As Jim Harper noted yesterday, the questioning during yesterday’s oral arguments in United States v. Antoine Jones suggested to most observers that the Supreme Court is acutely concerned about the dangers of leaving police use of location tracking technology completely unregulated by the Fourth Amendment. “If you win this case,” Justice Stephen Breyer told the government’s lawyer, “then there is nothing to prevent the police or government from monitoring, 24 hours a day, the public movement of every citizen of the United States.… You produce what sounds like Nineteen Eighty‐​Four.” Yet, as I observed in a previous post, the Court has a wide array of rationales to choose from if it decides to rule in favor of Antoine Jones, and each has different implications for the larger question of how the Constitution will limit a whole array of location tracking technologies.

The simplest and narrowest ruling against the government here — one that seemed to appeal to Justice Scalia — would focus not on the monitoring, but only on the physical intrusion on property involved in the installation of the tracking device. That is, I think, absolutely right as far as it goes, but would only prolong the deeper questions, since there are many high‐​tech ways to track someone without physically attaching anything to their property, from the cell‐​phone tracking already in widespread use, to robotic surveillance insects in the foreseeable future. Many on the Court would seemingly prefer to avoid a game of technological whack‐​a‐​mole, and find a principle to regulate location monitoring itself. The appeals court in this case relied on what’s come to be called the Mosaic Theory, which holds that prolonged monitoring may invade privacy, even if all the specific journeys — all the tiles in the “mosaic” — are public when considered in isolation.

The virtue of the Mosaic Theory is that it captures a deep underlying truth about our complex, socially embedded “reasonable expectations of privacy.” The defect is that courts and police need clear rules: Any principle that requires ad hoc assessment of complex and contextual social facts is a nonstarter. That’s why, after roasting the government’s attorney, the Court tried to force Jones’ lawyer to articulate some kind of bright line test that would distinguish permissible from impermissible monitoring. If tracking someone 24/7 for a month violates people’s expectation of privacy, but following them on a single trip doesn’t, how do you draw the line? Really, this is just a special case of the notoriously difficult “Sorites Problem” in philosophy: Sometimes you confront a gradient or continuum where things on one end obviously do have some property (like “being a privacy violation”), things on the other end don’t, but you can move in arbitrarily tiny steps from one end to the other, and it seems absurd to pick any particular point as a binary boundary.

But this doesn’t seem like it ought to be a problem if we’re thinking clearly — and the Court has already found a perfectly satisfactory way of dealing with it in another famous case: Kyllo v. United States. In ruling that the use of thermal imaging scanners to detect marijuana‐​growing lamps in a home violated the Fourth Amendment, Justice Scalia considered the objection that (much like GPS trackers), the scanners would often reveal only information that could have been obtained by ordinary observation from a public space:

The fact that equivalent information could sometimes be obtained by other means does not make lawful the use of means that violate the Fourth Amendment. The police might, for example, learn how many people are in a particular house by setting up year‐​round surveillance; but that does not make breaking and entering to find out the same information lawful.

The key point here is that there’s a difference between the information exposed by an investigative method, and the investigative method itself. The Court in Kyllo did not say that police may use thermal imaging scanners just to the extent that they reveal information about the home that could also, in principle, be obtained by some other legitimate method (though perhaps with greater difficulty or expense); they opted to regulate the technology categorically. It may be that people knowingly expose their movements to the public, but that’s not what’s initially being monitored here. What police who use GPS tracking are monitoring is signals emitted by a surreptitiously installed tracking device. And that information was certainly not knowingly exposed to the public by Antoine Jones, or most drivers.

It’s an interesting question whether even ordinary visual surveillance in public — whether by a “tail” or by technologies in general public use, like video cameras — could become so prolonged and invasive as to trigger Fourth Amendment protection, but this isn’t ordinary visual surveillance, which relieves the Court of the need to answer that question here. They can regulate the technological method categorically, even if we can envision uses that would be practically equivalent to other, permissible methods in terms of the type and quantity of information obtained. In those cases, of course, police are welcome to simply use those other methods. The main appeal of GPS tracking, of course, is that it provides far more information than is feasible for most police departments to obtain by visual observation — and in those cases, a magistrate judge can make the specific decision about a “reasonable” duration of surveillance. Since the officers in this case actually did get a judicial warrant before installing their tracker — they just failed to install it before the warrant expired—there’s no reason to think that this constitutes some unreasonable burden.

The apparent problem with this approach is that the Court’s opinion in Knotts seems to have rejected such categorical regulation. But, as no less an authority than the inventor of GPS himself stressed in an amicus brief filed by the Center for Democracy and Technology, the tracking “beepers” at issue in Knotts are a fundamentally different technology from GPS trackers. Though they actually monitor radio signals rather than visual impressions — just as GPS does — they are best thought of as augmenting ordinary visual observation by officers who are physically present in a nearby car or aircraft. It might be true in a particular case that police could not effectively tail a suspect without risking detection but for the beeper, because they’d have to follow too closely, but the central method of surveillance here is still ordinary visual observation. The beeper, in this case, is more like a flashlight than a thermal imager. GPS does not augment visual observation; it replaces visual observation. That, I think, means that there’s just no need to puzzle over the permissible quantity of information that can be obtained as though we were still talking about a physical tail with a little technological boost. We’re not. It’s an utterly different method that can be regulated categorically, and without regard to whether some different and unregulated method might yield some of the same information. It’s true that the reason you want to regulate this method is that it has the capability to gather more information, more easily and cheaply, in a way that exceeds people’s reasonable expectations. But that doesn’t mean you have to solve the Sorites Problem and pick some particular marginal quantum of information as the boundary, where monitoring below that level is consistent with social expectations of privacy, and monitoring above that level is not. Rather, you can say that because of its capacity for such intrusive monitoring, people reasonably expect not to be subject to that method at all. The Fourth Amendment, recall, does not just protect against unreasonable searches, but the right to be “secure” against unreasonable searches—and that security would hardly be protected if a method intrinsically capable of “unreasonable” intrusiveness were permitted without a warrant, and all of us had to trust police to determine the point at which its application in each particular case crossed into “unreasonableness.”

The fuzziness of the boundary here, in other words, is no argument at all for deference. It’s an argument for imposing a categorical warrant requirement on a class of technologies and letting the issuing magistrate evaluate the bounds of reasonableness in the instance. As in Kyllo, you draw the bright constitutional line by looking at the capabilities of a technological type, not by trying to craft an impossible rule that permits specific uses of the type that are informationally equivalent to other, permissible methods. It is the capability of the technology to reveal an intrusive “mosaic” that justifies its categorical treatment as a search, and as long as it’s clear that many typical uses of that technology would fall on the wrong side of “reasonable,” the tough question of “exactly how many tiles does it take?” is one the Court can happily decline to answer.