Heading reference and land navigation system

Claim 1

A self-contained strapped-down heading reference system for use in a land-based vehicle to provide a visual indication to vehicle operators of directional vehicle heading relative to a fixed reference azimuth axis, the heading system comprising: a housing for mounting the heading system in the vehicle, wherein the housing is oriented so as to define a roll axis corresponding to the direction of vehicle travel, a pitch axis perpendicular to the roll axis and forming a horizontal plane therewith when the vehicle is on level terrain, and a yaw axis mutually perpendicular to the pitch and roll axes and parallel to the azimuth axis when the vehicle is on level terrain; gyroscopic means mounted in the housing for sensing the rate of vehicle rotation about the yaw axis and the pitch axis, and for generating rate signals indicative of the sensed vehicle rotation; accelerometer means for measuring the magnitude of vehicle acceleration only along the roll and pitch axes and for generating acceleration signals indicative of the misalignment of the vehicle relative to a horizontal plane formed by the pitch and roll axes; circuit means coupled to the gyroscopic means and responsive to the rate signals for generating force signals corresponding to the magnitude of forces necessary to align the gyroscopic means to an initial null position; processor means responsive to the force signals and acceleration signals for generating a heading signal representative of the angular deviation of the direction of the vehicle relative to the azimuth axis; measuring means connected to the processor means for generating a displacement signal so as to provide a ground-referenced displacement of the vehicle relative to an initialization position; and the processor means is responsive to the displacement signal for compensating the heading signal in accordance with the distance travelled by the vehicle.