writes with this story about the proliferation of companies willing to sell tracking information and systems. 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 ... It is unclear which governments have acquired these tracking systems, but one industry official ... said that dozens of countries have bought or leased such technology in recent years. This rapid spread underscores how the burgeoning, multibillion-dollar surveillance industry makes advanced spying technology available worldwide. "Any tin-pot dictator with enough money to buy the system could spy on people anywhere in the world," said Eric King, deputy director of Privacy International.