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The paper deals with the use of output anticipations to relax restrictions of the existing unknown input observer (UIO) design methods. The anticipation signals are treated as additional outputs of the original system, and a modi® ed existence condition is presented. The modi® ed condition is shown equal to the condition of system inversion. A computer simulation shows that the proposed UIO using the anticipation signals extends the applicability of UIO to a larger class of dynamic systems.

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The problem of estimating the angular rate of a satellite is considered. A nonlinear observer based on the state-dependent Riccati equation method is proposed. A sufficient stability condition for the convergence of the estimation error is presented. This condition is related to a state-dependent algebraic Riccati equation. The Riccati equation is derived by transforming nonlinear error dynamics into a Lipschitz nonlinearity. Observer gains are obtained from this state-dependent algebraic Riccati equation. Numerical simulations are presented to demonstrate the proposed method.