If they were able to spoof the GPS signal, backup navigation systems wouldn't be activated.
Yes it would if you design it correctly. If you have 3 independent ways of navigation they take a a vote of where to go: 2 can always overrule 1. Thus if they just Jammed the GPS other nav system would have taken it to base. Having significant errors between at least 2 navigation systems, should initiate a self destruct of the drone. Thus Iranians would have to perfectly spoof ground beacons and GPS to get it — very unlikely. Also, its important to run the monitoring stations (that NSA responsibility) next to Iran to detect when they turn on their spoofing/jamming transmitters and deal with them — not that Obama would.