Police say nightclub manager Antoine Jones was the ringleader of a cocaine trafficking operation, and they had enough evidence to convince a jury he was guilty. There's just one problem: a key part of the government's case came from a GPS tracking device the police secretly attached to Jones' car. Jones argues that the installation and use of the device violated his Fourth Amendment rights. On Tuesday, the Supreme Court heard Jones's argument.

This is the high court's first opportunity to address the constitutionality of prolonged, warrantless GPS tracking. In the 1983 case of US v. Knotts, the court ruled that the government didn't need a warrant to hide a short-range electronic transmitter in a drum of chemicals and use it to track a suspect. But in an August 2010 decision, the United States Court of Appeals for the DC Circuit ruled that the Knotts holding didn't apply to Jones's case. The tracking device in Knotts was used for just a few hours to track a suspect during a single trip. In contrast, the police tracked Jones's car continuously for 28 days. In 1983, the Supreme Court specifically said it would withhold judgment on more aggressive "dragnet" surveillance activities for a future case.

That future has now arrived. Tracking technology has advanced rapidly in the last three decades. Devices can be as small as a credit card, and they can be monitored from a great distance.

Every step you take

The government's lawyer, deputy solicitor general Michael Dreeben, argued that because Jones's car was in a public place, the installation of the GPS device wasn't a search under the Fourth Amendment. The police can legally conduct around-the-clock human surveillance of a suspect, and the government argued that installing a tracking device on a suspect's car was just a more efficient—but equally legal—way of accomplishing the same result.

This argument seemed to alarm several of the justices. "If you win this case then there is nothing to prevent the police or the government from monitoring 24 hours a day the public movement of every citizen of the United States," Justice Breyer said. "And no one, at least very rarely, sends human beings to follow people 24 hours a day. That occasionally happens. But with the machines, you can. So if you win, you suddenly produce what sounds like 1984."

"This case does not involve 24-hour surveillance of every citizen of the United States," Dreeben countered. "It involves following one suspected drug dealer as to whom there was very strong suspicion." But he wasn't able to explain how pervasive surveillance would have to get before it became constitutionally problematic.

Chief Justice Roberts asked if the government's theory would allow the police to install tracking devices on the cars of the members of the Supreme Court. Dreeben said it would. He suggested that legislatures might want to place limits on such surveillance, but argued that it didn't raise any constitutional concerns.

Justice Kennedy asked whether the government's theory would allow the installation of a GPS tracking device on a suspect's overcoat. Again, Dreeben argued that it would, provided that it only report information about the suspect's movements in public places.

I'll be watching you

Stephen Leckar, speaking for defendant Jones, focused on the point that the police had trespassed on his client's property when it installed the tracking device. He argued that this action by itself constituted a violation of the Fourth Amendment. This position seemed to win the support of Justice Scalia, who is a skeptic of the "reasonable expectation of privacy" standard the high court first articulated 45 years ago.

But the other justices were unimpressed with this argument. After all, they pointed out, it wouldn't be a constitutional problem if the police placed an inert, credit-card-sized device on the underside of a suspect's car. Conversely, future technological advances, such as satellite surveillance or cell phone tracking, may enable the police to conduct equally invasive searches without a technical trespass.

Justices pressed Leckar on this point, using the example of surveillance cameras at intersections. "Suppose the police suspected someone of criminal activity and they had a computer capacity to take pictures of all the intersections that he drove through at different times of day, and they checked his movements and his routes for 5 days," he said. "Would that be lawful?"

Leckar answered that even pervasive camera surveillance would be constitutional because "people understand nowadays that there may be video cameras out in public space." In contrast, Jones didn't expect that his car would be tracked 24 hours a day for almost a month, so according to Leckar tracking him was unconstitutional.

Of course, this has a troubling circularity to it. It seems to suggest that GPS tracking would become legal under the Fourth Amendment if it became sufficiently commonplace that people started expecting it. But the same reasoning could be used to justify almost any surveillance, provided that the government publicize the fact that the surveillance was being conducted. Surely the legality of surveillance depends on more than just whether people know it's occurring.

The mosaic theory

We found the argument frustrating because the Supreme Court gave very little attention to the "mosaic theory" articulated by the DC Circuit. In its decision last year, the lower court argued that comprehensive information about a person's movements revealed far more information about intimate details of a suspect's life than any specific trip did:

Repeated visits to a church, a gym, a bar, or a bookie tell a story not told by any single visit, as does one‘s not visiting any of these places over the course of a month. The sequence of a person‘s movements can reveal still more; a single trip to a gynecologist‘s office tells little about a woman, but that trip followed a few weeks later by a visit to a baby supply store tells a different story. A person who knows all of another‘s travels can deduce whether he is a weekly church goer, a heavy drinker, a regular at the gym, an unfaithful husband, an outpatient receiving medical treatment, an associate of particular individuals or political groups — and not just one such fact about a person, but all such facts.

This argument seems right to us. Unfortunately, neither attorney seemed interested in defending it. Dreeben, of course, argued that the government should have unfettered discretion to do as much tracking as it liked. For his part, Leckar focused on the specific details of his client's situation: the fact that the tracking device was installed without his client's consent. Perhaps that argument will get his client off the hook. But it won't provide much protection for the rest of us, because if the Supreme Court rules on the narrow facts of the case, the police will simply shift to other techniques for gathering the same data