The second control term, UDES, is formulated as a feed-forward term designed to cancel the effect of the inertial input on the inertial element. From the Principle of Optimal Disturbance Accommodation (61, exact cancellation of W depends only upon the instantaneous value of W and not upon any future behavior of the unknown disturbance. For this problem it is assumed that exact cancellation of the inertial input can be achieved by proper selection of the control input.