Law enforcement will need to obtain a court order to place a GPS device on cars they want to track, according to the Supreme Court, which felt that to do otherwise would violate the Fourth Amendment. However, the justices have different ideas as to why. "The government had told the high court that it could affix GPS devices on the vehicles of all members of the Supreme Court, without a warrant," Wired's David Kravets wrote in his report on the decision, which he called "arguably the biggest Fourth Amendment case in the computer age." All the justices felt the government's attaching a GPS device to a criminal suspect's car to track his movements violated his constitutional rights, but as Ars Technica points out, they arrived at their decisions via differing routes that spoke to their political bents:

While the result was unanimous, the reasoning was not. A five-judge majority led by Justice Scalia, and including most of the court's conservatives, focused on the physical trespass involved in attaching the device to the car. Three of the court's liberals signed a concurrence by Justice Alito, a conservative, that would have taken a stronger pro-privacy stance, holding that extended warrantless tracking itself violates the Fourth Amendment regardless of whether the government committed a trespass to accomplish it.

Justice Sotomayor straddled the line. She signed onto the majority opinion, but also filed a separate concurrence in which she endorsed both Scalia's concerns about physical trespass and Justice Alito's broader concerns about the dangers of warrantless GPS tracking.