You see it on TV cop shows — the bad guy’s lie about his whereabouts that night is refuted by the cell phone records, records that show his call was picked up by a tower in the neighborhood of the murder. It’s reassuring to know the good guys can lock up killers with such technology. But what if the tower tracking information does not put the suspect in the neighborhood as definitively as police claim? Or what if it is used to inappropriately prejudice a jury against a defendant, such as when an out-of-towner is shown to have been in a neighborhood known for drug dealing?

Yet again, emerging technology is threatening the constitutional right to privacy. If that is too extreme a conclusion, then at the very least, cell phone tracking is providing law enforcement with new tools to incriminate suspects, leaving civil libertarians to play catch-up and persuade courts that some standards must be set for gathering that information.

A case now before the 3rd Circuit Court of Appeals in Philadelphia may provide guidance on when police can access such cell phone records.

The Justice Department argues that police need show only “reasonable grounds” to get the records if they believe they are relevant to an investigation. But a year ago, a federal magistrate judge in Pittsburgh disagreed, and said prosecutors must meet a “probable cause” standard, which is higher.

U.S. Magistrate Judge Lisa Pupo Lenihan wrote that “citizens continue to hold a reasonable expectation of privacy in the information the government seeks regarding their physical movements-locations — even now that such information is routinely produced by their cell phones … the government’s investigatory search of such information continues to be protected by the Fourth Amendment’s warrant requirement.”

The Justice Department appealed the ruling to the 3rd Circuit.

Location data are recorded about once every seven seconds whenever a cell phone is turned on. And newer, “smart” phones have GPS chips that make locating a user even easier. But Zachary Heiden, the Maine Civil Liberties Union’s legal director, says some experts say the tracking results are “absolutely junk science.”

But beyond that, he believes the standard now in place — reasonable grounds, or heightened suspicion — is too low. Judges understand that threshold to mean something more than a hunch. Probable cause, Mr. Heiden explains, means police have facts that lead them to believe it is more likely than not the suspect is guilty. Even that standard is fairly low, he says.

“The right to privacy is a large, abstract construct,” Mr. Heiden said, which people readily believe in. But when that right is chipped away in “small intrusions,” there is little public outrage.

“Freedom of movement and association don’t mean much if the government is always able to track us,” he says.

Law enforcement needs to gather information to protect the public. But the fourth Amendment provides a balance between a blank check for police and complete privacy. “It only works if there’s a standard for that,” Mr. Heiden correctly asserts.