In what could potentially be a major (if limited to one state) privacy decision, Maryland's Court of Special Appeals has ruled that carrying around a cellphone with GPS features doesn't mean that the police can simply use it to track people's physical locations without a warrant or the protection of the Fourth Amendment.
The court is referring to cell tower simulating devices we've come to know as "stingrays." Use of these tools has spread to law enforcement offices across the country, and the police have been doing everything in their power to keep their uses secret. From The Baltimore Sun:
"We conclude that people have a reasonable expectation that their cell phones will not be used as real-time tracking devices by law enforcement, and — recognizing that the Fourth Amendment protects people and not simply areas — that people have an objectively reasonable expectation of privacy in real-time cell phone location information," the judges wrote.
"The court's opinion is a resounding defense of Fourth Amendment rights in the digital age," added Nathan Freed Wessler, a staff attorney who specializes in technology and privacy with the American Civil Liberties Union. "The court's withering rebuke of secret and warrantless use of invasive cell phone tracking technology shows why it is so important for these kinds of privacy invasions to be subjected to judicial review.
Dan Kobrin, an attorney with the public defender's office who argued the case before the court, said the impact is "enormous" and "will hopefully curb abuse of this device and bring it out into the sunlight." But he said it was unclear whether the ruling can be applied retroactively.
The state's attorney general's office can appeal the decision to the state's supreme court but is still looking over the ruling at the moment, according to the Sun.
Baltimore is one of the cities where police had been using the stingrays and had been concealing that fact from the courts and defendants as part of a non-disclosure agreement to get permission from the Department of Justice to use them. They even agreed to drop cases in order to prevent disclosure of the tools. You can read that non-disclosure agreement here.
You can read the court's ruling here.