Enhanced Observed Time Difference (E-OTD) is a method for increasing localization accuracy in GSM networks. Since positioning in mobile communications networks depends essentially on information from the base station and positioning accuracy depends on the size of the radio cell, E-OTD is a method that evaluates the propagation time differences of the signals from multiple base stations in the cell phone and thus increases positioning accuracy.
With E-OTD, at least three base stations transmit a positioning signal simultaneously, which is received by the cell phone with slight time differences. The travel time differences detected by the cell phone are evaluated and sent back to the base station. The accuracy of the time-of-flight measurement depends on the reception conditions and thus determines position accuracy. A prerequisite is that the cell phones must be designed for this operation.
UMTS uses Observed Time Difference of Arrival (OTDOA), a procedure similar to E-OTD. In this procedure, the mobile device measures the propagation time differences between three base stations and evaluates it accordingly. Uplink Time Difference of Arrival (UTDOA) is different, as several base stations measure the propagation time differences of the mobile device.