In a rare decision, the Florida Supreme Court ruled last Friday that law enforcement must get a warrant in order to track a suspect’s location via his or her mobile phone.

Many legal experts applauded the decision as a step in the right direction for privacy.

"[The] opinion is a resounding defense of our right to privacy in the digital age," Nate Freed Wessler, staff attorney with the American Civil Liberties Union, said in a statement. "Following people’s movements by secretly turning their cell phones into tracking devices can reveal extremely sensitive details of our lives, like where we go to the doctor or psychiatrist, where we spend the night, and who our friends are. Police are now on notice that they need to get a warrant from a judge before tracking cell phones, whether using information from the service provider or their own ‘stingray’ cell phone tracking equipment."

To be clear, the ruling does not specifically mention stingrays—the devices designed to sweep up, pinpoint, and intercept cellular data in real-time. Providers can achieve the same location-tracking ends on their own. However, the legal reasoning is the same regardless of the means.

"The decision as I read it quickly stands for the proposition that probable cause is required for real-time cell site location information," Brian Owsley, a former federal judge in Texas who is now a law professor at Indiana Tech, told Ars. "This is a significant decision, but not by any means the first time that a court has concluded this. Regarding stingrays, it has significance implicitly in that it can limit the use of stingrays by requiring probable cause to be demonstrated before they can be deployed."

Cops didn't ask for permission to track suspect

The case involves a suspect named Shawn Alvin Tracey, who was convicted in a lower court for possession of more than 400 grams of cocaine. In December 2007, Tracey was traveling to Broward County to pick up more supply, but officers tracked his movements by using real-time cell site location (CSLI) and eventually arrested him. At trial, Tracey attempted to suppress evidence gathered as a result of his movements being tracked, but this was denied. He appealed all the way up to the Sunshine State’s Supreme Court.

Prosecutors, meanwhile, cited earlier case law and argued that Tracey had no reasonable expectation of privacy while traveling on public roads. Plus, local authorities did have authorization to conduct a "pen register" and "trap and trace device" against his phone, thus obtaining the numbers received and dialed.

But as the Florida Supreme Court observed in its decision:

The application did not seek authority—or provide facts establishing probable cause—to track the location of Tracey’s cell phone in either historical or real time; and the order did not ask for access to real time cell site location information. For some unexplained reason, the cell phone information given to officers did include real time cell site location information on Tracey’s cell phone, which the officers then used to track him. … we conclude that Tracey had a subjective expectation of privacy in the location signals transmitted solely to enable the private and personal use of his cell phone, even on public roads, and that he did not voluntarily convey that information to the service provider for any purpose other than to enable use of his cell phone for its intended purpose.

Hanni Fakhoury, a staff attorney at the Electronic Frontier Foundation and former federal public defender, believes the decision can have a positive impact.

"I think it's a significant decision that will have practical effect beyond just police in Florida," he said by e-mail. "The Fourth Amendment protects against unreasonable searches, which means a subjective expectation of privacy that is considered objectively reasonable in society. As we've argued, the more states that require police get a warrant to track a person's location, the more objectively reasonable it becomes to expect location info remains private, requiring police to get a warrant across the country."