In November, an important case before the Supreme Court—United States v. Antoine Jones—will take up the question of whether the warrantless GPS tracking of an automobile violates the Fourth Amendment’s prohibition on unreasonable searches and seizures. (Our colleague Jim Harper has penned an excellent amicus brief arguing that it does.) This comes as the press have focused increasingly on the legally controversial practice of using cell phones and other location‐​aware mobile devices to track suspects. Perhaps unsurprisingly, both the press and many civil liberties advocates are linking these issues together. But it’s important to recognize that they’re actually distinct in many ways: What implications this ruling has for location tracking of phones will depend crucially on why they decide the way they do, because of the differences in the technologies involved. So I want to consider several possible outcomes in the Jones case, and what implications they would have for the broader question of location tracking under the Fourth Amendment.

Just to review the facts: This case involved a drug trafficking suspect on whose car the police surreptitiously installed a GPS tracking device, which they proceeded to monitor for nearly a month, giving them a map of the car’s movements 24 hours a day during the period. Police had sought a court order authorizing the installation, but they installed it outside the jurisdiction covered by the order, and after the order had already expired. Two cases from the early 80s involving more primitive tracking technology—Knotts and Karo—suggest that the constitutional line is at the boundary between public spaces: Tracking a person or their property into a private location, such as a residence, is a Fourth Amendment “search” requiring a warrant, but monitoring limited to the public movements of (say) a vehicle is not, because we have no “reasonable expectation of privacy” in activities that we “knowingly expose” to the general public. (As an intuitive test of this reasoning, ask yourself if, upon finding a GPS tracker that had been affixed to your private car weeks or months ago, you would really react in the same way as if you’d been told another driver had observed some particular commute.)

Still, there are important differences between the tracking technology in use when those cases were decided and modern GPS tracking devices, and still further differences between those and the various forms of cell phone location tracking. As an amicus brief authored by technology experts stresses, the “beepers” used in Knotts were rather like short‐​range geiger counters: They were a supplement to ordinary visual surveillance by police, who still had to follow along, observe where the suspects went, and record their observations. The beepers just ensured that the suspect would not lose the “tail.” In practice, the extent of such monitoring would have to be pretty similar to what any person could normally achieve by unassisted observation—limited in scope, duration, and (crucially) the number of people who could simultaneously be monitored. Tracking via GPS is potentially far more sweeping.

Cell phone tracking, however, has the capacity to be still more invasive. Because people typically carry their phones on their person—and not just when driving in public—it provides a far more detailed picture of a person’s activities, and when the location method used is precise enough, poses a greater risk of crossing the boundary between monitoring in public and private spaces. And while GPS tracking makes feasible the simultaneous tracking of many more people than could be physically tailed, there are still practical limits. Because nothing needs to be “installed” for cell surveillance, in principle it could be used to track entire populations—and indeed, it’s at least possible that tracking on this scale is already happening. When police use so‐​called “stingray” or “triggerfish” technology to track a phone, it is otherwise akin to the use of a GPS tracking device, in that these technologies are deployed by police to directly track a suspect, except that instead of planting a device that emits radio signals, they intercept the signals already being emitted by the suspect’s own phone.

When, instead, tracking is done with the assistance of a telecom, there is a further difference: Police are accessing information that has already been obtained (and, often, would already be retained by) a third‐​party corporation for the non‐​governmental purpose of providing cell service. This form of monitoring also opens the door to retroactive tracing. Government documents released recently under a Freedom of Information Act request reveal that some telecoms now routinely keep years worth of customer location records—though it is unclear whether this includes data from multiple towers (allowing tracking with GPS‐​level precision) and whether these records are continuous, or only cover location at the time a phone is being used to make a call. With these differences in mind, let’s consider different ways the court might rule in Jones, and what it might mean for cellular tracking.

The worst outcome for privacy advocates in this particular case, of course, would be for the Court to simply conclude that GPS trackers are fundamentally no different from the beepers in Knotts: There is no Fourth Amendment privacy interest in location data pertaining to public movements, regardless of how precise, comprehensive, or infeasible to obtain by conventional visual surveillance, and regardless of the technological means by which the information is gathered (assuming it does not involve some other illegal act, such as trespass). This would weigh heavily against finding a Fourth Amendment obstacle to most cell phone tracking that relied on information from a single cell tower, though as computer scientist Matt Blaze explained to Congress last year, even location to the nearest tower is becoming increasingly precise as telecoms deploy “microcells” to cope with ever heavier wireless data traffic. It would not necessarily preclude Fourth Amendment regulation of cell tracking that used triangulated cell tower data, or the handset’s native GPS technology, given the vastly greater risk that such tracking would “follow” suspects into private spaces—forbidden by the ruling in Karo. (This is one reason automobile GPS trackers are often configured not to generate data unless the car is in motion.) It might, however, be technologically feasible in some cases to “filter” such tracking using the known coordinates of public streets, sidewalks, malls, parks, and other non‐​private spaces. If the Court were to rule against Jones, these would constitute a kind of virtual “green zone,” potentially allowing police to monitor freely, so long as recording automatically ceased whenever a suspect’s phone ventured off the public portions of the map. (A similar rule applies to “pen‐​register” surveillance of communications metadata: A monitoring device may be used to capture dialed phone numbers without a full Fourth Amendment warrant, but must be designed so as to avoid capturing any communications content—including touchtone digits entered after a call has connected.)

