This was an interesting incident considering the drone was also a high altitude 'Stealth' drone. So detection of the 'Stealth Drone' itself was even more sophisticated than the GPS 'Spoofing'. GPS 'Spoofing' might be more difficult than Differential GPS but probably no more sophisticated than implementing Real Time Kinematic RTK GPS based on phase measurements to get centimeter accuracy

As most Stealth aircraft will use AESA spread spectrum RADAR emitters i.e. for ground mapping imagery, I suspect that these radio signals were detected using an array of ground RADAR Warning Receivers RWR based on Software defined radio which digitises the microwave spectrum, which can give a directional information and when triangulated will pinpoint and track the AESA RADAR emitter. Its amazing what you can do with a Sony Playstation 3.

The same thing can be achieved using Networked PC Digital TV cards with firmware upgrades costing a few thousand dollars sometimes referred to as passive RADAR.

An aircraft intercept of the stealth drone with the GPS spoofing kit would then have gotten the stealth drone to follow it (follow me mode) back down to its air field.

GPS spoofing is actually an important defense area. i.e. A JDAM bomb could be redirected to behind the lines of the military who dropped it or at the very least missing its intended GPS coordinates.

The cost of this type of electronic equipment is dropping like a stone, take for example the Chinese made Tecsun PL360 radio receiver (around $45) based on the si4734 DSP chip

http://www.silabs.com/products/audiovideo/amfmreceivers/Pages/Si473435.aspx

Yes a complete digital signal Intermediate Frequency baseband processor for SW,MW,LW and Stereo FM Digital Radio Reciever!