Q: How can a mobile be located using Timing Advance and Cell ID information?
A. This method of positioning uses the existing Timing Advance parameter used in GSM and GPRS operation. This value is known to the serving BTS (Base Transceiver Station) which enables a distance from the BTS to be calculated in approximate units of 125m. The accuracy of the system is enhanced when a site is sectored as this enables the mobile to be located on an arc of approximately 120° (assuming tri-sectored site). To determine the position of a mobile in the idle mobility management state, a special call not noticed by the subscriber (no ringing tone) is established. The Cell ID and associated Timing Advance can then be calculated.
R. My understanding is that the Timing Advance parameter is only used locally on the BTS, regardless of whether it is measured with SACCH or PTCCH. So how can this information be shared by multiple bases to perform mobile location?
R. Where is this calculation performed - at the BSC? How would the TA values get to the BSC?
R. Why do you say the distance is calculated to units of 125m? The Timing Advance measurement uses units of symbol delays (0-63). Each delay equates to 1100 round trip meters or 550m one-way. How do you get 125m?
The BTS can optionally include the MS timing offset parameter in the Measurement Result messages it sends to the BSC. This would enable it to increase the accuracy of possible distance measurements.
The timing on the BTS includes both QN (Quarter symbol number) ranging from 0 - 624 and BN (Symbol number) ranging from 0 - 156. Depending on implementation, this quarter symbol could be used to bring the resolution from approx. 500m to 125m. Note that the use of this is implementation specific.