Iran could have stationed its newly acquired Russian Electronic Warfare (EW) truck mounted system, known as Avtobaza, near the nuclear facility. The Russian export is designed to manipulate the guidance and communications system of U.S. weapons. Using that system, Iran might have jammed the command-control link between the U.S. drone and the commercial satellite the drone uses to link back to its pilot.
When the drone can’t talk to its pilot, after a while, it aborts its mission and goes home. To find its way home, the drone uses signals from the Global Positioning System (GPS) satellites. Unfortunately, the signal strength of the GPS satellites is relatively weak and a strong signal from something like the Russian EW systems can overpower it. This technique has been frequently demonstrated and allows something like the Russian trucks to “spoof” the GPS signal, pretending to be the satellite and providing false data to GPS receivers.
To avoid being spoofed, U.S. military systems listen to a different frequency than civilian GPS receivers, an encrypted channel from the satellite. How could Iran get around that? Iran could have gotten its hands on the encryption key used on U.S. drones, perhaps in the wreckage of the Reaper drones it has already shot down. Alternatively, Iran could have jammed the military GPS frequency, forcing the RQ-170 to shift to the civilian GPS channel.