The plant model contains the open loop dynamics of the inertial instrument and any dynamics associated with the control or measurement process. These plant states are assumed to be controllable. From the control input, U, and observable from the measurement, Y The disturbance model can take on various forms depending upon the characteristics of the anticipated disturbances (e.g., vehicle dynamics, mechanical isolator characteristics). These fictitious states are observable from the measurement by construction. By inspection these states are not controllable from the-control input.