Angle of arrival (AOA) is a network-based method of determining position that does not require a mobile device upgrade to operate. In AOA, a mobile device's signal is received by multiple base stations (see Figure 5.8). The base stations have additional equipment that determines the compass direction from which the user's signal is arriving. The information from each base station is sent to the mobile switch, where it is analyzed and used to generate an approximate latitude and longitude for the mobile device.
An advantage of AOA is that it supports legacy handsets. Disadvantages include the fact that every base station needs to have an equipment upgrade. Users might also be concerned about privacy issues because they are not able to disable positioning from the handset.
A commercially available solution is available from Trueposition (http://www.trueposition.com), supporting both AOA and TDOA.