It is believed that the US drone taken down in Iran last year was hacked through spoofing technique. A few days ago, American researchers from University of Texas also used the same technique to hack into a flying drone.
The explanation behind this technique is that a flying drone mistakes hacker's signal to that it's suppose to recieve from the satellite hence it makes an adjustment to the location in the hacker's signal. The hackers can then take control of the drone and either crash it some where or redirect to another location.
(http://www.wired.com/images_blogs/dangerroom/2011/02/reaper.jpg)
image source: wired.com
Drones are mainly used by military to spy or sometimes direct missiles to certain locations. A flying drone is directed remotely from the control station and recieves the signals from the satellite. It also sends back signals with it's GPS cordinates to the control station continuously. This allow the control crew to monitor it's movement. For spying drones, pictures and other information gathered are sent to the soldiers on the ground.
Spoofing technique has been proven to be effective and reports states that it is even worse when hackers are able to determine how to un_encrypt the drones. They can load dangerous weapon and direct it to the wrong people.
bbc