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...