We formulate and study the problem of designing nonlinear observers whose error dynamics are input-to-state stable (ISS) with respect to additive output disturbances as long as the plant's input and state remain bounded. We present a reduced-order observer design which achieves this quasi-ISS property when there exists a suitable state-independent error Lyapunov function. We show that our construction applies to several classes of nonlinear systems previously studied in the observer design

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... ature. As an application of this robust observer concept, we prove that quantized output feedback stabilization is achievable when the system possesses a quasi-ISS reduced-order observer and a state feedback law that yields ISS with respect to measurement errors. A worked example is included. H. Shim is with ASRI and the