Time Difference of Arrival Location Determination

The Time Difference of Arrival (TDOA) method of locating a wireless caller is just one of several that may be selected by wireless carriers to meet Phase II requirements by October, 2001. The Angle of Arrival, location pattern matching and GPS methods are additional methods being considered. There are also hybrid location methods that use both TDOA and AOA technology. The TDOA, AOA and location pattern matching methods are network based, while the GPS method is handset based.

Using this technique:

• A wireless subscriber can use any handset (digital, analog, TDMA, CDMA, no special add-ons) to make a 911 call.
• The wireless phone's signal is received at various antenna sites. Since each antenna is a (usually) different distance from the caller, the signal arrives at a (very) slightly different time. The technique requires signal timing information from at least three different antenna sites.
• The receivers, synchronized by an atomic clock, send the caller's voice call and timing data on to the mobile switch, where the times are compared and computed to generate a latitude and longitude for the caller.
• The caller's voice call and the latitude and longitude are then sent to the PSAP for use by the dispatcher.

This technique is used by True Position in their Phase II products.

Wireless 911 page