This article was originally published on the Cato Institute's @Liberty blog. The views expressed below do not necessarily represent the views of Ars Technica. You can find the original post here

In the summer of 2006, agents of the Drug Enforcement Agency used GPS tracking technology to locate drug courier Melvin Skinner’s prepaid phone, ultimately seizing more than 1,000 pounds of marijuana from Skinner’s mobile home. The judges on the Court of Appeals for the Sixth Circuit then apparently smoked all of it before issuing their ruling in United States v. Skinner this week, because the opinion approving DEA’s use of GPS technology in this case is easily one of the most muddled examples of legal reasoning I’ve ever encountered—a surreal potpourri of factual misunderstandings, inapt analogies, sloppy and selective appeals to precedent, and logical leaps worthy of Nijinsky.

A very brief summary of the case: DEA was already investigating a drug trafficking organization, and through the use of lawful wiretaps learned that a courier code-named "Big Foot" would be driving a large shipment of marijuana from Tucson, Arizona to Mooresburg, Tennessee in his mobile home. "Big Foot" was using a prepaid (or "burner") mobile phone purchased for him by his co-conspirators, which meant one thing the DEA didn’t know was Big Foot’s identity, because the prepaid phone wasn’t registered in his name. (While this makes them appealing to drug dealers, they’re also very popular with ordinary, law-abiding citizens: prepaid phones now account for 25 percent of mobile phone subscriptions.) Agents then obtained a court order—but not a search warrant based on probable cause—to "ping" the phone’s GPS chip and precisely track its location in realtime. Tracing it to a truck stop near Abilene, Texas, authorities brought drug dogs to sniff the perimeter of the mobile home, and when the dogs alerted the agents to the presence of drugs, they performed a search—finding the drugs and arresting Skinner, now revealed as "Big Foot."

The Sixth Circuit’s Fourth Amendment analysis is disturbing right from the outset. "If a tool used to transport contraband gives off a signal that can be tracked for location," the argument begins, "certainly the police can track that signal. The law cannot be that a criminal is entitled to rely on the expected untrackability of his tools." This is bizarre and circular: It suggests that criminals categorically lack Fourth Amendment privacy interests in any "tool" they use to conduct criminal activity, in which case no search would violate the Fourth Amendment if it actually turned up evidence of criminal conduct. But the whole point of requiring a warrant is to let a neutral magistrate determine whether there’s probable cause to believe such conduct will be uncovered. The court hastily acknowledges this in a footnote, clarifying that there’s no expectation of privacy for anyone in cell phone GPS data, but beginning in this way suggests the court is reasoning backward to a desired conclusion, based on Skinner’s now-established guilt.

The court proceeds through a series of lazy and underdeveloped analogies:

Otherwise dogs could not be used to track a fugitive if the fugitive did not know that the dog hounds had his scent. A getaway car could not be identified and followed based on the license plate number if the driver reasonably thought he had gotten away unseen. The recent number of cell phone technology does not change this. If it did, then technology would help criminals but not the police. It follows that Skinner had no expectation of privacy in the context of this case, just as the driver of a getaway car has no expectation of privacy in the particular combination of colors of the car’s paint.

But it does not follow at all. "What a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection," the Supreme Court explained in the seminal case of Katz v. United States, "But what he seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected." Any member of the public can buy a dog and follow a scent. Any member of the public can view and copy down a license plate number. Any member of the public can view the external paint job of a car. But any member of the public cannot just track the GPS signal of a random cell phone—and if they could, most of us would be extremely wary about carrying cell phones. Unlike all these other examples, GPS tracking as employed here depends crucially on the ability of police to invoke state authority—a seemingly salient distinction the court fails to take any note of.

Finally, the judges move on from these dubious analogies and cite an actual precedent: United States v. Knotts. In Knotts, police had placed a relatively short-range tracking "beeper" in a can of chloroform sold to suspected drug manufacturers. Having monitored the sale of the canister, police followed the car in which the suspects had placed it, using the beeper to supplement their visual observation of the car’s public journey, and ultimately relying on it to recover the trail when they lost the suspects. The Supreme Court held that no Fourth Amendment expectation of privacy had been violated, because the location of the car police had been tailing was information exposed to any observer on the road. Here too, the court reasoned, Skinner’s RV was moving along public roads visible to any member of the public.

