We techies know this, but most people don't ... Cell phones, whenever on (and there's some question about whether a few of them do so even when off) periodically "check in" with the cellular carrier. This happens both on a time basis (perhaps every ten minutes) or when moving from one tower area to another.
(- paper edition had some good graphics -)
How cell phone helped cops nail key murder suspect
BY ALISON GENDAR and ADAM LISBERG; DAILY NEWS STAFF WRITERS Wednesday, March 15th, 2006
No one saw the fiend who dumped Imette St. Guillen's body by the muddy shoulder of a dead-end Brooklyn road.
But a tall, blue electronic sentinel stands just around the bend and sources say it puts prime suspect Darryl Littlejohn at the scene about
2 1/2 hours before St. Guillen's corpse was found.The NYPD traced Littlejohn to that lonely corner of East New York, off the Belt Parkway, by tracing the invisible "pings" that his T-Mobile cell phone sent to the antenna-studded tower, sources said.
The big, blue tower apparently took notice that Littlejohn's cell phone was nearby, even though he wasn't making a call and it stored that information, which was later retrieved from T-Mobile by cops.
"It's a way to track people that is stronger than relying on witnesses," a police source said.
[ snip ]But even when you are not making a call, your phone is sending out a regular "check" signal. In major cities you will be within range of several phone masts. Computers can compare the signal strength emitted by your phone and the tiny time lags at each aerial, giving the operator several accurate reading s.
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