Error cancelling scanning optical angle measurement system

Abstract

An attitude determining system for vehicles, such as spacecraft or for ground-based environments measures the angle between a reference and the line-of-sight to a star or other celestial object of known position or to aircraft and missiles. The system includes a rotating scanning telescopic sensor, and a fixed mirror positioned so that the sensor scans both the direct and the reflected line-of-sight of the celestial object in the course of each rotation. Associated transducers and electronic circuits determine the relative angular positions of the telescopic sensor at the instants it detects the direct and the reflected radiation from the celestial object. A computation based on the two angles gives the angular position of the celestial object relative to the vehicle reference. The system cancels or substantially reduces errors due to optical or electronic inaccuracies which identically affect the direct and the reflected angular position measurements.