The U.S. Supreme Court confronts the digital age again on Wednesday when it hears oral arguments in a case that promises to have major repercussions for law enforcement and personal privacy.

At issue is whether police have to get a search warrant in order to obtain cellphone location information that is routinely collected and stored by wireless providers.

Cellphone thieves caught because they used ... cellphones

The irony of the case before the court, Carpenter v. United States, is that it involves massive cellphone thefts and a string of armed robberies at Radio Shacks in Michigan and Ohio. The robbers entered the stores, guns drawn, herded patrons to the back, loaded up laundry bags with new smartphones, and then later sold their booty to fences for tens of thousands of dollars per haul.

In April of 2011, police arrested four men, one of whom confessed that he and a shifting group of 15 others had robbed nine different stores over the previous year. The suspect identified Timothy Carpenter as one of the ringleaders. The thieves all pleaded guilty, except for Carpenter and his half-brother.

At their trial, the icing on the prosecution's case was the cellphone location information recorded by Carpenter's wireless provider for each of the calls he placed or received on the dates of the robberies.

This was seven years ago, and several smartphone generations ago, too. The information used at Carpenter's trial was not exactly precise. It did not record where he was when he texted, or where he was when his phone was not in use. But when he made or received calls, the cellphone towers nearby recorded his general location, with an accuracy range of about a half mile to 2 miles. And those calls matched up rather nicely with the vicinity of the robberies.

While there was also eyewitness and video evidence against Carpenter, the painfully irrefutable evidence was the cell-site location information, according to Carpenter's trial lawyer, Harold Gurewitz.

The cellphone location data, he says, is "the kind of evidence that, in the end, is the most difficult to argue to a jury that they shouldn't credit, because the records are what they are."

Does the Constitution's ban on unreasonable searches require a warrant?

The question before the Supreme Court is whether the cops should have gotten a search warrant in order to obtain the cell location information. A warrant would have required them to show a judge that they had probable cause to believe those records contained evidence of a crime. What the police did instead was obtain a court order under the federal Stored Communications Act, which is easier.

In this case, as in others, prosecutors argue that the Supreme Court has long viewed information shared by a consumer as fair game without a warrant. Even before the Stored Communications law was enacted, the high court ruled that you lose your Fourth Amendment right to privacy when you share information with a third party, like the phone company.

Fourth Amendment scholar Orin Kerr contends that the idea of tracking someone's movements in public is not new. The police, for instance, tail a suspect, or check on his alibi. Only when they search the suspect's home or person do they have to get a court-approved warrant.

Kerr contends that the cell-cite location records at issue in this case "are basically the network equivalent of public observation that traditionally would not be protected" by a warrant requirement.

After all, he notes, the cell-site location information is not maintained by government decree. Rather, wireless providers keep the data recorded by cell towers in order to monitor and improve their service.

Nathan Freed Wessler of the American Civil Liberties Union is challenging that argument in the Supreme Court. This kind of cellphone technology "really changes the game and threatens to upend our expectation of privacy in the digital age," he says. After all, he argues, this wasn't a case of the police following a shady person.

"They decided after the fact they wanted to try to tie him [Carpenter] to a crime," Wessler says, "and never before in the history of this country has the government had the power to press rewind on someone's life and chart out where they were going over the course of four months."

Four months and nearly 13,000 calls, to be precise.

Technological advancements and the Fourth Amendment

The mere fact that the phone data used to convict Carpenter was held by his service provider does not change the equation, the ACLU's Wessler maintains. Indeed, he notes that tracking a smartphone gets more and more precise by the day, and in some cases, law enforcement is able to pinpoint the building or the office a suspect is standing in.

Wessler concedes that the government may be able to secure that information without a warrant for a short time, and under certain conditions. He thinks the limit for warrantless tracking should be 24 hours. But after that, he argues, it is an unconstitutional search unless the police obtain a search warrant.

Professor Kerr counters that the path between the government's law-enforcement power and privacy rights is not a one-way street.

"This is a technology that can be used to facilitate crime, and also can be used by the government to help solve the crime," Kerr says, "and those, I think, roughly balance each other out."

Indeed, he notes that with private service providers increasingly encrypting data content, it may soon be impossible for the law enforcement to obtain the content of calls and texts even with a search warrant.

With encryption on the rise, Kerr says it could well be that the only smartphone data the government will have ready access to will be records like those at issue in this case — cell-site records, or automatic license-plate reader records, or video taken by surveillance cameras installed in public places.

New rules for a digital age

In the meantime, the Supreme Court has in recent years laid down some new rules for the digital age. In 2012, the court ruled that if the police use a GPS tracking device to monitor a person's life, they need a search warrant. And in 2014 it ruled that if they seize a smartphone at the time of an arrest, in order to view its contents, they need a warrant for that, too.

So the Carpenter case is just the latest battle in what promises to be a long technological and legal war.

Later this term, the justices will hear another case testing whether an email provider, in this case Microsoft, must comply with a search warrant and turn over email that is stored outside the country.

Up until now, the Supreme Court has stuck with the framework it adopted nearly 40 years ago that distinguishes between material in one's home or car, and material that is out in the open, or shared with others. But as Justice Sonia Sotomayor suggested in a case five years ago, the entire framework used in the past may well be "ill-suited to the digital age."

She said that because people now "reveal a great deal of information about themselves in order to carry out mundane tasks," it may be time to reconsider past decisions that allow police to get information without a warrant from third parties like phone companies or banks or email providers.

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