In January the Supreme Court said tracking a car by attaching a GPS device to it is a "search" under the Fourth Amendment. But because the decision hinged on the physical intrusion required by that technique, it left unclear what limits the Constitution imposes on surveillance that does not involve touching the target's property, such as cellphone tracking. In the absence of clear guidance, a recent report from the American Civil Liberties Union (ACLU) suggests, law enforcement agencies are making up the rules as they go along.
The ACLU asked 383 agencies about their policies for obtaining location information from cellphone providers and received about 200 responses. "Law enforcement agencies' tracking policies are in a state of chaos," the organization reported. Some police departments do warrantless tracking only in life-threatening emergencies, but many do it routinely, while others let cellphone carriers decide how much legal authorization is necessary.
As examples of permissive policies, the ACLU cited Lincoln, Nebraska, where police "obtain even GPS location data (which is more precise than cell tower location information) without demonstrating probable cause," and Wilson County, North Carolina, where police "obtain historical cell tracking data where it is 'relevant' to an ongoing investigation—a standard lower than probable cause." By contrast, police in Hawaii County, Wichita, and Lexington, Kentucky, have a general policy of obtaining a warrant based on probable cause before tracking a cellphone.
Attitudes about the privacy of cellphone location data vary along with the policies, as reflected in documents quoted by The New York Times. While training notes from the California District Attorneys Association enthused that "subtler and more far-reaching means of invading privacy have become available to the government" thanks to modern technology, a Nevada training manual emphasized that warrantless cellphone tracking "IS ONLY AUTHORIZED FOR LIFE-THREATENING EMERGENCIES" and warned that "continued misuse by law enforcement agencies will undoubtedly backfire."