Enhanced-observed time difference (E-OTD) is similar to time difference of arrival (TDOA), but is a handset-based positioning solution rather than a network-based solution. E-OTD takes the data received from the surrounding base stations to measure the difference in time it takes for the data to reach the terminal (see Figure 5.7). The time difference is used to calculate where the mobile device is in relation to the base stations. For this to work, the location of the base stations must be known and the data sent from the different base stations must be synchronized.
Base stations are typically synchronized using fixed GPS receivers. Accuracy of E-OTD is expected to be as good as 50 meters using GSM and even greater with 3G networks. E-OTD requires additional memory and processing power in the handset, and can be used both when the terminal is idle and when the device is handling a call. E-OTD does have one major advantage over simple GPS in that it works indoors and in overcast weather conditions.
Commercially available solutions are available from Ericsson (http://www.ericsson.com) and Cambridge Positioning (http://www.cursor-system.com).