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Originally Posted by dhitchco
First, the most obvious is to use radio frequency "beacons" such as cell phone that can emit a "beacon" which can be triangulate the player's position on the field. Even easier would be to use a 900Mhz low-power belt-worn transmitter and two receivers to triangulate the player's position. You could also do the same thing via light (e.g. lasers) but they would require "aiming" the light at the player each time you wanted to triangulate.
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There's basically two ways to triangluate if you have some sort of emitter on the person and two receivers off-field. These receivers either have to be able to measure the angle (or bearing) to the player from each sensor position OR they each have to be able to measure the distance to the player. And of course the distance between the receivers and their exact location needs to be known in advance. When you talk about using RF (cell phone or 900MHz), are you thinking of measuring the distance or the angle? Measuring angle would be tricky but theoretically possible I suppose, perhaps using a highly directional antenna and sweeping it back and forth across the court. Distance would be nearly impossible. To measure distance you'd have to measure the time-of-flight of the radio signal and if you're talking about measuring things in the range of 10s to 100s of feet you'd need extremely accurate timing. RF travels at roughly the speed of light, meaning that it travels 1 foot every nanosecond. So, you'd have to be able to measure the time of flight of the radio transmission with accuracy down to the nanosecond to be able to measure distance with about a foot of accuracy.
I suppose you could try to use signal strength as a way of estimating distance from the sensor, but that has many problems - other players blocking the signal would make the player appear father away, the difference in signal strength between 10 feet away and 20 may be too small to measure, etc.
You'd also have to contend with reflections and interference and an assortment of other problems. The bottom line is that there's a lot of good theory to be explored in solving this problem, but a practical implementation is extremely difficult.