Victory! Government must get a warrant to use GPS tracking devices on our cars.

The following guest blog was written by ACLU of Massachusetts privacy rights coordinator Kade Crockford.

In a breathtakingly important decision we've anticipated for months, the Supreme Court today decided, in a unanimous ruling, that the government needs a warrant before placing a GPS tracking device on a car. A number of lower courts had disagreed on the question, raising the profile of the US v. Jones case at the nation's highest court.

The decision today is a major victory for privacy advocates and for all people who want to see a clear translation of basic constitutional rights in the digital age.

While it was the right call on the specifics of the case in question, the decision didn't go far enough. Unfortunately, the ruling does not extend to GPS tracking by other means, such as mobile phone tracking, although a number of justices wish that it had. Justices ultimately wrote three concurring opinions, with Scalia's narrow ruling serving as the majority. Justice Alito wrote one, arguing that the ruling should have extended to other types of GPS monitoring, including that of mobile phones. Justice Sotomayor raised the all important issue of third party information holders in her concurring opinion.

While the justices all agreed that the warrantless tracking in the Jones case violated the Constitution, they differed as to why. Scalia's majority ruling argued that the government had violated the Constitution because, in installing the GPS device on his car, it had invaded Mr. Jones' physical property. Justices Roberts, Kennedy, Thomas and Sotomayor joined Scalia in the majority opinion.

But Justice Alito criticized the narrowness of Scalia's ruling, arguing in a concurrence for four judges that it inappropriately tied 18th century legal constructs to 21st century technologies. Alito is onto something big.

The majority ruling's reliance on the invasion of Mr. Jones' private property leaves unsettled the question of whether the government can access our location or other private data when we do not physically possess it, or when the government never physically invades our homes or cars in order to spy on us.

The New York Times highlights key passages of Sotomayor's concurrence, in which she raises just these issues, warning that the Court did not deal with the question of third party content holders such as telecommunications companies. "It may be necessary to reconsider the premise that an individual has no reasonable expectation of privacy in information voluntarily disclosed to third parties," she wrote.

People disclose the phone numbers that they dial or text to their cellular providers; the URLs that they visit and the e-mail addresses with which they correspond to their Internet service providers; and the books, groceries, and medications they purchase to online retailers...I for one doubt that people would accept without complaint the warrantless disclosure to the government of a list of every Web site they had visited in the last week, or month, or year.

Given the government's keen interest in mobile phone tracking and obtaining information about us from third party holders without warrants, these issues are likely to come before the Court more directly. Since most people in the United States travel with their mobile phones 24/7, and use the internet to communicate and store private information every single day, these questions could not be more important.

For more information, see Marcy Wheeler's analysis on the Jones decision.