An approach to model error compensation in the control of nonrigid spacecraft

Summary This paper seeks to improve the convergence properties of state estimators as used in spacecraft controller designs, when the linearized model upon which the estimator is based is subject to parameter errors, truncated modes, and neglected disturbances. Instead of choosing orthogonal mode shapes multiplied by time varying coefficients (a conventional approach for modeling elastic modes) to represent the truncated modes, the “model error vector” discussed herein is approximated, over short “observation windows” τ units long, by functions which are orthogonal over the interval τ, where the coefficients of the orthogonal functions are automatically updated via the use of real time measurements from the system. The device which updates the coefficients of the orthogonal functions is called an orthogonal filter and takes on the form of a state estimator for the synthetic modes of a “model error system” which generates the orthogonal functions. The method is illustrated for a 15th order model of a flexible spacecraft, resulting in 2nd, 3rd, and 4th order controllers.