Check your pocket. You’re probably carrying a tracking device that will allow the police — or even the Trump administration — to track every move that you make.

If you use a cellphone, you are unavoidably revealing your location all the time. Cellphones typically receive service by connecting to a nearby communications tower or other “cell site,” so your cellular provider (and, potentially, the police) can get a decent sense of where you are located by tracking which cell site your phone is currently connected with. Many smartphone users also use apps that rely on GPS to precisely determine their location. That’s why Uber knows where to pick you up when you summon a car.

Nearly a decade ago, in Carpenter v. United States (2018), the Supreme Court determined that law enforcement typically must secure a warrant before they can obtain data revealing where you’ve been from your cellular provider. On Monday, April 27, the Court will hear a follow-up case, known as Chatrie v. United States, which raises several questions that were not answered by Carpenter.

For starters, when police do obtain a warrant allowing them to use cellphone data, what should the warrant say — and just how much location information should the warrant permit the police to learn about how many people? When may the government obtain location data about innocent people who are not suspected of a crime? Does it matter if a cellphone user voluntarily opts into a service, such as the service Google uses to track their location when they ask for directions on Google Maps, that can reveal an extraordinary amount of information about where they’ve been? Should internet-based companies turn over only anonymized data, and when should the identity of a particular cellphone user be revealed?

More broadly, modern technology enables the government to invade everyone’s privacy in ways that would have been unimaginable when the Constitution was framed. The Supreme Court is well aware of this problem, and it has spent the past several decades trying to make sure that its interpretation of the Fourth Amendment, which constrains when the government may search our “persons, houses, papers, and effects” for evidence of a crime, keeps up with technological progress.

As the Court indicated in Kyllo v. United States (2001), the goal is to ensure the “preservation of that degree of privacy against government that existed when the Fourth Amendment was adopted.” More advanced surveillance technology demands more robust constitutional safeguards.

But the Court’s commitment to this civil libertarian project is also precarious. Carpenter, the case that initially established that police must obtain a warrant before using your cell phone data to figure out where you’ve been, was a 5-4 decision. And two members of the majority in Carpenter, Justices Ruth Bader Ginsburg and Stephen Breyer, are no longer on the Court (although Breyer was replaced by Justice Ketanji Brown Jackson, who generally shares his approach to constitutional privacy cases). Justice Neil Gorsuch also wrote a chaotic dissent in Carpenter, suggesting that most of the past six decades’ worth of Supreme Court cases interpreting the Fourth Amendment are wrong. So it’s fair to say that Gorsuch is a wild card whose vote in Chatrie is difficult to predict.

It remains to be seen, in other words, whether the Supreme Court is still committed to preserving Americans’ privacy even as technology advances — and whether there are still five votes for the civil libertarian approach taken in Carpenter.

Geofence warrants, explained

Chatrie concerns “geofence” warrants, court orders that permit police to obtain locational data from many people who were in a certain area at a certain time.

During their investigation of a bank robbery in Midlothian, Virginia, police obtained a warrant calling for Google to turn over location data on anyone who was present near the bank within an hour of the robbery. The warrant drew a circle with a 150-meter radius that included both the bank and a nearby church.

Google had this information because of an optional feature called “Location History,” which tracks and stores where many cellphones are located. This data can then be used to pinpoint users who use apps like Google Maps to help them navigate, and also to collect data that Google can use to determine which ads are shown to which customers.

The government emphasizes in its brief that “only about one-third of active Google account holders actually opted into the Location History service,” while lawyers for the defendant, Okello Chatrie, point out that “over 500 million Google users have Location History enabled.”

The warrant also laid out a three-step process imposing some limits on the government’s ability to use the location information it obtained. At the first stage, Google provided anonymized information on 19 individuals who were present within the circle during the relevant period. Police then requested and received more location data on nine of these individuals, essentially showing law enforcement where these nine people were shortly before and shortly after the original one-hour period. Police then sought and received the identity of three of these individuals, including Chatrie, who was eventually convicted of the robbery.

Chatrie, in other words, is not a case where police simply ignored the Constitution, or where they were given free rein to conduct whatever investigation they wanted. Law enforcement did, in fact, obtain a warrant before it used geolocation data to track down Chatrie. And that warrant did, in fact, lay out a process that limited law enforcement’s ability to track too many people or to learn the identities of the people who were tracked.

The question is whether this particular warrant and this particular process were good enough, or whether the Constitution requires more (or, for that matter, less). And, as it turns out, the Supreme Court’s previous case law is not very helpful if you want to predict how the Court will resolve Fourth Amendment cases concerning new technologies.

The Court’s 21st-century cases expanded the Fourth Amendment to keep up with new surveillance technologies

The Court’s modern understanding of the Fourth Amendment, which protects against “unreasonable searches and seizures,” begins with Katz v. United States (1967), which held that police must obtain a warrant before they can listen to someone’s phone conversations. The broader rule that emerged from Katz, however, is quite vague. As Justice John Marshall Harlan summarized it in a concurring opinion, Fourth Amendment cases often turn on whether a person searched by police had a “reasonable expectation of privacy.”

