Shatter by Michael Robotham Page A

walkie-talkie, but while a walkie-talkie can transmit perhaps a mile and a CB radio about five miles, the range of a mobile phone is huge because it can hop between transmission towers without losing the signal.’
    He holds out his hand. ‘Show me your phone.’
    I hand it to him.
    ‘Every mobile handset identifies itself in two ways. The Mobile Identification Number (MIN) is assigned by the service provider and is similar to a landline with a three-digit area code and a seven-digit phone number. The Electronic Serial Number (ESN) is a 32-bit binary number assigned by the manufacturer and can never be changed.
    ‘When you receive a cal on your mobile, the message travels through the telephone network until it reaches a base station close to your phone.’
    ‘A base station?’
    ‘A phone tower. You might have seen them on top of buildings or mountains. The tower sends out radio waves that are detected by your handset. It also assigns a channel so you’re not suddenly on a party line.’
    Oliver’s fingers are stil tapping at keys. ‘Every cal that is placed or received leaves a digital record. It’s like a trail of breadcrumbs.’
    He points to a flashing red triangle on the screen.
    ‘According to the cal log, the last time Mrs Wheeler’s mobile received a cal was at 12.26 on Friday afternoon. The cal was routed through a tower in Upper Bristol Road. It’s on the Albion Buildings.’
    ‘That’s less than a mile from her house,’ I say.
    ‘Most likely the closest tower.’
    Ruiz is peering over his shoulder. ‘Can we see who cal ed her?’
    ‘Another mobile.’
    ‘Who owns it?’
    ‘You need a warrant for that sort of information.’
    ‘I won’t tel ,’ replies Ruiz, sounding like a schoolboy about to sneak a kiss behind the bike shed.
    ‘When did the cal end?’ I ask.
    Oliver turns back to the screen and cal s up a new map, covered in numbers. ‘That’s interesting. The signal strength started to change. She must have been moving.’
    ‘How do you know?’
    ‘These red triangles are the locations of mobile phone towers.
    In built up areas they’re usual y about two miles apart, but in the country there can be twenty miles between them.
    ‘As you move further away from one tower the signal strength diminishes. The next base station— the tower you’re moving towards— notices the signal strengthening. The two base stations coordinate and switch your cal to the new tower. It happens so quickly we rarely notice it.’
    ‘So Christine Wheeler was stil talking on her mobile when she left her house?’
    ‘Looks like it.’
    ‘Can you tel where she went?’
    ‘Given enough time. Breadcrumbs, remember? It might take a few days.’

    Ruiz has suddenly become interested in the technology, pul ing up a chair and staring at the screen.
    ‘There are three missing hours. Perhaps we can find out where Christine Wheeler went.’
    ‘As long as she kept the phone with her,’ replies Oliver. ‘Whenever a mobile is turned on it transmits a signal, a “ping”, looking for base stations within range. It may find more than one but wil latch onto the strongest signal. The “ping” is actual y a very short message lasting less than a quarter of a second, but it contains the MIN and ESN of the handset: the digital fingerprint. The base station stores the information.’
    ‘So you can track any mobile,’ I say.
    ‘As long as it’s turned on.’
    ‘How close can you get? Can you pinpoint the exact location?’
    ‘No. It’s not like a GPS. The nearest tower could be miles away. Sometimes it’s possible to triangulate the signal from three or more towers and get a better fix.’
    ‘How accurate?’
    ‘Down to a street: certainly not a building.’ He chuckles at my incredulity. ‘It’s not something your friendly service provider likes to advertise.’
    ‘And neither do the police,’ adds Ruiz, who has started taking notes, boxing off details with doodled circles.
    We know Christine Wheeler