The law of unintended consequences: build a cellular-phone network and get a sophisticated surveillance system along with it. At least that is what may happen in the U.K., thanks to England's contract research and development firm Roke Manor Research and aeronautics company BAe Systems. The two are working on a way of using the radio waves broadcast by the world's mobile-phone base stations as the transmission element of a radar system. They call it Celldar.

Radar works by transmitting radio pulses (or pings) and listening for an echo. Measuring the Doppler shift of the echo can give an object's distance and speed. Celldar proposes to take advantage of U.K. base stations, which transmit radio waves from known locations in a known microwave frequency band. Instead of erecting a radar transmitter, a Celldar operator would only need to set up passive receivers that can measure the cellular-network radio waves reflected from nearby objects and process the data. Because they would not transmit, Celldar receivers can, according to BAe Systems, be smaller and more mobile than traditional systems--and undetectable. Celldar operators would not require the cooperation of the cell-phone-network operators, either.

This article was originally published with the title Connect the Pings.

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