Head tracking relative to a moving vehicle or simulator platform using differential inertial sensors

Inertial trackers have been successfully applied to a wide range of HMD applications including virtual environment training, VR gaining and even fixed-base vehicle simulation, in which they have gained widespread acceptance due to their superior resolution and low latency. Until now, it has been impossible to use inertial trackers in applications which require tracking motion relative to a moving platform, such as motion-base simulators, virtual environment trainers deployed on board ships, and live vehicular applications including helmet-mounted cueing systems and enhanced vision or situational awareness displays. This paper describes a new technique which makes it possible to use inertial head- tracking systems on-board moving platforms by computing the motion of a `tracking' Inertial Measurement Unit (IMU) mounted on the HMD relative to a `reference' IMU rigidly attached to the moving platform. Detailed kinematic equations are derived, and simulation results are provided for the particular case of an inertial tracker with drift correction by means of ultrasonic ranging sensors, but the conclusions can be applied to hybrid inertial trackers involving optical, magnetic, or RF drift correction as well.