While the justices unanimously agreed police violated a suspect's Fourth Amendment rights, the Court split on whether the government can track you without a device.
Last fall the Supreme Court heard a case concerning whether police in Washington DC were in violation of the Constitution when they tracked the car of a suspected drug dealer's wife using a GPS device they installed on the car. Today the Court decided that the police did violate the Fourth Amendment, which protects people from "unreasonable search and seizures" and it did so unanimously.
But that's about the extent of the Justices' agreement. They split among three different opinions, and that of the majority -- authored by Justice Scalia and signed by Chief Justice Roberts and Justices Kennedy, Thomas, and Sotomayor -- is as narrow as can be, resting on the specifics of this particular case but saying little as to whether GPS tracking is more broadly permissible. The government's act of physically attaching the device to the undercarriage of the car is a physical trespass, the Court held, something clearly prohibited by the Fourth Amendment as it was originally understood in the 18th century. But, and here is the decision's narrowness, it "may be that achieving the same result through electronic means, without an accompanying trespass, is an unconstitutional invasion of privacy. ... [The] present case does not require us to answer that question." Meaning, in the majority's opinion, the question of GPS tracking is still up in the air.