Helmet tracker

Abstract

A method and apparatus are provided for determining the orientation of an object relative to a moving or moveable platform. The object may for example be a helmet worn by a pilot of an aircraft in which orientation of the helmet relative the aircraft while in flight may usefully be known, in particular when determining the position of space-stabilised symbols being displayed in an associated helmet-mounted digital display system. The method and apparatus rely upon a combination or orientation rate data output by an inertial sensor arrangement associated with the object and orientation data output by a non-inertial tracker system associated with the object. Orientation data of the moveable platform is also used to reference the non-inertial tracker output to inertial space. A correction to the rate data output by the inertial sensor arrangement is determined using rate data derived from data output by the non-inertial tracker referenced to inertial space using orientation data of the moveable platform in inertial space. The corrected rate data are used to determine a predicted orientation for the object at some future time to take account of latency in the tracker process and in any user process relying upon the output of the method, for example a digital helmet-mounted display system displaying space-stabilised symbols.