Enhanced Observed Time Difference measures the differences in time that signals from the base stations take to reach both the handset and a fixed point in the network. This information is then sent from the handset and the fixed point to a mobile location center where a latitude and longitude are computed and sent to the PSAP. The main difference between GPS-location services and E-OTD is that GPS uses a constellation of satellites maintained by the U.S. department of defense, whereas E-OTD uses a mathematical algorithm to identify the location of the caller based on the time signal between base stations as well as a triangulation scheme. A major driver for E-OTD implementation in the U.S. was the E-911 mandate made by the FCC.
Image courtesy of Cambridge Position System
Source: http://www.mobileinfo.com/locationbasedservices/E_OTD.htm
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