The Court fleshed out what this phrase means in later cases. Though Katz held that the actual contents of a phone conversation are protected by the Fourth Amendment, for example, the Court held in Smith v. Maryland (1979) that police may learn which numbers a phone user dialed without obtaining a warrant. The Court reasoned that, while people reasonably expect that no one will listen in on their phone conversations, no one can reasonably think that the numbers they dial are private because these numbers must be conveyed to a third party — the phone company — before that company can connect their call.

Similarly, while the Fourth Amendment typically requires police to obtain a warrant before searching someone’s home without their consent, if a police officer witnesses someone committing a crime through the window of their home while the officer is standing on a public street, the officer has not violated the Fourth Amendment. As the Court put it in California v. Ciraolo (1986), “the Fourth Amendment protection of the home has never been extended to require law enforcement officers to shield their eyes when passing by a home on public thoroughfares.”

As the sun rose on the 21st century, however, the Court began to worry that the fine distinctions it drew in its 20th-century cases no longer gave adequate protection against overzealous police.

In Kyllo, for example, a federal agent used a thermal-imaging device on a criminal suspect’s home, which allowed the agent to detect if parts of the home were unusually hot. After discovering that parts of the home were, in fact, “substantially warmer than neighboring homes,” the agent used that evidence to obtain a warrant to search the home for marijuana — the heat came from high-powered lights used to grow cannabis.

Under cases like Ciraolo, this agent had a strong argument that he could use this device without first obtaining a warrant. If law enforcement officers may gather evidence of a crime by peering into someone’s windows from a nearby street, why couldn’t they also measure the temperature of a house from that same street? But a majority of the justices worried in Kyllo that, if they do not update their understanding of the Fourth Amendment to account for new inventions, they will “permit police technology to erode the privacy guaranteed by the Fourth Amendment.”

Devices existed in 2001, when Kyllo was decided, that would allow police to invade people’s privacy in ways that were unimaginable when the Fourth Amendment was ratified. So, unless the Court was willing to see that amendment eroded into nothingness, they needed to read it more expansively. And so the Court concluded that, when police use technology that is “not in general public use” to investigate someone’s home, they need to obtain a warrant first.

Similarly, in Carpenter, five justices concluded that law enforcement typically must obtain a warrant before they can use certain cellphone location data to track potential suspects.

Under Smith, the government had a strong argument that this data is not protected by the Fourth Amendment. Much like the numbers that we dial on our phones, cellphone users voluntarily share their location data with the cellphone company. And so Smith indicates that cellphone users do not have a reasonable expectation of privacy regarding that data.

But a majority of the Court rejected this argument, because they were concerned that giving police unfettered access to our location data would give the government an intolerable window into our most private lives. Location data, Carpenter explained, reveals not only an individual’s “particular movements, but through them his ‘familial, political, professional, religious, and sexual associations.’” Before the government can track whether someone has attended a union meeting, interviewed for a new job, or had sex with someone their family or boss may disapprove of, it should obtain a warrant.

Why a cloud of uncertainty hangs over every Fourth Amendment case involving new technology

One of the most uncertain questions in Chatrie is whether the Kyllo and Carpenter Court’s concern that advancing technology can swallow the Fourth Amendment is still shared by a majority of the Court. Again, Carpenter was a 5-4 decision, and two members of the majority have since left the Court. One of those justices, Ginsburg, was replaced by the much more conservative Justice Amy Coney Barrett.

Justice Anthony Kennedy, who dissented in Carpenter, was also replaced by Justice Brett Kavanaugh. Chatrie is Kavanaugh’s first opportunity, since he joined the Court in 2018, to weigh in on whether he believes that advancing technology demands a more expansive Fourth Amendment.

And then there’s Gorsuch, who wrote a dissent in Carpenter arguing that Katz’s “reasonable expectation of privacy” framework should be abandoned, and that the right question to ask in a case about cellphone data is whether the phone user owns that data. After a long windup about Fourth Amendment theory, Gorsuch’s dissent concludes with an unsatisfying four paragraphs saying that he can’t decide who owned the cellphone data at issue in Carpenter because the defendant’s lawyers “did not invoke the law of property or any analogies to the common law.”

Because Gorsuch’s opinion focuses so heavily on high-level theory and so little on how that theory should be applied to an actual case, it’s hard to predict where he will land in Chatrie. (Though it’s worth noting that Chatrie’s lawyers do spend a good deal of time discussing property law in their brief.)

All of which is a long way of saying that the outcome in Chatrie is uncertain. We don’t know very much about how several key justices approach the Fourth Amendment. And the Court’s most recent Fourth Amendment cases suggest that lawyers can no longer rely on precedent to predict how the amendment applies to new technology.

But the stakes in this case are extraordinarily high. If the Court gives the government too much access to this information, the Trump administration could potentially gain access to years’ worth of location data on anyone who has ever attended a political protest. As the Court said in Carpenter, the government can use your cellphone to track all of your political, business, religious, and sexual relations.

At the same time, the police should be able to track down and arrest bank robbers. So, if there is a way to use cellphone data to assist law enforcement without intruding upon the rights of innocents, then the courts should allow it. The Fourth Amendment does not imagine a world without police investigations. It calls for police to obtain a warrant, while also placing limits on what that warrant can authorize, before they commit certain breaches of individual privacy.

The question is whether this Court, with its shifting membership and uncertain commitment to keeping up with new surveillance technology, can strike the appropriate balance.