You probably know, or should know, that your cellphone is tracking your location everywhere you go. But whether law enforcement officials should have access to that data is at the center of a constitutional debate.

Matt Blaze, a professor of computer and information science at the University of Pennsylvania, says location tracking is key to how the cell system operates.

"As you move around, your phone is constantly checking to see whether the tower that it's currently registered with is the best one, or whether there's a better tower with a stronger signal coming in range," he says. Cellphone companies store that information so they can deliver better service.

That's handy for the police. Law enforcement agencies across the country already subpoena phone location data regularly. The district attorney for Suffolk County, Mass., regularly asks phone companies for cellphone location information.

The subpoenas are "part of almost every major case, including homicide, in some cases, sexual assault, drug trafficking cases," says Jake Wark, a spokesman for the office.

While the National Security Agency has conceded that it does collect records of U.S. phone traffic, it says it does not currently track the location of cellphones. But the agency also says that it would be legal to collect that information.

In Massachusetts, A Test Case

In many states, the use of that data has led to a movement to protect cellphone location information. One cellphone search, in particular, could serve as a test case for civil liberties groups challenging law enforcement's access to such information.

Shabazz Augustine stands accused of murdering a former girlfriend nine years ago. Massachusetts state prosecutors want to use information they got about the location of his cellphone at the time.

Matt Segal, legal director of the ACLU of Massachusetts, told the state's high court that the evidence should be thrown out, because police got it using a simple subpoena, not a search warrant.

"All the government has to show is that the information they're requesting is relevant and material to an ongoing investigation," Segal says.

That standard is too low, he says, and encourages searches before a crime is committed — like the collection of nearly all the nation's phone traffic by the NSA. The government relies on that same relevance standard to justify collecting bulk phone records.

Now, groups like the ACLU are arguing in court that widespread use of cell location data shows that digital information needs stronger protections. Segal says it should be released only when it meets a higher standard: probable cause to show someone has committed a crime.

"What we're focused on is the possibility that governments are obtaining this kind of location information on many people who have not committed crimes," Segal says.

The 'Third-Party Rule'

The New Jersey Supreme Court has already backed the idea that cellphone location information is so revealing, it should be better-protected. Montana, Maine and other states have also passed laws backing the same approach.

Those state efforts have given hope to civil libertarians that they can get the federal courts to go along — something that might put chains on the NSA. But Orin Kerr, a law professor at George Washington University, says, not so fast.

"The state cases have no direct impact on federal standards, because they're interpreting state constitutions, not the federal Constitution," he says.

Federal courts have ruled consistently that the location of your cellphone is just like any other business record: already out of your possession, and not protected by the Fourth Amendment. Until the U.S. Supreme Court says otherwise, Kerr says, information you give to a provider is fair game.

"So far, courts have applied the third-party rule across the board. So as long as the information is disclosed to a third party, that's it," he says.

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