  This brief memo reviews the theory of Unknown Input Observers (UIO) for state
estimation in systems subject to disturbance inputs that are not known a
priori. One main advantage of the UIO is that the observer structure naturally
decouples the plant inputs from the state estimation process. This advantage is
highlighted in three example dynamic systems commonly found in control theory.
It is shown that the estimation error for all three systems asymptotically
approaches zero by application of the UIO under the influence of dynamic
disturbances.
