Uplink Time Difference of Arrival (U-TDOA)

This is sometimes referred to simply as Time Difference of Arrival (TDOA). Time Difference of Arrival relies on the fact that each cell site is generally a different distance from the caller and that signals travel with constant velocity. Therefore, each signal arrives at the cell site at slightly different times. Using these properties, a signal defines a locus of points on a circle around a base station on which a mobile could be located. Then, using synchronized receivers, the times can be compared and a latitude and longitude can be computed and sent to the PSAP. At least three different receivers are needed for TDOA to work. TDOA works with any handset – digital, analog, TDMA, GSM, CDMA, etc.

Source: http://www.iwi.uni-hannover.de/lv/ucc_ws04_05/riemer/pic/tdoa.gif