For sale: Systems that can secretly track where cellphone users go around the globe

Makers of surveillance systems are offering governments across the world the ability to track the movements of almost anybody who carries a cellphone, whether they are blocks away or on another continent.

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.

While 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, 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.