The report develops the algorithm for deriving attitude,heading,and navigation information from a strapdown inertial system. Beginning with the fundamental physical relationships,it develops all required equations and progresses to the onboard implementation of the algorithm. Significant features of the algorithm include: (1) Computations performed in the wander azimuth coordinate frame to provide a system capable of operating in the polar regions; (2) Separation into four loops of different iteration rates. This maintains rapid,accurate updating of the direction cosine matrix involving vehicle attitude,while processing other information and extracting display data at appropriately slower rates; (3) Fourth order Runge-Kutta integration of quaternions,using second order rate extraction,to update the attitude direction cosine matrix; (4) Specification of the computations that require double precision for adequate performance; (5) Third order damping of the vertical channel by means of barometric altimeter data. The applicability of this algorithm to a range of vehicle and mission environments is indicated,the required adaptations being easily performed for each particular implementation. (Author)
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