Phone mast speed traps
Celldar - short for cellphone radar - is being developed by two British companies, Roke Manor Research and BAE Systems.
It uses a "passive radar" technique that, instead of bouncing signals off objects, looks for disturbances in the radio environment.
Celldar works out the position of objects in an area by comparing the signals reflected from them by random radio "noise" with those it receives directly from a mobile phone base stations whose positions are known.
Because the system is passive, the "target" has no way of knowing it is being tracked.
For this reason, the technology has attracted military interest and US defence contractors.
On the road, motorists would be given no clue that they are being tracked.
There would be no speed cameras by the side of the road, and no police radar traps to watch out for.