Vehicle tracking systems are increasingly being used to help police recover stolen cars. In the past six months they have helped retrieve a £256,600 Rolls-Royce Phantom and a £313,565 Mercedes-McLaren SLR, and many insurance companies now insist that expensive cars are fitted with them.

The units are not the same as immobilisers. The purpose of a tracker is not to prevent theft but to increase the chances of recovery. To do this they use the same GPS satellites as the navigation systems fitted to modern cars, which enable them to keep a constant fix on the vehicle.

The unit itself is a small box, hidden somewhere within the vehicle’s body, containing a small electronic beacon that is in constant communication with a manned monitoring station via a satellite. When a car is stolen the monitoring centre notifies police in the area where the car is travelling. The police then use their own positioning computers to locate it.

There are two types of tracking device. The simplest are passive versions. These rely on the vehicle’s keeper to inform the monitoring station of a theft. Active systems are more advanced. These are primed automatically when the key is removed from the ignition. They can sense if the ignition has been bypassed — if the vehicle is hot-wired, for example — and send an alert. Some systems also feature movement sensors to detect if a static car is being loaded onto a transporter.

Some trackers can also deal with the scenario in which thieves have gained possession of the car keys, either by carjacking or a house break-in. These use a credit card-sized transponder that the driver keeps about them at all times. If the car and driver are separated, and the system senses the car is moving, the monitoring station is alerted.

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