http://www.wired.com/dangerroom/2012/07/drone-hijacking/all/

On the evening of June 19, a group of researchers from the
University of Texas successfully hijacked a civilian drone at the White Sands
Missile Range in New Mexico during a test organized by the Department of
Homeland Security.

The drone, an Adaptive Flight Hornet Mini, was hovering at around 60 feet,
locked into a predetermined position guided by GPS. Then, with a device that
cost around $1,000 and the help of sophisticated software that took four years
to develop, the researchers sent a radio signal from a hilltop one kilometer
away. In security lingo, they carried out a spoofing attack.

“We fooled the UAV (Unmanned Aerial Vehicle) into thinking that it was rising
straight up,” says Todd Humphreys, assistant professor at the Radionavigation
Laboratory at the University of Texas.

Deceiving the drone’s GPS receiver, they changed its perceived coordinates. To
compensate, the small copter dove straight down, thinking it was returning to
its programmed position. If not for a safety pilot intervening before the drone
hit the ground, it would have crashed.