Enhanced Observed Time Difference
A position-location method for GSM networks in which the phone measures the difference in the time of arrival signals from different towers. Essentially, the phone triangulates its position using signals from the towers, the reports the position back to the network.
E-OTD is one of several technologies that carriers can deploy to meet E-911 requirements.
Unlike competing U-TDOA technology, the phone actively participates in the location process. Therefore, E-OTD only works with phones that specifically include E-OTD technology.
Most major U.S. carriers that tested E-OTD for E-911 compliance decided against it and opted for U-TDOA technology instead.
The main proponent and provider of E-OTD technology is a company called Cambridge Positioning Systems (CPS).