Since the timing scheme in the GSM standard has been defined very tightly, the mobile station must send its data packet to the base station before the start of the receive time slot so that it arrives there within the receive window. The time by which the transmit burst must be advanced, i.e. the Timing Advance, is determined by the base station and communicated to the mobile station.
Timing Advance is directly dependent on the distance between the mobile station and the base station. The TA value increases with the distance between the two stations and can assume values between 0 and 63, which corresponds to runtimes of up to 233 µs. This highest value corresponds to a cell radius of 35 km. Since the TA signal is a distance-dependent signal, it is also used for position determination in GSM networks. It is evaluated by the Serving Mobile Location Center( SMLC) for locating the mobile device. The hybrid method in combination with Angle of Arrival( AoA) and Cell Sector( CS) for sector determination provides good position results.