Your phone knows where you were

Your phone knows where you were

1. To stay connected, a cell phone keeps connecting to the nearest of its carrier’s towers, and the carrier records which tower and when. Over a few months these records add up to a detailed history of where a person has been. In Carpenter v. United States the US Supreme Court decided whether investigators can obtain that history from the carrier without a warrant. The answer shapes how well the data companies keep about us is protected.

2. The case began in 2011 in Detroit, where police arrested four men suspected of robbing electronics and cell phone stores. One of them confessed that over four months the group had robbed nine stores in Michigan and Ohio. He named fifteen accomplices and gave the FBI the phone numbers of some of them. That led investigators to Timothy Carpenter, and they set out to learn where his phone had been.

3. Prosecutors asked the court not for a warrant but for a simpler order under the Stored Communications Act. It required only reasonable grounds to believe that the records were relevant to an investigation. Two carriers handed over records for 127 days and for 2 more days: 12,898 location points in all, an average of 101 a day. At trial, maps of these points showed that Carpenter’s phone was near four of the robberies. He was sentenced to more than 100 years in prison.

4. Before trial Carpenter moved to suppress the carriers’ records because they had been obtained without a warrant. He relied on the Fourth Amendment to the US Constitution, which protects people, their houses, papers and effects against unreasonable searches and seizures. A warrant is a judge’s permission to search; under the Amendment it issues only upon probable cause and must particularly describe what is to be searched for. So the Court had to decide whether asking a carrier where a phone had been is a search.

5. The answer was not obvious, because the government relied on an old rule known as the third-party doctrine: information a person voluntarily hands over to someone else, such as a bank or a phone company, the state can obtain without a warrant. In the 1970s the Supreme Court held so for bank records and dialed numbers. Cell-site records, the government insisted, are just such business records of the carrier. The court of appeals agreed and upheld the conviction.

6. On June 22, 2018 the Supreme Court sided with Carpenter by five votes to four. It held that acquiring cell-site records is a search within the meaning of the Fourth Amendment, so investigators generally need a warrant for it. The order the prosecutors used is not enough, because its standard is much lower. The judgment of the court of appeals was reversed and the case remanded for further proceedings. Chief Justice John Roberts wrote for the majority.

7. The Court declined to extend the old rule to these records and explained why. Dialed numbers or bank records reveal little about a person, while cell-site records add up to an almost complete chronicle of movements, because the phone goes with its owner both home and to the doctor. Carriers keep them for up to five years, so anyone’s past can be traced. Besides, the phone leaves this trail by itself, without any action by its owner, and getting by without a phone today is hard.

8. For investigators the judgment sets a new requirement: to compel a carrier to hand over a subscriber’s location history, they must go to a judge for a warrant and show probable cause. The simpler order is no longer enough for such records. The Court left an exception for exigent circumstances, such as pursuing a fleeing suspect, an imminent threat to people or a risk that evidence will be destroyed. Then the records can be obtained without a warrant.

9. For ordinary people what matters is the principle. Under the old rule, information a company keeps about you could usually be obtained by the state free of the Fourth Amendment’s limits. The Court said this does not work for a detailed chronicle of movements: the fact that a third party collects it does not strip it of protection. At the same time, the Court said, in the great majority of investigations the government will still be able to obtain records from companies without a warrant.

10. The Court itself called its decision narrow. It concerns records of past movements and does not answer whether a warrant is needed to track a phone in real time or to get a list of all phones that connected to one tower. The third-party doctrine for bank records and dialed numbers still stands, and the decision does not touch security cameras. Nor does it mean Carpenter was acquitted: the Court only reversed the court of appeals and sent the case back.

11. One thought is worth taking from the case: data about you does not lose protection just because a company keeps it, but the protection depends on how much detail it reveals about your life. When someone says US police can no longer find out where a phone has been, ask what exactly is meant: records from past months, tracking right now or a street camera. The Court answered only the first.