Compensator to maintain observability while decoupling

In many linear multivariable systems, the loss of complete observability is the price of decoupling by state feedback. A design procedure is presented for a compensator by which either all modes or only the unstable modes of the system are made observable; these would be unobservable without the compensator. This additional dynamic system is not a compensator in the common sense, since it is driven by the state of the plant and does not serve as a state estimator.