The US Supreme Court today unanimously blocked GPS tracking by law enforcement without a warrant, at least in certain cases.  The decision concluded that GPS tracking did constitute a “search” within the meaning of the fourth amendment, but various groupings of Justices offered differing rationales.

A good summary of the case is available at Wired’s Threat Level.

“We hold that the government’s installation of a GPS device on a target’s vehicle, and its use of that device to monitor the vehicle’s movements, constitutes a ‘search,’” Justice Antonin Scalia wrote for the five-justice majority. The majority declined to say whether that search was unreasonable and required a warrant.

All nine justices, however, agreed to toss out the life sentence of a District of Columbia drug dealer who was the subject of a warrantless, 28-day surveillance via GPS.

I was particularly impressed by this dicta from Justice Sotomayor, which seems to get at the current reality of modern digital life:

Physical intrusion is now unnecessary to many forms of surveillance.  [In the case of G.P.S. devices], I would ask whether people reasonably expect that their movements will be recorded and aggregated in a manner that enables the government to ascertain, more or less at will, their political and religious beliefs, sexual habits, and so on.

[Further,] it may be necessary to reconsider the premise that an individual has no reasonable expectation of privacy in information voluntarily disclosed to third parties.

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.