h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

One thing that might be worth mentioning is the ability for the networks (in exceptional circumstances) to determine position to much greater accuracies than just the BTS and sector (antenna) the phone is logged into.

The way that the GSM radio channel works during a call requires the same frequency to be shared by up to 16 simultaneous calls. To achieve this each phone transmits for a short time (bursts) in a specific time period or slot. Each handset on the channel will transmit in a different slot.

The further away the handset is from the BTS antenna the earlier the handset must begin its burst in order for the transmission to arrive at the BTS antenna within its correct slot time.

In the GSM implementation this is taken care of by a parameter called Time Advance (Ta), normally a number from 0 to 63. A handset next to the BTS antenna will have a Ta of 0 , whereas a handset at the maximum nominal range of 35Km will have a time advance of 63.

In effect each increment of the Ta is a measure of distance (in multiples of 550 metres) from the BTS antenna to the handset.

This alone cound narrow a handset to a few locations however for more accuracy (in kidnap or other extreme situations for example) it is possible for the network to force a handoff (pass the call in progress) to another BTS in the area and get another Ta measurement. You only need 3 such Ta measurements from different BTS to have a fair idea of a hansets location to an area of about 500m in any direction.

Does this happen in practice, probably not except in special circumstances as required by law enforcement authorities. I can think of a recent (Sept 2002) case here in the UK where the police have pleaded for the abductors of two children to call a number from the abductees mobile.. something I found a bit odd at the time until I thought about it some more.

Cheers

Wullie