The Supreme Court handed down a landmark opinion today in Carpenter v. United States, ruling 5-4 that the Fourth Amendment protects cell phone location information. In an opinion by Chief Justice Roberts, the Court recognized that location information, collected by cell providers like Sprint, AT&T, and Verizon, creates a “detailed chronicle of a person’s physical presence compiled every day, every moment over years.” As a result, police must now get a warrant before obtaining this data.

This is a major victory. Cell phones are essential to modern life, but the way that cell phones operate—by constantly connecting to cell towers to exchange data—makes it possible for cell providers to collect information on everywhere that each phone—and by extension, each phone’s owner—has been for years in the past. As the Court noted, not only does access to this kind of information allow the government to achieve “near perfect surveillance, as if it had attached an ankle monitor to the phone’s user,” but, because phone companies collect it for every device, the “police need not even know in advance whether they want to follow a particular individual, or when.”

For years, the government has argued that the sensitive nature of this data doesn’t matter; the mere fact that it’s collected by phone companies makes it automatically devoid of constitutional protection.

This argument is based on an outdated legal principle called the “Third Party Doctrine,” which was developed by the Supreme Court in two main cases from the 1970s involving records of phone calls and bank transactions. Courts around the country had long been deeply divided on whether the Third Party Doctrine should apply to cell phone location information or whether the invasiveness of the tracking it enables should require a more privacy-protective rule.

...there is a “world of difference between the limited types of personal information addressed in” prior Supreme Court cases and “the exhaustive chronicle of location information casually collected by wireless carriers today.”

EFF has been involved in almost all of the significant past cases, and in Carpenter, EFF filed briefs both encouraging the court to take the case and urging it to reject the Third Party Doctrine. We noted that cell phone usage has exploded in the last 30 years, and with it, the technologies to locate users have gotten and continue to get ever more precise.

Thankfully, in Carpenter, Justice Roberts rejected the government’s reliance on the Third Party Doctrine, writing that there is a “world of difference between the limited types of personal information addressed in” prior Supreme Court cases and “the exhaustive chronicle of location information casually collected by wireless carriers today.” The Court also explained that cell phone location information “is not truly ‘shared’ as one normally understands the term,” particularly because a phone “logs a cell-site record by dint of its operation, without any affirmative act on the part of the user beyond powering up.”

We were pleased that the Court cited our amicus brief in its opinion and agreed with many of the points we raised. In particular, Justice Roberts noted that because cell phones generate a record of location information all the time and “because location information is continually logged for all of the 400 million devices in the United States—not just those belonging to persons who might happen to come under investigation—this newfound tracking capacity runs against everyone.” What’s more, cell phone tracking enables the government to compile an “exhaustive chronicle of location information” so that “unlike the nosy neighbor who keeps an eye on comings and goings, [phone carriers] are ever alert, and their memory is nearly infallible.”

As we pointed out, this means that the government can engage in long-term monitoring. In Carpenter, for example, the government obtained 127 days of the defendant’s cell phone records from MetroPCS—without a warrant—to try to place him at the locations of several armed robberies around Detroit. Other cases have involved even longer periods of time. In a footnote, the Supreme Court declined to reach the question of whether very short periods of tracking, less than the 7 days used at trial in Carpenter, might not be covered by the Fourth Amendment. We think the right rule is to require a warrant for any cell phone tracking, but that will have to wait for another day.

Perhaps the most significant part of today’s ruling for the future is its explicit recognition that individuals can maintain an expectation of privacy in information that they provide to third parties. The Court termed that a “rare” case, but it’s clear that other invasive surveillance technologies, particularly those than can track individuals through physical space, are now ripe for challenge in light of Carpenter. Expect to see much more litigation on this subject from EFF and our friends.