The authors present a system identification technique that represents the structural dynamics of large, flexible space structures in terms of a reduced-order modal model. The finite-element method is used to reduce the order of the physical model, and then the model is transformed from a physical to a modal form. The maximum-likelihood method is used to parameterize the modal model on the basis of experimental observations of the structural dynamics. The test article consists of a softly supported dynamic model of a flexible beam and reflector grillage attached to the space shuttle, called SCOLE. The control objective includes directing the line of sight of the antenna like configuration toward a fixed target, under conditions of limited control authority and random disturbances. The identification technique is shown to be very effective in modeling SCOLE and for active controller designs. The method has some convergence problems but has the potential to be extremely useful for solving the system identification problem of other large flexible spacecraft.
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