According to top-secret documents provided by former NSA contractor Edward Snowden, and interviews with U.S. intelligence officials, the National Security Agency, Fort Meade, Maryland, gathers nearly 5 billion records a day on the locations of cellphones around the world, enabling the agency to track the movements of individuals, and map their relationships, in ways that would have been previously unimaginable.
"The records feed a vast database that stores information about the locations of at least hundreds of millions of devices, according to the officials and the documents, and new projects created to analyze that data have provided the intelligence community with what amounts to a mass surveillance tool.
"The NSA does not target Americans’ location data by design, but the agency acquires a substantial amount of information on the whereabouts of domestic cellphones “incidentally,” a legal term that connotes a foreseeable but not deliberate result.
"One senior collection manager, speaking on the condition of anonymity but with permission from the NSA, said 'we are getting vast volumes' of location data from around the world by tapping into the cables that connect mobile networks globally and that serve U.S. cellphones as well as foreign ones. Additionally, data are often collected from the tens of millions of Americans who travel abroad with their cellphones every year.
"In scale, scope and potential impact on privacy, the efforts to collect and analyze location data may be unsurpassed among the NSA surveillance programs that have been disclosed since June. Analysts can find cellphones anywhere in the world, retrace their movements and expose hidden relationships among the people using them" (http://www.washingtonpost.com/world/national-security/nsa-tracking-cellphone-locations-worldwide-snowden-documents-show/2013/12/04/5492873a-5cf2-11e3-bc56-c6ca94801fac_story.html, accessed 12-06-2013).
The Washington Post provided an excellent information graphic on how NSA tracks people at this link.