Position tracker is one of the most critical input devices for human-machine interface utilities in virtual reality and other human-machine interation systems. It determines the opition and orientation of an object of interest （such asthe user’s head） and passes the information to the host comuter in real time

which whws the virtual worid on the bois ofthe current visual point of the user. A dynamic position tracker based on continuous amplitude-modulated ultrasonic wave isdeveloped by means of the interferometric techniques. Its principles

technical implementation and experimental results are discussed. A prototype virtual reality system using the ultrasonic position tracker as an input device is also constructed and presented in the paper.