Today the Third Circuit Court of Appeals ruled that law enforcement agents must obtain a warrant based on probable cause to attach a GPS device to a car and track its movements. The case, United States v. Katzin, is the first in which a federal appeals court has explicitly held that a warrant is required for GPS tracking by police. The ACLU submitted an amicus brief in the case (joined by the ACLU of Pennsylvania, Electronic Frontier Foundation, and National Association of Criminal Defense Lawyers) and presented oral argument to the court in March.

In this case, police suspected three brothers, Harry, Mark, and Michael Katzin, of robbing several pharmacies. Without getting a warrant from a judge, FBI agents attached a GPS tracking device to Harry Katzin's car in order to follow its movements. The government used the GPS device to track the Katzins as they drove to and from another pharmacy, and arrested them as they drove away. Before trial, the Katzins argued that police had violated their Fourth Amendment rights by using the GPS tracker without a warrant, and the district court agreed. Today's ruling affirms that decision.

Last year, in United States v. Jones, the Supreme Court unanimously held that attaching a GPS tracker to a car to follow its movements is a search under the Fourth Amendment. The Court did not decide whether police need a warrant to conduct such a search, however, and the government argued in this case that an exception to the usual requirement of a warrant should apply. The Third Circuit rejected the government's arguments, explaining that GPS tracking is a "vastly broader endeavor" than the kinds of limited searches that courts have allowed without warrants. In particular, the court held that the "automobile exception" to the warrant requirement does not apply to GPS tracking. The exception is designed to permit warrantless searches of cars to reveal contraband before the cars can drive away. But, as the court explained, that narrow exception does not "permit [police] to leave behind an ever-watchful electronic sentinel in order to collect future evidence" without judicial oversight.

The court also rejected a second argument made by the government: that the so-called "good faith" exception should permit use of evidence derived from the GPS tracking, even if it violated the Fourth Amendment. Because the GPS tracker was attached to the Katzins' car before the Supreme Court decided Jones, and before the Third Circuit had addressed the issue, the government argued that FBI agents couldn't have known that a using a GPS device might raise questions under the Fourth Amendment, and therefore they acted in good faith by choosing not to seek a warrant. The court explained that this does not excuse police from the requirement of getting a warrant:

Where an officer decides to take the Fourth Amendment inquiry into his own hands, rather than to seek a warrant from a neutral magistrate — particularly where the law is as far from settled as it was in this case — he acts in a constitutionally reckless fashion. Here, law enforcement personnel made a deliberate decision to forego securing a warrant before attaching a GPS device directly to a target vehicle in the absence of binding Fourth Amendment precedent authorizing such a practice. . . . Excluding the evidence here will incentivize the police to err on the side of constitutional behavior and help prevent future Fourth Amendment violations.