Unfortunately, this falls apart as soon as you begin thinking about it for a moment. The information that is exposed to the general public, in all these cases, is that a car with a particular external appearance is at such-and-such location at such-and-such a time. Having already observed their lojacked canister being loaded into the suspect’s car, that was the very information the police needed to maintain their tail.

Skinner presents a very different situation. It’s true that any member of the public could observe that Skinner’s RV was one of probably hundreds traveling on public highways in mid July of 2006. But that was not the information investigators relied upon here. What the GPS tracking here revealed was the non-publicly observable fact that one particular mobile home, which police had never encountered before, contained a particular phone believed to be in close proximity to illegal drugs. These are importantly different facts. The Secret Service even relies on that difference to help protect the life and safety of the president. Anyone can observe dozens of limos or SUVs with tinted windows leaving the White House on public roads each day, but which of these contain the president is not so exposed. Because the salient fact for the purposes of the police investigation was not the location of a particular vehicle on public streets, but rather the location of a particular personal effect in a private mobile home, the relevant precedent isn’t Knotts, but rather United States v. Karo. The government might then argue that the contents of a mobile home aren’t entitled to the same high level of protection as the contents of the residence at issue in Karo, but having incorrectly framed the issue, the Sixth Circuit panel never takes up that question.

The court does at least gesture in the direction of the idea that it might somehow matter that police didn’t initially know the identity of "Big Foot," and had not previously observed his vehicle. They deem this immaterial with a truly breathtaking bit of hand-waving:

As for not knowing his identity, this is irrelevant because the agents knew the identity of Skinner’s co-conspirators and could have simply monitored their whereabouts to discover Skinner’s identity. Using a more efficient means of discovering this information does not amount to a Fourth Amendment violation.

It is, I think, an open question what police could or could not have discovered in a parallel universe where they employed a completely different set of investigative methods (as opposed to the physical tail actually employed in Knotts and supplemented by a beeper), but it’s not clear why this is really germane. That a postal letter might have been retrieved from the trash of a suspect who never shreds his correspondence does not make it any less an illegal search to intercept the unopened letter. Remarkably, the court does not deign to even mention the 2001 Supreme Court case Kyllo v. United States, which contemplated and rejected a similar argument. Writing for the majority, Justice Scalia explained that the use of thermal imaging to detect marijuana growing lights in a garage was not immunized from Fourth Amendment scrutiny by the fact that other permissible means might have revealed facts about the temperature of a home:

The dissent’s comparison of the thermal imaging to various circumstances in which outside observers might be able to perceive, without technology, the heat of the home–for example, by observing snow melt on the roof [...]–is 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.

That argument seems to present at least a potential problem for the court’s reasoning here, and seems more generally relevant insofar as it concerns the use of technology to gain information about the contents of a home, but again, Kyllo is not even mentioned.

Since this is a case involving phones, the Sixth Circuit also takes a stab at breezily invoking Smith v. Maryland—the basis for the much-criticized "third-party doctrine"—where a suspect was held to lack an expectation of privacy in dialed phone numbers that had been voluntary exposed to the phone company, which routinely retained that information in its ordinary business records for billing purposes. Without much analysis the court asserts that "[s]imilar reasoning compels" a parallel conclusion here.

But on closer examination, the fact that both cases involve phones is about as far as the similarities go. Dialed numbers are information actively and consciously exposed to the phone company by the user, and then retained in billing records as a matter of course, independently of any government investigations. Unfortunately, as both Orin Kerr and Jennifer Granick note, the court seems fuzzy on the very different characteristics of the GPS technology used here. The GPS chip contained in the phone—which apparently the suspects were unaware of—would not normally transmit any information to the phone company at all. Rather, the chip would have calculated its precise location coordinates and transmitted them to the company only in response to a "ping" initiated by law enforcement.

To be sure, the company might be physically capable of sending such a ping on its own, just as it would be physically capable of intercepting the contents of a phone call. And if it had built the phone with a secret capability to be remotely activated as a microphone, it would also be physically capable of remotely gathering information about the user’s activities in that way as well. Fortunately, the Fourth Amendment is not limited to pointlessly prohibiting only physically impossible surveillance. The technological capabilities of the phone company or the government do not determine what has been "knowingly exposed"—and it seems clear here that Skinner did not knowingly expose, to either the general public or the phone company, the precise GPS coordinates of his phone.

The Fourth Amendment status of the kind of GPS tracking employed here is hardly a slam dunk either way. There are strong arguments on both sides, and the issues involved are complex. Alas, this opinion doesn’t even begin to address them adequately.