It would be interesting to know how the drone was captured.
Through 'Iranian physics'.
Military GPS signals are encrypted so not possible to spoof.
True. Most people here do not understand the difference between jamming and spoofing. Until now.
One possibility - the GPS may have been jammed and the drone may be programmed to attempt a landing when it runs out of fuel. If I recall correctly the drone was somewhat damaged when it tried to land.
The drone's topside bumps are antenna housings and because of their locations -- topside -- they are shielded from most (not all) of any attempt to jam.
If we are to be generous and say that Iran know the drone's general air location/direction, then the power to overwhelm the drone's topside antenna at 30-40k ft altitude would have to be something sufficiently powerful like an airborne jamming pod and constant lest any gap allow a reconnection between aircraft and remote operator. But let us be that generous.
So what happens next when the aircraft detect a loss of connection? It depends on the programming sophistication.
The most primitive would be to continue on current heading/altitude until fuel is exhausted. This is premised on the hope that distance will take the aircraft out of the jamming radius and will allow a reconnection. But it would also incur the risk that the aircraft would be so far out that no reconnection would be possible.
The next option would be to orbit at a constant altitude and hope the jamming signals would stop. Fuel exhaustion is still a possibility before a reconnection.
The next option would be to land. Whether the ground below is 'hostile' or 'friendly' is a matter of opinion. For the aircraft, it is simply a destination.
The next option would be the 'return-to-base' (RTB) capability. This require a much more sophisticated avionics package that would include an unassisted/uncorrelated guidance system, aka the inertial navigation system (INS), coupled to the flight controls system (FLCS). The system would redirect the aircraft to its ingress heading, meaning the direction from which it came from, and hope that INS drift is not too great during the RTB flight. INS drift is a serious problem over distance. That is why the INS is assisted/correlated by an external source such as the GPS or astronav (star sighting). The RTB capability is casually referred to as the 'homing pigeon' feature by the UAV community.
The RQ-170 does have the RTB capability. But assuming that the one shown by Iran is
THE ONE that we lost, then it is more reasonable to have an educated guess that the drone experienced a deeper software failure than from jamming. What we do not know is how long into the mission did the drone lost its remote connection. Long enough and it is possible that the drone exhausted its fuel in RTB because the human operator would not allow his charge to fly too long to have no fuel margin for the return home flight. It lost connection deep into the mission, decided to orbit for a while to give reconnection a chance, then decided to RTB and crashed because of no fuel.
But this explanation deny Iran the glory of deliberate capture and relegate possession to mere luck. Hence the spectacular assertions of 'hacking' to 'shoot down'.