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Cellphone 'radar' tracks traffic flow


09:45 27 October 03

(Exclusive from New Scientist Print Edition. )

Signals from cellphone masts can be used to track aircraft, monitor traffic congestion and spot speeding motorists without tipping them off that they are being watched.

The radar-like system, which is still being developed, has provoked media reports of the start of a huge extension of Big Brother-style surveillance - privacy campaigners have complained that it could be used to track individual people. But radar experts say such fears are unfounded.

Conventional radar works by transmitting a signal, listening for the reflection and using the time taken for the round trip to work out the object's distance. More sophisticated systems can work out the object's speed from characteristic changes to the signal's frequency, known as Doppler shifts. But such radar systems are expensive, and the signals they send out are easy to detect.

An alternative technique, called passive radar, gets round these problems. Instead of broadcasting its own signal, a passive radar system listens in to the cacophony of radio signals in the environment and monitors the way moving objects change them.

The US defence company Lockheed Martin is developing a system called Silent Sentry which exploits the signals from radio and television masts to spot aircraft and ships.

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