From Experimental Robotics
Method of determining a POSITION using DISTANCES from THREE or MORE locations/landmarks/beacons/transmitters/etc.
Same process used in GPS.
If you were looking for HOW we used this, check Radio Localization
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THREE readings are required to determine a 2D position, unless you wish to do something crazy like the Robosapien guys, or use the Earth as an imaginary sphere like in GPS.
You will need FOUR readings for 3D positioning.