Minimal-order observers for linear multivariable systems with unmeasurable disturbances

The problem of designing an observer to reconstruct the state vector of a linear multivariable system for a general class of unmeasurable and inaccessible disturbance inputs is considered. Existence of an observer is determined by a set of necessary and sufficient conditions which are related to the structure of the system and disturbance model. A minimal-order observer is presented along with an outline of some currently available synthesis techniques.