Fine Pointing Control for the Orbiting Astronomical Observatory (Oao)

High pointing accuracy requirements are imposed on the control system of the OAO. One tenth arcsec angular accuracy can be achieved in orbiting spacecraft using star references, long focal length optical systems, and relatively small torquing devices. Noise problems are introduced when a single error sensor is required to track stars with greatly different magnitudes. As star magnitude decreases, the compensating increase in the gain by the automatic gain control decreases the signal-to-noise ratio and degrades the pointing accuracy. A linear range of at least one arcmin will eliminate or reduce many restrictions on initial acquisition of a particular star. These restrictions are introduced by high initial satellite rates and/or unbalanced inertial wheel torques. Relatively small voltage rates out of the sensor may be detected by a suitably designed nonlinear lead circuit with low frequency corners.