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Last Updated: Wednesday, 3 August 2005, 08:19 GMT 09:19 UK
Tracking a suspect by mobile phone
Hussain Osman (picture made available by Italian police after his arrest)
Hussain Osman was staying at his brother's flat in Rome
Calls made from a mobile phone led Italian police to one of the suspects in the failed suicide bombings in London on 21 July.

Officers used a combination of tracking and tapping Hussain Osman's calls to locate him.

It is relatively simple to follow a person via their mobile phone. Plenty of companies offer technology to do so, legally and with the phone user's knowledge.

However, the same process can be employed by police to locate the user without alerting them. Detectives need to first identify the phone and then find out where it is.

Tracking SIM and handset

In the case of Mr Osman, British police had provided their Italian counterparts with two mobile numbers linked to him.

The Italian police were able to keep tabs on him despite the fact that he changed his SIM card along the way. This is because the phone identifies itself in two ways when it connects to the mobile phone network.

The SIM card sends its unique IMSI number - standing for International Mobile Subscriber Identity. This starts with the country code of the user's account, followed by the network code and finally the telephone number.

The second number is the IMEI - International Mobile Equipment Identity. This is the number of the handset and remains constant even if the SIM card is changed.

Mobile phones transmit these numbers each time they make a call and when they regularly "check in" to the local base stations.

Once the police know which mobile they are looking for, it is not hard to locate.

1) Phone sends signal to nearby base stations
2) Positioning software performs a triangulation calculation on the information from the base stations
3) The data is converted into a geographical location

The mobile telephone network is divided into cells with a base station at the centre of each cell.

The base station which processes the call provides the first clue to location, giving police an idea of the general area in which to look.

But other base stations can also make contact with the phone and once information from several base stations has been gathered, the location of the phone can be narrowed down using triangulation.

In built up areas where the base stations are close together this can be to within a few hundred metres; in rural areas, the system is less accurate.

Phone tapping

As well as tracking Mr Osman, Italian police listened in to his calls.

They say they heard him speaking in an Ethiopian dialect spoken among people from the Somali/Eritrean border and this encouraged them to believe they had the right man.

One way tapping a mobile phone is with a device called an IMSI-catcher.

This pretends to be a legitimate base station of the mobile phone network and tricks the phone into routing its call via the IMSI-catcher where it can be passed on for decryption.

Once received, the IMSI-catcher passes the call on to the network, so the suspect is none the wiser he is being monitored.

However, Italian police may have just contacted the mobile phone company to intercept the call.

Police mobile phone tapping is particularly prevalent in Italy and the country's state mobile phone network, TIM, recently appealed to magistrates to cut the number of requests as it had reached the limit it could cope with.

Italian police explain how Osman Hussain was traced

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