RF/Ultrasonic transmission: the person would have a RF/ultrasonic transmitter, and the camera would have multiple receivers. The camera would know how to align to the object due to variances in signal strength.
Time-of-arrival or phase measurement might work, but signal strength probably will not.
Before the GPS technology, people used to uss LORAN, a system to track ships with time-of-flight of RF signals to various towers and a master tower. You can do it with long distances, nautical miles, but meters of distance is too short. It only takes a bunch of nanoseconds to travel and the difference in time of flight is too short.
I've seen ultrasound tracking. Google "ben heck show episode with suitcase". He made a robot suitcase that follows him around. It's around 2 meters of distance but could work up to say 6 meters.
Nintendo wii remote control can also work. It gives you all IR objects (up to 6). so if the person is cooperating and always wears IR leds around his chest and back, the remote can tell where he is, up to a certain angle range. Still the range is a few meters and you need brighter IR leds for longer distance, say 10-20 meters.