US Patent 7301648 Self-referenced tracking

A new tracking technique is essentially “sourceless” in that it can be used anywhere with no set-up, yet it enables a much wider range of virtual environment-style navigation and interaction techniques than does a simple head-orientation tracker. A sourceless head orientation tracker is combined with a head-worn tracking device that tracks a hand-mounted 3D beacon relative to the head. The system encourages use of intuitive interaction techniques which exploit proprioception.