State-deviation-estimation circuit employing a phase-locked-loop phase

Abstract

A system (34) for determining deviations in the state of motion of a projectile (10) from its intended state includes a comparison module (38) that receives the outputs S of a sensor array (36). The comparison module (38) converts the sensor outputs to a measurement vector Z.sub.m and computes the deviation of this measurement vector from an intended measurement vector Z.sub.0 received from a control system. The comparison module (38) then determines the difference E.sub.Z between this measured deviation and the deviation predicted by a Kalman filter (44, 46). In generating the measurement vector Z.sub.m from the outputs of the sensor array (36), the comparison module (38) 'de-spins' the array outputs in accordance with the output of a phase reference (42), whose purpose is to indicate the phase with respect to gravity of the spin of the projectile (10) about its longitudinal axis. The Kalman filter's state-deviation estimator (46) weights the vector output of the comparison module (36) and adds it to the output of the Kalman filter's state-deviation predictor (44) to provide an updated state-deviation estimation E.sub.X (t:t), which the state-deviation predictor (44) then uses as a basis for its next cycle. The output of the comparison module (38) constitutes a measure of the performance of the state-deviation predictor (44) and is used as a correction factor for the phase reference (42) so that the system acts as a phase-locked loop to lock the phase reference (42) onto the projectile spin without the need for an external sensor to determine the direction of gravity.