EFF: “The Supreme Court handed down a landmark opinion today in Carpenter v. United States, ruling 5-4 that the Fourth Amendment protects cell phone location information. In an opinion by Chief Justice Roberts, the Court recognized that location information, collected by cell providers like Sprint, AT&T, and Verizon, creates a “detailed chronicle of a person’s physical presence compiled every day, every moment over years.” As a result, police must now get a warrant before obtaining this data. This is a major victory. Cell phones are essential to modern life, but the way that cell phones operate—by constantly connecting to cell towers to exchange data—makes it possible for cell providers to collect information on everywhere that each phone—and by extension, each phone’s owner—has been for years in the past. As the Court noted, not only does access to this kind of information allow the government to achieve “near perfect surveillance, as if it had attached an ankle monitor to the phone’s user,” but, because phone companies collect it for every device, the “police need not even know in advance whether they want to follow a particular individual, or when.”

…in Carpenter, Justice Roberts rejected the government’s reliance on the Third Party Doctrine, writing that there is a “world of difference between the limited types of personal information addressed in” prior Supreme Court cases and “the exhaustive chronicle of location information casually collected by wireless carriers today.” The Court also explained that cell phone location information “is not truly ‘shared’ as one normally understands the term,” particularly because a phone “logs a cell-site record by dint of its operation, without any affirmative act on the part of the user beyond powering up.”