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There is an amazing ignorance of how trackable mobile phones are. On the one hand the location based services crew all think that your precise map reference can be determined instantly from GPS, and at the other end of the scale people in dramas blithely use mobile phones without realising that they are leaving a trail of their locations as they go.

To dispel the GPS myth you really need to try it. Something like 80% of mobile phone calls are made indoors. GPS doesn't work at all indoors. Forget what you are told about increasing sensitivity and how it is getting better and ask for a demonstration. GPS works reasonably well on the dashboard of a car with a full view through the windscreen. It works less well on the passenger seat.

Stand in the middle of a room in a building and you have no hope. Put the phone in your pocket or handbag, a ridiculous thing to do I know, and you'll lose the fix. Getting a new fix will take between a few seconds and a few minutes when you have retrieved the phone.

All this means that you can't track anyone who has a GPS phone. Those ideas of kid trackers fail on any kind of practicality when you need the phone kept out in the open.

Conversely the only film I've ever seen where there was a good realisation of the traceability of phones was The Bourne Ultimatum. The hero bought a pre-pay phone for cash, made a call, dumped the phone before the systems could be fired up to do the location tracking and then another one bought.

It's more usual for criminals in films to dump phones after they've made it to the lock up or wherever. By then it is too late. The mobile network has to know where you are so that the call can reach you. The phone checks in with the network so that when someone calls you the network knows where to look, first in the cell where you were last seen and then in adjoining cells. If you've made a call the details of where you were when you did it are kept for months. This is essential for billing disputes. Some tariffs charge differently depending on where you were and where you were calling when you made the call. The idea of long distance calling has become a little distorted with mobile but plenty of networks offer Home Zone where your home cell or cells give a rate that is competitive with the landline. There is even a GSM standard for it and you'll get an icon that looks like a house from the Monopoly board game.

So anyone with access to your billing records knows where you were when you called. That includes the police who request something like a couple of thousand cell and billing look-ups a day in the UK across the five networks.

A phone that's off is invisible to the network but one that's on, even if it's not in a call can be located quickly enough. It will check in of its own accord, or if you send an SMS the phone will wake up and acknolwledge. Drama writers don't have any grasp of this, if they did their characters would leave phones on buses and trains going in the wrong direction, hidden under the seat of a transcontinenal bus or left on silent and posted somewhere.

Unfortunately proper criminals understand all this. They know about using secure VoIP software that even the Police can't crack, things like Cellcrypt. But just as drama writers don't understand the limitations the useful services like kid tracking, are not as practical as we might like. There are cell based tracking services but they have been surprisingly unsuccessful.

Cat Keynes publishes her thoughts on the mobile phone industry every Sunday at www.catkeynes.com you can read the column  the previous Friday by subscribing here. Follow me on Twitter here.

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