Live in Delaware, New Jersey, or Pennsylvania? You can rest a little bit easier today, knowing that police need a warrant before putting a GPS tracker on your car to monitor your movements. The Department of Justice has declined to appeal a Third Circuit Court of Appeals ruling that the police violated the Fourth Amendment rights of electrician Harry Katzin when they placed a GPS tracker on his van without a warrant.

When the police placed the tracker on Mr. Katzin's van, the Third Circuit hadn't yet ruled on whether or not the government needed a warrant for such a search. Police thought they were on safe legal footing, relying on other circuit court decisions that said they could go ahead without a warrant. That was in December 2010, years before the Supreme Court would rule in Jones that placing a GPS device on a car constituted a physical intrusion deserving of Fourth Amendment protection.

Today, the DOJ filed a petition to the Third Circuit arguing that, while it doesn't challenge the court's opinion on the constitutionality of the GPS tracking, the officers who placed the tracker on Katzin's car did so in good faith, believing that what they were doing was legal. The DOJ therefore wants the Third Circuit to allow the evidence against Katzin to be used in trial. The court's opinion on this question is critically important, especially given how rapidly surveillance technologies develop as opposed to how slowly the law changes around digital privacy.

In its brief today, the Department of Justice is basically saying, "Ok, we won't appeal the substance of your ruling -- this warrantless GPS tracking violated the Fourth Amendment. But how could the police officers have known this at the time? We believe they acted in good faith, and therefore the GPS evidence against Katzin, even though we now know it was obtained illegally, should still be viable in court to use against him."

If the court disagrees with the government on the 'Good Faith Exception' issue here, it will send a strong message to both police departments and legislatures. To police, it would say that pushing the limits of the Fourth Amendment to its absolute extremes won't work -- and that getting a warrant for most surveillance is the safest bet, if they want to use evidence in court. To legislatures, such an opinion would make clear that they need to put the pedal to the metal with respect to passing comprehensive digital privacy legislation. It's important that police have the power they need to investigate serious crimes, and they shouldn't be confused about the state of the law.

Technology changes much faster than privacy law, and more often than not that means police push warrantless use of cutting-edge surveillance technologies to the max, to the detriment of our privacy rights. The Third Circuit now has an opportunity to send a message to police departments and legislatures both: the Fourth Amendment applies in the digital age, so if you want to spy on someone -- no matter what new fancy tool you use -- you're gonna need a warrant for that.

Cross-posted from the ACLU of Massachusetts's Privacy SOS blog.