Time difference of arrival
Time difference of arrival (TDOA) is an electronic technique used in direction finding and navigation, in which the time of arrival of a specific signal, at physically separate receiving stations with precisely synchronized time references, are calculated. In the military context, it is part of both electronic warfare and measurement and signature intelligence.
One commercial application for TDOA is locating a cellular telephone based on comparing when the signal arrives at different cell towers. The technique requires no additional circuitry in the telephone, since it uses the standard signal. It may be supplemented with angle of arrival information if the cell towers have directional receiving antennas.
TDOA should not be confused with time of arrival (TOA). Even though a TDOA call flow would look virtually the same as a TOA call flow, there is a difference in how the location is calculated. TDOA and TOA are similar, but there is a difference. TOA differs in the fact that it uses the absolute time of arrival at a certain base station rather than the difference between two stations. The distance can be directly calculated from the time of arrival because signals travel with a known velocity. Time of arrival data from two base stations will narrow a position to two points and data from a third base station is required to resolve the precise position
It is also used in passive radar systems, which are especially attractive for countering stealth, and also make the ground radar more survivable because the specific transmitters it uses may not be possible to determine. 
- U-TDOA – Uplink Time Difference of Arrival, TruePosition
- Arend G. Westra (October 2009), "Radar versus stealth: passive radar and the future of U.S. military power", Joint Forces Quarterly