About 100 million Americans carry smartphones capable of emitting location data almost continuously. Even some less-sophisticated devices have such capacity, as do the navigation systems in automobiles and some laptop computers. Worldwide, 154 million smartphones were shipped to consumers in just the past three months, according to International Data, a market analysis firm. (The Global Positioning System functions often can be switched off, but that deactivates some phone features.)
Changing technology has long strained the legal strictures of the Fourth Amendment, whose prohibition on "unreasonable" searches and seizures was born of 18th-century law and guides the legal standards for when police can tap phones, use tracking devices and monitor a suspect's Internet activity.
Cellphones always have been trackable to some degree, as users moved among towers that carried the signals necessary to make the devices work, creating an electronic record in the process. But GPS technology is far more sophisticated, narrowing locations typically to within a few feet. Many smartphones relay location data to central servers throughout the day, as users check traffic, search for nearby restaurants or scan weather maps.
Combined with information from toll booths, credit card machines and security cameras, people in highly wired nations often move within a web of data that can allow governments to pinpoint individual movements down to the second.
The location data become even more potentially valuable when associations among people can be mapped, as they are on social media networks. A British research team tracking 25 volunteers in a Swiss town used GPS data, text messages and calling history to pinpoint current movements and predict where individuals were likely to be 24 hours in the future.
Researcher Mirco Musolesi, who teaches computer science at the University of Birmingham, said police could use location data to track suspects, predict crime hot spots, even anticipate political protests if enough potential participants are known. Downloading all the data on a single cellphone tower, as can be done easily with current technology, could allow police to identify those present at a demonstration.