How do police track cell phones?

Written by keith evans Google
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How do police track cell phones?
Antenna towers evaluate the signal strength of mobile phones. (Duncan Smith/Photodisc/Getty Images)

Cell phone users who want to understand how their phones can be tracked must first understand how the phones allow mobility. In each cellular service area, dozens of cell phone towers maintain bidirectional communication with nearby wireless phones. When a cell phone is turned on, its signal is received by two, three or more nearby wireless towers known as "cells." When the cell phone user makes or receives a call, the cellular network analyses the phone's position and determines which tower, or cell, is best positioned to provide wireless service. As a result of this overlapping service coverage, any mobile phone that is turned on maintains connections with several nearby towers. The phone does not have to be actively engaged in a call to be connected to cells, but it must be turned on; phones in the "off" position or those with no batteries do not register with the cellular carrier's network and cannot be tracked.

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Triangulation locates phones

Police rely on principles of triangulation to track down the phone (and, presumably, its owner). Because the nature of the mobile phone network allows the phone to communicate with a number of nearby antenna towers, and for each tower to evaluate the signal strength of the phone, network analysis software can estimate the distance of the phone from each tower. If the phone communicates with three or more towers, triangulation software can use the phone's signal strength from each tower to estimate the geographic position of the phone on a 3-dimensional plane. Triangulation is not an exact science, however, and software programs are able only to estimate the phone's position rather than precisely pinpoint its exact location. Still, triangulation allows police to place mobile phones -- and, presumably, the mobile phone user -- in general neighbourhoods, either in real time or in recent history.

Newer phones use GPS

While triangulation based on an antenna tower signal strength is reasonably reliable, newer mobile phones use a different technology to identify their locations. In metropolitan areas, where there are high numbers of towers, distances can be pinpointed to within 50 metres. To achieve mobile phone tracking with this level of accuracy, many carriers and phone manufacturers began incorporating Global Positioning System (GPS) triangulation capabilities into handsets.

How GPS works

GPS location works much like cellular triangulation; rather than relying on three local cells, though, GPS relies on very precise signals from 12 or more satellites in low-Earth orbit. With many more reference points, the phone can identify its own location to within a few feet. The phone then uses a software application that runs in the background (and is not visible to the user), to report its location back to the wireless service provider. Because the phone reports its location based on GPS positioning, wireless carriers can relay a relatively precise location for the phone to emergency response teams, police and other government officials.

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