NY Times 
April 1, 2012
WASHINGTON — Law enforcement tracking of cellphones, once the province mainly of federal agents, has become a powerful and widely used surveillance tool for local police officials, with hundreds of departments, large and small, often using it aggressively with little or no court oversight, documents show.
The practice has become big business for cellphone companies, too, with a handful of carriers marketing a catalog of “surveillance fees” to police departments to determine a suspect’s location, trace phone calls and texts or provide other services. Some departments log dozens of traces a month for both emergencies and routine investigations.
With cellphones ubiquitous, the police call phone tracing a valuable weapon in emergencies like child abductions and suicide calls and investigations in drug cases and murders. One police training manual describes cellphones as “the virtual biographer of our daily activities,” providing a hunting ground for learning contacts and travels.
But civil liberties advocates say the wider use of cell tracking raises legal and constitutional questions, particularly when the police act without judicial orders. While many departments require warrants to use phone tracking in nonemergencies, others claim broad discretion to get the records on their own, according to 5,500 pages of internal records obtained by the American Civil Liberties Union  from 205 police departments nationwide.