Finding a cellphone.
Not only does the cellular base station control the power output of the phone's radio transmitter, it is also possible to trace the position of a phone.
The system knows which cell the phone is in: but it also knows which sector of the cell it is in. You'll have noticed the cells have multiple antennae pointing in specific directions -- these are sectors.
As the power setting used by the phone transmitter is set by the base (as the signal from the phone weakens, it cranks up the power), it follows that the distance from the base can be approximated. So the vector (sector) and distance (power) are known.
Add to that the fact that the phone, although only in communication with one cell, may be visible and known to others, you can triangulate the phone's position.
Such evidence has been used in court (the Soham murders, for example).
Last edited by derekl; 31st Aug 2006 at 23:13.