Cell Phones Use Cells
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 bi-directional 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 analyzes 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.
Triangulation Locates Phones
Police rely on principles of triangulation to track down the phone (and, presumably, its owner). Because the nature of the cellular network allows the phone to communicate with a number of nearby cell towers, and for each cell 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 cell phones—and, presumably, the cell phone user—in general neighborhoods, either in real time or in recent history.
Newer Phones Use GPS
While triangulation based on cell tower signal strength is reasonably reliable, newer cell phones use a slightly different technology to identify their locations. According to the San Francisco Chronicle, federal legislation from 2001 required wireless service providers to accurately identify and place phones on their network to within 328 feet. To achieve cell phone tracking with this level of accuracy, many carriers and phone manufacturers began incorporating Global Positioning System (GPS) triangulation capabilities into telephone handsets. 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.