Now let’s consider some different ways the Court could rule in Jones’ favor. The Fourth Amendment prohibits unreasonable seizures as well as searches, and so the Court could take the path suggested by the dissent in Karo (and in Cato’s own amicus brief):

The attachment of the beeper, in my judgment, constituted a “seizure.” The owner of property, of course, has a right to exclude from it all the world, including the Government, and a concomitant right to use it exclusively for his own purposes. When the Government attaches an electronic monitoring device to that property, it infringes that exclusionary right; in a fundamental sense, it has converted the property to its own use. Surely such an invasion is an “interference” with possessory rights; the right to exclude, which attached as soon as the can respondents purchased was delivered, had been infringed. That interference is also “meaningful”; the character of the property is profoundly different when infected with an electronic bug than when it is entirely germ‐​free.

In a similar spirit, the court could focus, as Justice Brennan’s partial concurrence in Knotts did, on the “physical intrusion of a constitutionally protected area,” which may trigger Fourth Amendment safeguards “even if the same information could have been obtained by other means.” In theory, a ruling on these grounds could raise the bar for physical installation of GPS trackers without affecting cell tracking that involves no physical intrusion. But maybe only in theory.

The wrinkle here is that while normally “search” and “seizure” are separate issues—police may briefly “seize” your laptop while wating for a court order to “search” it, or “search” your property without “seizing”—in this case it seems harder to disentangle the questions. Suppose, for instance, a police officer rudely disposed of his bubblegum by sticking it to your car’s tire or undercarriage. Whatever else we might want to say about this, it strains plausibility to call it a Fourth Amendment seizure. If a GPS tracker is different, it’s precisely because it enables further monitoring—which means the question is likely to be bound up with whether that monitoring itself invades privacy interests. The Court might still want to focus on the “seizure” aspect for the following reason: There are messy arguments (which I’ll examine below) over whether location tracking might be permissible up to a point, and only become a search when it is too protracted or precise, or depending on contingent facts about whether it actually does end up following the suspect into a private space. Ruling that installation of a tracker constitutes a seizure insofar as it creates the capability for more intrusive monitoring by police (and perhaps others with the right equipment) provides a convenient bright line. This kind of reasoning would be unlikely to affect cell tracking, however, where there is no independent invasion of any property interest in the phone that can be distinguished from the monitoring itself.

Next, the court could go the Kyllo route. This case involved use of infrared imaging cameras to detect the telltale heat signature of marijuana grow lamps. As the dissent in that case observed, public evidence that the suspect’s garage was unusually warm could have been obtained by various means that clearly did not offend the Fourth Amendment. Police might, for instance, have observed snow melting more quickly over one part of the roof, and noted this fact as one component of the “probable cause” justifying a physical search warrant. Justice Scalia, writing for the majority, deemed this “quite irrelevant”:

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.

Things get a little bit hairy here: The reasoning in Kyllo leans heavily on the idea that information about the interior of the home is due the very highest level of presumptive deference, and there is perhaps a whiff of circularity to Scalia’s logic. But the core idea is that using sense‐​enhancing technology beyond what is in general public use may be an offensive means of gathering information, even if the particular information obtained is not categorically private, insofar as it could in principle be observed by unaided senses. Given that many states have either passed explicit laws against nonconsensual GPS tracking, or treat it as a form of “stalking,” it seems unlikely that this type of use of GPS technology would become common among the general public.

While it may seem slightly ad hoc, this sort of “general public use” rule provides a kind of neat Kantian way of solving the notorious problem of legally determining “reasonable expectations of privacy” without risking circularity: If we are not prepared to countenance routine use of a method of information gathering by ordinary citizens, then it violates our “reasonable expectations” even if it might otherwise seem analogous to forms of monitoring that are not searches. This relieves the Court of the responsibility of determining purely on the strength of its own intuitions about which features of a technology are relevant to the Fourth Amendment analysis, or when a method of information gathering is “different enough” from traditional, unaided methods to be treated differently. Instead, the Court can defer to public attitudes as: If this is not a method of observation we’re comfortable with everyone using, we will treat it as a search whether or not we can specify precisely why it is more objectionable than alternative methods that are not searches. We note that the public regards them differently, and treat this as prima facie reasonable.

There are, of course, distinctions we can point to. We’ve already noted that GPS allows more detailed and protracted monitoring than would be practical by physical tailing, even aided by a beeper. But it also, crucially, permits tracking in many situations where, even if a person is technically in a “public” space, a reasonable person would be fairly confident that they were not being monitored. Someone driving deep into remote woods at night, or along an empty desert highway at noon, would often reasonably expect that they were not observed—and might not realistically be subject to unaided observation without making it obvious that they were being followed. Beeper tracking, recall, was only effective in contexts where it would be clear that visual monitoring was probable. Since this difference in contextual expectations would not neatly track the distinction between public and private spaces, it might be enough—bolstered by the rarity and frequent legal disapproval of nonconsensual GPS—to support a general warrant requirement for the class of technologies, without regard to whether a particular instance did or did not entail such a contextual violation.

