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arstechnica.com wrote:A federal appeals court ruled today that the authorities must get a warrant before obtaining a suspect's mobile-phone location history. It's a decision squarely at odds with other appellate rulings and one likely to force the Supreme Court's hand.

The ruling (PDF) from the 11th U.S. Circuit Court of Appeals concerns a Florida man, Quartavious Davis, sentenced to life for a string of robberies. His 2012 conviction rested largely on mobile phone records that pegged his location near six of seven heists.

"The court’s opinion is a resounding defense of the Fourth Amendment's continuing vitality in the digital age," said Nathan Freed Wessler, the American Civil Liberties Union attorney who argued the case before the court. "This opinion puts police on notice that when they want to enlist people’s cell phones as tracking devices, they must get a warrant from a judge based on probable cause."

It's the first time a US circuit court of appeals has said cell phone location data is constitutionally protected. However, the circuit's ruling only covers Alabama, Florida, and Georgia.

At least three other federal circuits, there are 13 altogether, have ruled the other way. Wednesday's decision likely means the Supreme Court must step in and settle the issue. The justices often decide issues of national importance if there is a split among the circuits.

In all of the cases, the authorities argued that so-called cell-site records are not constitutionally protected and are business records that telcos may hand over if the government asserts there are reasonable grounds to believe the data is relevant to an investigation.

The decision comes as the government has adopted warrantless cell-site tracking as a surveillance tool in the aftermath of a Supreme Court ruling that said the authorities needed a warrant to affix GPS trackers to vehicles. In that case (from two years ago), the justices ruled that the actual installation of a GPS device on a vehicle was the equivalent of a search usually requiring a warrant.

In the surveillance case decided this week, the Florida defendant argued that the public maintains a reasonable expectation of privacy that their public movements won't be catalogued for the government unless a probable cause warrant is issued by a judge. In this case, there was no warrant.

"Thus, the exposure of the cell site location information can convert what would otherwise be a private event into a public one. When one’s whereabouts are not public, then one may have a reasonable expectation of privacy in those whereabouts," the court ruled.

The appeals court, however, let stand the defendant's conviction based on the theory that the authorities had a "good faith" belief that no warrant was required.