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A self-contained midcourse guidance system is assumed which uses a combination of optical and inertial equipment. This system would utilize the optical equipment in making automatic subtense measurements on nearby bodies. Position and velocity perturbations would then be determined by making a maximum likelihood fit to this subtense measurement data. Velocity corrections would be introduced at several points along the trajectory to bring the vehicle back to the reference trajectory. The inertial system would be used in controlling the velocity corrections, and the platform would serve as a stable base for the optical equipment. This report gives estimation errors, perturbations, and propulsive fuel requirements for this system on a typical circumlunar mission of 8.7 days' duration.