A set of emitters and receivers are used with a known relationship between the emitters and between the receivers. The emitters are pulsed in sequence and the time lag to each receiver is measured. Triangulation gives the position. Drawbacks to ultrasonics are low resolution, long lag times and interference from echoes and other noises in the environment. Logitech and Transition State are two companies that provide ultrasonic tracking systems.
Magnetic trackers use sets of coils that are pulsed to produce magnetic fields.
The magnetic sensors determine the strength and angles of the fields. Limitations of these trackers are a high latency for the measurement and processing, range limitations, and interference from ferrous materials within the fields. However, magnetic trackers seem to be one of the preferred methods. The two primary companies selling magnetic trackers are Polhemus and Ascension. Optical position tracking systems have been developed.