When the Supreme Court said in United States v. Jones that planting a GPS device on a vehicle is a physical intrusion that amounts to a Fourth Amendment "search," the government should have gotten the memo: the police have to get a probable cause warrant issued by a neutral magistrate before installing one of those devices on a car to track a person's location. Amazingly, that hasn't stopped the government from coming up with new ways to argue that it can use GPS devices to track people's movements without a warrant. So we teamed up with the ACLU and the National Association of Criminal Defense Lawyers in an amicus brief filed last week urging a federal appeals court to reject these arguments.

In the case, United States v. Katzin, FBI agents installed a GPS device on a minivan without a search warrant to track the movements of three brothers suspected of committing a string of robberies. The trial court wouldn't let the government use the the GPS evidence to prosecute the brothers because the government violated the Fourth Amendment when collecting it. The government appealed to the Third Circuit Court of Appeals, arguing that the agents didn't need to get a warrant to gather the GPS evidence because the Supreme Court's decision in Jones held only that warrant is required to install a GPS device, but didn't reach the question of whether a warrant is required to monitor a person's movements using that device. The government also claimed that the officers planted the GPS device before the Supreme Court decided Jones, and so weren't required to get a warrant, since they were relying in good faith on the law as it existed at that time.

As our amicus brief explains, these arguments are nonstarters. First, the Supreme Court has made clear that warrantless "searches" are per se unreasonable and a violation of the Fourth Amendment. Although there are some limited exceptions to this otherwise absolute rule, none of them apply to GPS evidence. The government claimed that the "automobile exception" -- which allows officers to search a car without a warrant if they have probable cause that they will find contraband or evidence of a crime -- applies to GPS searches. But this blows a huge hole through an otherwise narrow exception. The automobile exception is about making sure contraband and evidence don't drive away, not about providing a mechanism for law enforcement to track people.

Second, as the trial court explained, at the time of the GPS installation in this particular case, courts across the country were considering the constitutionality of GPS tracking, and some were finding it blatantly unconsitutional. Most damning, in the case that would eventually lead to the Supreme Court decision in Jones, the D.C. Circuit decided that the warrantless installation and use of a GPS device violated the Fourth Amendment just four months before the FBI planted the GPS device here.

We hope the Third Circuit rejects the government's newest attempt at erasing the long established privacy protections enshrined in our Constitution. If the cops want to use a GPS device to collect details about a criminal suspect's every move, they have to get a search warrant supported by probable cause, period.