TDOA is a technique used in mobile networks to allow the network to obtain location information for a particular subscriber. The technique is often used to support the stringent location accuracy requirements associated with E911, which mandate that the network is able to identify the location of a subscriber to a specific degree of accuracy. TDOA bases its measurements on the time it takes for a radio signal to propagate between the mobile phone and the cell site. Receivers at the cell site termed LMUs (Location Measurement Units) will take the time measurements which can then be turned into an estimated distance. Providing the service provider has the measurements from several different cell sites surrounding the subscriber (three or more), a technique termed multilateration can be used to pinpoint the location to within an accuracy of approximately 50m (although this figure can vary).