ABSTRACT Integration of a commercial inertial navigation unit into a dynamic positioning control loop for offshore drilling rigs will provide improved operation and a self-contained backup in the event of signal loss from the primary acoustic or taut wire systems. Tests have demonstrated an average position error buildup of less than two feet per minute for up to one hour of pure inertial operation. INTRODUCTION The accuracy and reliability of the dynamic positioning system for deep water drilling rigs are strongly dependent upon the sensors in the control loop which measure the rig position and velocity. The prime reference for position control of the rig is an acoustic beacon, mounted near the well head, whose signals are picked up by receivers under the rig. This reference is usually augmented by taut wire angle sensors which are also tied to anchors on the ocean floor. Other more remote position references, such as Loran C or navigation satellites, are unreliable and less accurate, and therefore not normally considered for primary reference control. While the riser itself is a potential reference, its dynamic motion in deep water needs to be better understood. As the drilling rigs begin to operate in depths of 2, 000 to 6,000 feet, the ocean bottom acoustic and taut wire references become less reliable because of transmission noise and the increased possibility of their complete loss. There is a strong desire by the operators of the dynamically positioned rigs to have a 'sky hook' available to provide a temporary reference should these signals be completely lost, and to provide a smoothing medium to suppress their high frequency measurement noise. An inertially stabilized platform can provide such a reference when integrated properly with other position references. An inertial reference unit and digital computer together form a dead reckoning navigation system. The velocity is determined through the integration of accelerometer outputs; another integration determines the system position. The constants of integration, which are the initial velocity and position, must be supplied to the system. In addition, the platform alignment with respect to the earth must be known by the computer in order to establish a reference coordinate system in which to make the measurements and perform the computations. On board the drilling rig, the inertial reference unit (IRU) platform can be leveled to the local gravity vector to provide a vertical reference to the acoustic pickups.