Observed Time Difference of Arrival (OTDoA) is a method for locating mobile devices in UMTS. OTDoA is based on the time difference measurement of the radio frame of three base stations by the mobile device.
Like Time of Arrival( ToA) and Difference Time of Arrival( TDoA), Observed Time Difference of Arrival (OTDoA) works on a time basis and measures the different propagation times of the radio signal transmitted by multiple base stations. OTDoA measurement requires two cellular stations and the mobile device, between which triangulation is used to calculate the user location. High-precision clocks are used to determine the small time differences in which the user's uplink radio signal reaches the different mobile radio stations. The slight time differences that lie between a pilot signal or the start of the radio frame are measured. The measured values are sent to the Serving Radio Network Controller( SRNC), which evaluates them and uses them to determine the position. The measurable position accuracy is between 20 m and 200 m.