up is a safety issue. An aircraft is probably relatively immune to ground-based jamming if its antenna pattern only looks up and not down. As for radio hams jamming the signals, I'd say that was unlikely - you're looking more for a technical equivalent to the idiot who smashes up railway signalling gear or finds it funny to play with laser pointers under a flight path.