The pilots knows the same as the passengers: That if the hijackers take the cockpit, odds are nobody lives, so in a crisis, his best strategy for both personal survival and for saving the most passengers is to get the plane on the ground as soon as possible.
If a hijacker "just" wants to perform an old-school hijack, ie. not use the plane as a 9/11-style missile, his best odds are convincing the captain of that fact without taking the cockpit. The hijacker can verify that the captain is complying using a smartphone with GPS.
Is that an actual limitation of the GPS network, or a limitation of the client-side software.
My understanding is that GPS satellites broadcast a timestamp, and the receivers use that information to compute their position. Assuming that this is accurate, devices should still be able to get the signal at higher altitudes. The only way I can think of this not working is if the geometry works out so that at 10K feet you are no longer in ranges of 4 satellites. I suppose this could be done my calibrating how wide to make the signal (which would make the signals stronger on the ground, but require more satellites for full coverage). This would likely also require having been thought of back when the GPS network was planned.
Also, I would imagine that planes use GPS, in which case it would have to be a client-side restriction.
Client side (may be in firmware or software): GPS above certain elevations or speeds requires a special export license that your smartphone probably doesn't have. An unlicensed device is required to output no fix (but it has to be able to calculate that the parameters are outside the allowed range).
How difficult would it be for someone to work around these restrictions? If I have an unlocked Android phone, would I be able comment out the range check and recompile the gps software?
If a hijacker "just" wants to perform an old-school hijack, ie. not use the plane as a 9/11-style missile, his best odds are convincing the captain of that fact without taking the cockpit. The hijacker can verify that the captain is complying using a smartphone with GPS.