The Iranian engineer told the Christian Science Monitor that "The GPS navigation is the weakest point. By putting noise [jamming] on the communications, you force the bird into autopilot. This is where the bird loses its brain."
In other words, the drone thought it was in one place (its home base in Afghanistan) when it was actually in another (Iran). The hacking methodology used on the drone, the engineer said, was developed with the use of earlier downed drone aircraft.
At the same time, a separate source said that he believed the images of the drone that Iran says it captured showed a "fake" drone. A former Pentagon official, who requested anonymity as he is not authorized to speak about the drone, told the USA Today that video footage of the drone showed an aircraft that is not only the wrong color, but one that has also has welds along the wing joints that do not conform to the stealth design of the RQ-170 Sentinel.
He also questioned why the landing gear was obscured when displayed by the Iranian military in the video. However, the Iranian engineer speaking to the CSM explained that:
Could Iran have done what it says it did? Western military experts and previously published studies indicate that GPS spoofing as described by the Iranian engineer is indeed possible. Former US Navy electronic warfare specialist Robert Densmore said, "Even modern combat-grade GPS [is] very susceptible” to manipulation. "I wouldn't say it's easy, but the technology is there."