Here's how cellphones give away your every move

The technology works by exploiting an essential fact of all cellular networks: They must keep detailed, up-to-the-minute records on the locations of their customers to deliver calls and other services to them. Surveillance systems are secretly collecting these records to map people’s travels over days, weeks or longer, according to company marketing documents and experts in surveillance technology.

The world’s most powerful intelligence services, such as the National Security Agency and Britain’s GCHQ, long have used cellphone data to track targets around the globe. But experts say these new systems allow less technically advanced governments to track people in any nation — including the United States — with relative ease and precision.

Users of such technology type a phone number into a computer portal, which then collects information from the location databases maintained by cellular carriers, company documents show. In this way, the surveillance system learns which cell tower a target is currently using, revealing his or her location to within a few blocks in an urban area or a few miles in a rural one.

Washington Post