A U.S. federal appeals court ruled Wednesday that law enforcement officials need to have a warrant to access phone location data from cellular carriers.
The ruling, which is the first of its kind in the U.S., raises the bar that such requests must clear and puts the information beyond the reach of court orders.
The U.S. Court of Appeals for the 11th Circuit handed down its decision as part of an appeal lodged by Quartavious Davis, who was sentenced to almost 162 years behind bars for a string of violent robberies.
As part of its evidence against Davis, prosecutors presented cell site location data that included 11,606 location records in addition to details of calls made and received by his phone.
While not as exact as GPS-based location data, the location information identifies the cell tower used to handle the call and can help to establish that a phone was in a general area, the size of which depends on the tower and local surroundings.
Davis appealed his conviction, saying the procurement of the data went against his Fourth Amendment rights. The "Stored Communications Act" allows law enforcement access to the data with a warrant or court order, and they used the latter in the Davis case.
The appeals court decided that a court order isn't sufficient and that "warrantless gathering of his cell site location information violated his reasonable expectation of privacy."
It also dismissed a U.S. government assertion that the data is less worthy of protection because it isn't as precise as GPS data.
"We are not sure why this should be significant. We do not doubt that there may be a difference in precision, but that is not to say that the difference in precision has constitutional significance," the judges said in their decision.
"We hold that cell site location information is within the subscriber's reasonable expectation of privacy. The obtaining of that data without a warrant is a Fourth Amendment violation."
The decision was hailed by civil liberties groups.
"The court's opinion is a resounding defense of the Fourth Amendment's continuing vitality in the digital age," said Nathan Freed Wessler, an attorney with the American Civil Liberties Union (ACLU) in a statement. Wessler argued the case before the appeals court in April.
"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. The court soundly repudiates the government's argument that by merely using a cell phone, people somehow surrender their privacy rights," the statement said.
The 11th Circuit covers Florida, Georgia and Alabama, but the ruling could help set precedent across the U.S.
For Davis, the victory is bittersweet. While agreeing that his Fourth Amendment rights were infringed, the court did not overturn his jail sentence, ruling that the information had been obtained at the time in good faith.
Martyn Williams covers mobile telecoms, Silicon Valley and general technology breaking news for The IDG News Service. Follow Martyn on Twitter at @martyn_williams. Martyn's e-mail address is firstname.lastname@example.org