This paper intents to add to the literature on 3D position and orientation Eacking systems by describing TNO's experience with tlie InterSense ftacking system that uses a combination of inefial and ultra-sound technology. From the results of a performance evaluation study and our practical experience with this system in military applications, the value of the system and its underlying inertial technology based hybrid concept is determined. The performance figures addressed in the study include noise and regisgation error characteristics. Orientation and position tracking performance results are provided for the InterSense system. The figures are compared with the figures for the Polhemus FASTRAK system. The hybrid tracking system concept as introduced by InterSense is of great value to virtual environment applications. The filter algorithms included in the InterSense fiacking system to combine the two different sensor types result in,a system that is both fast and noise free. The system is very well suited for most imrnersive applications. Registraúon error, however, is rather large, causing the system to be inadequate for augmented reality applications in its cunent implementation. TNO feels that the "on."¡ will evolve to an inertial technology based system combined with high accuracy auxiliary trackers that will meet the requirements for augmented reality applications.