Bluetrackr is a head tracking device. It allows you to track your head orientation & position in realtime. It's mainly used in games such as Elite Dangerous, Star Citizen, Flight simulators, etc to simulate head movement in a cockpit.
prototype-v1: There was an unknown problem with the SPI bus. Might be because there are two devices on the same SPI bus? prototype-v2: Ordered, waiting for parts.
Everything below is for v2.