Police cannot surreptitiously stick a GPS unit on your car and track your movements without a warrant, the US Court of Appeals for the District of Columbia has ruled. In an opinion published Friday, the court said that police use of GPS evidence to convict two individuals was a violation of the Fourth Amendment, and that people have a reasonable expectation of privacy when it comes to their movements over an extended period of time.

Warrantless GPS tracking has always been a contentious issue, with supporters arguing that an individual can make similar observations about the location of your car just by driving around town and noting that you're at home, you're at the grocery store, you're at the strip club, and so on.

Detractors, which include the Electronic Frontier Foundation and the American Civil Liberties Union, argue that it's one thing to note someone's car location and another to keep hourly data on every single stop you make along a specific route for days or months on end.

In this particular case, two nightclub owners, Antoine Jones and Lawrence Maynard, had been convicted on narcotics charges in part due to police-collected GPS data. Police had planted* the GPS unit on a car that was parked on private property, then tracked its whereabouts for a month. The government argued that the suspects had no reasonable expectation of privacy because their movements took place out in public.

The appeals court disagreed. "Society recognizes Jones‘ expectation of privacy in his movements over the course of a month as reasonable, and the use of the GPS device to monitor those movements defeated that reasonable expectation," wrote the court.

Both the ACLU and EFF applauded the decision, saying that the Supreme Court had not considered location tracking in such depth and for such a long period of time.

"GPS tracking enables the police to know when you visit your doctor, your lawyer, your church, or your lover," ACLU-NCA Legal Director Arthur Spitzer said in a statement. "And if many people are tracked, GPS data will show when and where they cross paths. Judicial supervision of this powerful technology is essential if we are to preserve individual liberty. Today's decision helps brings the Fourth Amendment into the 21st Century."

The decision does indeed help set a precedent for future cases, though similar decisions vary by state. In 2009, the Wisconsin Court of Appeals ruled that warrantless GPS tracking did not violate an individual's Fourth Amendment rights, while the New York Court of Appeals said that it did.

In Jones' case, his conviction was heavily dependent upon the GPS data collected by police; as a result, the court reversed his conviction. (Maynard wasn't so lucky, as there was other convincing evidence against him.)

* The court documents don't say what police used in this specific situation, but the court's opinion includes a description of a miniature GPS dart, a radio transmitter, and a battery in "a sticky compound material" that will stick to a vehicle when fired. Perhaps we're late to the party, but this is some serious Spiderman tech going on here.