Oliver Rabb almost sneaks up on us, appearing suddenly through a second door. Tall and bald, with big hands and a stoop, he seems to present the top of his head as he bows and shakes our hands. A study of tics and eccentricities, he is the sort of man who regards a bow tie and braces as practical rather than a fashion statement.
‘Ask away, ask away,’ he says.
‘We’re looking for calls made to a mobile number,’ replies Ruiz.
‘Is this investigation official?’
‘We’re assisting the police.’
I wonder if Ruiz is so good at lying because he’s met so many liars.
Oliver has logged onto the computer and is running through a series of password protocols. He types Christine Wheeler’s mobile number. ‘It’s amazing how much you can tell about a person by looking at their phone records,’ he says, scanning the screen. ‘A few years ago a guy at the Massachusetts Institute of Technology did a PhD project where he gave out a hundred free mobile phones to students and employees. Over nine months he monitored these phones and logged over 350,000 hours of data. He wasn’t listening to the actual calls. He only wanted the numbers, the duration, the time of day and location.
‘By the time he finished he knew much more than that. He knew how long each person slept, what time they woke, when they went to work, where they shopped, their best friends, favourite restaurants, nightclubs, hangouts and holiday destinations. He could tell which of them were co-workers or lovers. And he could predict what people would do next with eighty-five per cent accuracy.’
Ruiz looks over his shoulder at me. ‘That sounds like your territory, Professor. How often do you get it right?’
‘I deal with the deviations, not the averages.’
‘Touche.’
The screen refreshes with details of Christine Wheeler’s account and phone usage.
‘These are her call logs for the past month.’
‘What about Friday afternoon?’
‘Where was she?’
‘The Clifton Suspension Bridge- about five.’
Oliver starts a new search. A sea of numbers appears on the screen. The flashing cursor seems to be reading them. The search comes up with nothing.
‘That doesn’t make sense,’ I say. ‘She was talking on a mobile when she jumped.’
‘Maybe she was talking to herself,’ replies Oliver.
‘No. There was another voice.’
‘Then she must have had another phone.’
My mind trips over the possibilities. Where did she get a second mobile? Why change phones?
‘Could the data be wrong?’ asks Ruiz.
Oliver bristles at the suggestion. ‘Computers in my experience are more reliable than people.’ His fingers stroke the top of the monitor as if worried that its feelings might have been hurt.
‘Explain to me again how the system works,’ I ask.
The question seems to please him.
‘A mobile phone is basically a sophisticated radio, not much different to 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 call 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 still tapping at keys. ‘Every call 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 call log, the last time Mrs Wheeler’s mobile received a call was at 12.26 on Friday afternoon. The call 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 called her?’
‘Another mobile.’
‘Who owns it?’
‘You need a warrant for that sort of information.’
‘I won’t tell,’ replies Ruiz, sounding like a schoolboy about to sneak a kiss behind the bike shed.
‘When did the call end?’ I ask.
Oliver turns back to the screen and calls 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 usually 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 towardsnotices the signal strengthening. The two base stations coordinate and switch your call to the new tower. It happens so quickly we rarely notice it.’
‘So Christine Wheeler was still talking on her mobile when she left her house?’
‘Looks like it.’
‘Can you tell where she went?’
‘Given enough time. Breadcrumbs, remember? It might take a few days.’
Ruiz has suddenly become interested in the technology, pulling 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 will latch onto the strongest signal. The “ping” is actually 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 finished up at the Clifton Suspension Bridge on Friday afternoon. At some point she stopped using her mobile and picked up another. When did it happen and why?
Oliver pushes his chair away from the desk and rolls across the room to a second computer. His fingers flick