A Kyllo-like rule that shifted the focus from the intrinsically public or private characteristics of the information obtained to non‐​standard (and therefore un-“expected”) technological means of monitoring would probably similarly frown on “triggerfish” or “stingray” devices, which enable police to independently pick up radio signals emitted by a phone in order to track location—in much the same way the thermal imaging camera picked up radiation emitted by marijuana lamps within the home. But it would not necessarily create a barrier to the more common form of cell location tracking, involving cooperation from telecommunications companies. That cooperation is normally obtained by means of a court order—but the order is not necessarily (or, indeed, typically) a full Fourth Amendment warrant based on probable cause. This, the Court might argue, is more like detecting the presence of marijuana growing lamps by subpoenaing the billing records from the power company. According to an unfortunate line of Supreme Court rulings, citizens are typically presumed to waive their privacy interest in data held in such corporate records, even though it might constitute a search for police to obtain the same data directly. This logic, however, would only apply to location data that the provider was already in the practice of retaining for ordinary business purposes. If the police wanted to use telecom facilities to generate or retain more precise or detailed location data than was normal, Courts would be more likely to treat that as analogous to direct police monitoring.

Finally, the Court could follow the path carved out by the D.C. Court of Appeals and apply the reasoning of the so‐​called “Mosaic Theory,” which just to recap briefly, suggests that someone’s “reasonable expectation of privacy” may be violated by the totality of the government’s conduct (in this case, sustained 24‐​hour surveillance over the course of a month) even if it can be broken up into smaller component observations (following someone during a particular public journey, or for a much shorter period) that would not constitute an invasion of privacy. On this reasoning, neither GPS nor cell tracking would necessarily trigger the Fourth Amendment’s warrant requirement; rather, it would depend on the duration and scope of the monitoring.

While the same distinction between direct surveillance and monitoring via third‐​party records would still apply, the logic of the Mosaic argument might tend to cut against the implied waiver of privacy rights in third‐​party records, at least in some cases. That is, one could attempt a parallel argument that even if people knowingly and voluntarily expose approximate information about their cell phone’s location when they use a provider’s network—and even this is frankly dubious—this is not the same as voluntarily exposing a detailed map of their movements, such as might be generated sophisticated triangulation of many particular “check ins.” This type of argument would be weakened when monitoring involved more limited data from a single provider—such as a record of the nearest tower only at times when a call is made or ends. It would have additional force when the data was more detailed, and especially if police aggregated location information from multiple providers—a Verizon phone and a tablet on AT&T’s data network, say—to develop a more precise and detailed map of the target’s movements that would have been available from either provider alone.

Needless to say, adopting this reasoning would have far‐​reaching and unpredictable implications for many types of government information collection beyond location tracking, and constitute a fairly dramatic shift in Fourth Amendment jurisprudence. Thus, while I believe the logic of the Mosaic Theory is essentially sound, and that it captures an important truth about privacy, I find it extraordinarily unlikely that the Court will take this path—and I’d wager that they’ll look for almost any other principle for regulating location surveillance under the Fourth Amendment other than this one, for fear of creating chaos in the lower circuits.

This has been a long and involved post, so let’s review. I’ve identified four broad approaches the Court could take in this case, each of which have different implications for cell location tracking involving either “stingrays” or telecommunications records.

A ruling that simply rejects Jones’ Fourth Amendment claims would imply that relatively imprecise cell location tracking remains similarly unregulated, but would not affect the fundamental Knotts/Karo distinction between strictly public monitoring and tracking that crossed into private spaces. Thus it would not rule out the possibility that a warrant is required for highly precise location tracking, unless such tracking can be automatically limited to public spaces by some filter technology. An opinion based exclusively on a “seizure” rationale might not affect any type of cell tracking not involving physical intrusion, but any reasoning that found GPS tracking to be a constitutionally significant seizure would also tend to imply a privacy interest in location information. A Kyllo-like rule focusing on unusual (un-“expected”) technological methods of surveillance would almost certainly apply to direct police tracking via stingrays and triggerfish, but might not pose an obstacle to tracking based on telecom records, except when the police ask a telecom to generate or retain more detailed location data than it would for ordinary business purposes. Finally, a ruling relying on the Mosaic Theory—probably the least likely outcome—would not prohibit any location tracking technology per se, but require a warrant when tracking became more protracted and detailed than would be feasible using physical surveillance teams. There may, of course, be other options I have not considered—and these are not mutually exclusive—but they are the most obvious broad alternative approaches.

As should be apparent, then, the outcome of this location tracking case does not at all predetermine the Court’s response to the more worrying (and likely more pervasive) practice of cellular location tracking. The rationale the Court relies upon here, though, will at least hint at the shape Fourth Amendment regulation of different methods of cell tracking might take.