The positioning method known as uplink time of arrival (TOA) is based on the time of arrival of a known signal sent from the mobile device and received by three or more base stations (see Figure 5.9). The signal is the access burst created by having a mobile device perform an asynchronous handover.
Figure 5.9. TDOA.
The TDOA values are calculated by pairwise subtracting the TOA values at the SMLC. The position of the mobile device is then calculated by hyperbolic trilateration (see the earlier section "How GPS Works"), provided that the geographic coordinates of the measurement units are known and the timing offset between the measurement units used in the measurement are known.
Additional technical details are presented by the 3rd Generation Partnership Project (http://www.3gpp.org) in GSM 03.72: "Digital Cellular Telecommunications System (Phase 2+); Location Services (LCS); (Functional Description) “ Stage 2":
The uplink TOA method requires additional hardware (LMUs) to accurately measure the arrival time of the bursts. Different implementation options exist for this positioning method. For instance, it is possible to either integrate the measuring units in the BTSs or implement them as stand-alone units. If the measurement units are implemented as stand-alone units, the communication between the measurement units and the network is preferably carried out over the air interface. The stand-alone units can have separate antennas or share antennas with an existing BTS.
Similar to AOA, TDOA is a purely network-based solution and will support legacy handsets. The drawback, of course, is the installation of equipment in almost every base station, a potentially much more expensive proposition for the mobile operator. Another challenge with TDOA is that it requires that the mobile device be in range of at least three base stations. This is often not the case in rural or even some suburban areas.
A commercially available solution from Trueposition (http://www.trueposition.com) supports both AOA and TDOA.