Thanks for you help Daniel.. from what I can tell the martin lighting system uses a transmitter carried by the person that it tracks and is more $$$ and more advanced than I require. I need to be able to do this without the person in the room carrying anything and ideally for less than $100.. would I be able to modify the frequency of a few ultrasonic sensors so that I could use four of them in the same room to track movement and report it to max/msp without them interfering? or maybe use some very small delay so that none of them were "pinging" at the same time?