In a landmark decision in Commonwealth v. Rousseau, the Massachusetts Supreme Judicial Court ruled this week that people "may reasonably expect not to be subjected to extended GPS electronic surveillance by the government" without a search warrant -- whether they are driving the vehicle in question or not.

Police obtained a search warrant to install a GPS device on a car owned by a man suspected in a number of arsons throughout the state and tracked him while he drove the car with his friend and frequent passenger, Rousseau, for over 30 days. After being arrested and charged, both the owner and passenger Rousseau sought to challenge the GPS evidence, arguing that due to misrepresentations in the warrant application, the warrant was invalid. The trial court agreed the misrepresentations made the warrant invalid, but upheld the surveillance anyway, finding that neither the driver or the passenger had a reasonable expectation of privacy in their movements and that for the driver, the physical installation of the GPS device didn't trigger state or federal constitutional scrutiny.

After the trial court's decision in 2007, the Massachusetts Supreme Judicial Court ruled in 2009 in Commonwealth v. Connolly that the physical installation of a GPS device was a "seizure" under Article 14 of the Massachusetts constitution that required a search warrant. Then in 2012, the U.S. Supreme ruled in United States v. Jones that the physical installation of a GPS device was a "search" under the Fourth Amendment that required a search warrant. These decisions meant the driver had standing to challenge the installation of the GPS device on his car. But what about the passenger, Rousseau? Since he didn't own the truck, these decisions didn't determine whether he had a right of privacy in his public movements. That was the issue confronting the Massachusetts high court.

Thankfully, Rousseau had some powerful allies. Because although the majority opinion in Jones decided the constitutional issue by focusing on the government's physical interference with private property for surveillance purposes, in two separate concurring opinions written by Justices Sotomayor and Alito, five justices noted that people had a reasonable expectation of privacy in their public movements, meaning a search warrant was required before the police could engage in location surveillance.

Following that decision, we've argued to numerous courts that they should follow the Jones concurrences and find a reasonable expectation of privacy in a person's location -- meaning a search warrant is required before the police engage in cell site or GPS tracking. We did the same thing here, filing an amicus brief with the help of our friends at the Berkman Center for Internet and Society at Harvard Law School, explaining that everyone has a reasonable expectation of privacy in their public movements, whether they're sitting in the left or right side of the car.

Thankfully, the Massachusetts Supreme Judicial Court agreed with us and found that people have a reasonable expectation that their every movement won't be tracked 24/7 for days, weeks, and months on end. And while the decision only applies in Massachusetts, it's important for state courts and legislators to protect their citizens' privacy concerns and build momentum for other state courts and legislatures -- as well as federal courts and Congress -- to do the same. That way, we ensure advances in technology don't erode our privacy protections.