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How Do Cell Phones Reveal Your Location?

By Brendan I. Koerner
Posted Monday, May 12, 2003, at 5:00 PM ET

As part of its investigation into Jayson Blair's journalistic sins, the New York Times has pored over the reporter's cell-phone records. According to yesterday's lengthy list of corrections, those records indicate that Blair often falsified his whereabouts, claiming to be reporting from West Virginia or Maryland while still in New York. How do cell phones reveal their users' physical locations, and how did the Times get its mitts on Blair's records?

The simplest means of fixing a customer's location is to figure out which tower a call was routed through. When you ring someone up on your mobile, the signal seeks out the closest cell-phone tower. The tower that handled the call is typically logged (and stored indefinitely) on the wireless provider's computers, though it's not noted on the customer's monthly bill. In an urban area, each tower covers an area of approximately 1 to 2 square miles, so a caller's general location is fairly easy to pinpoint.

A newer, more precise method of tracking cell-phone users is via satellite. Many new handsets are equipped with Global Positioning System chips, which determine a caller's coordinates by receiving signals beamed down from a satellite array. The chip factors together the signals' different arrival times to calculate the phone's coordinates, using a mathematical process known as trilateration. GPS-enabled phones are becoming ubiquitous because of Enhanced 911, a Federal Communications Commission rule mandating that emergency operators must be able to trace wireless calls; 95 percent of cell phones must be E911 compliant by the end of 2005. At present, however, GPS data is typically not recorded for non-emergency purposes, unless the user has explicitly signed up for a location-based service.

Location data extrapolated from tower records is frequently used in criminal cases. It was vital, for example, to the prosecution of David Westerfield, who was convicted of murdering 7-year-old Danielle van Dam in San Diego. The killer's cell-phone usage revealed a bizarre travel pattern in the two days following the girl's disappearance, including a suspicious trip to the desert. In cases like this, wireless providers will not release a user's records without a court order, save for rare instances in which a kidnapping has taken place and time is of the essence.

The Blair situation, by contrast, does not involve a criminal proceeding. It is likely that he was using a Times-issued cell phone, a common perk at the Gray Lady. The newspaper, then, was merely requesting to view its own records, rather than those of a third party. There is no federal law that specifically addresses the right of an employer to view an employee's cell-phone records, though case law on workplace surveillance indicates that Blair likely had no reasonable expectation of privacy while using a company phone.

In the absence of concrete legal guidelines, the decision whether to release the records without a subpoena comes down to the wireless provider's whim. The identity of Blair's cell-phone company is a mystery at present; the Times did not respond to Explainer's inquiry.

Next question?

Explainer thanks the Electronic Privacy Information Center.

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Brendan I. Koerner is a contributing editor at Wired and a columnist for Gizmodo. His first book, Now the Hell Will Start, will be published by Penguin Press in May 2008.
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