Realtime Kinematic GPS Receiver

http://gizmodo.com/5405804/make-your-own-real-time-kinematic-gps-receiver-thats-accurate-to-one-centimeter

Researchers Tomoji Takasu and Akio Yasuda of Tokyo university have made a GPS receiver that is accurate to a centimeter. It displays real time information. They used beagle board as a processor.

On the link posted below, there are also instructions on how to make one yourself
http://gpspp.sakura.ne.jp/rtklib/rtklib_beagleboard.htm