MARTIN KASTE, BYLINE: And this is NPR's Martin Kaste. That broad hint, by Justice Alito, that it's time for Congress to tackle this issue did not go unnoticed on Capitol Hill.
SEN. RON WYDEN: They really laid out a red carpet.
KASTE: Senator Ron Wyden, a Democrat from Oregon, had already introduced the Geolocation Privacy and Surveillance Act - yes, the GPS Act - which would require warrants for geolocation data produced by phones or other personal gizmos. He says even though the ruling was a narrow one, it helps make the case that Americans need new privacy laws for new technologies.
WYDEN: You shouldn't have to come back to the United States Supreme Court every time there is one of these kinds of issues. And certainly, law enforcement deserves some clear guidance.
KASTE: Police and prosecutors generally aren't eager to see new warrant requirements. But when it comes to geolocation data, many have already anticipated the tighter rules.
Scott Rowland is assistant district attorney in Oklahoma County, and he says the tracking devices at the center of the case have become very common.
SCOTT ROWLAND: It used to be that these were just used on drug cases, murder cases. But they've become so cheap, so easy to use, and so reliable that I'm aware of a lot of burglary cases - serial burglary cases - where tracking devices are used.
KASTE: Rowland says he could tell this was coming. And he says forward-looking police departments have already started getting warrants for tracking devices, as a matter of course. To him, the bigger issue is what the ruling may mean for that device in your pocket.
ROWLAND: A cellphone is very much a tracking device.
KASTE: Even if a suspect turns off his phone's GPS function, investigators can get its location relative to cell towers. Lower courts differ on whether that requires a warrant. And that question was not answered, Rowland says, by yesterday's narrow ruling.
ROWLAND: But - but it seems to me that the four justices who concurred in this would go farther, to hold that a probable-cause warrant is required to do real-time tracking of a cellphone.
KASTE: And with that kind of shadow hanging over cellphone tracking, Rowland says he agrees it's now time for Congress to clarify matters. That promises to be a messy job. For one thing, Congress would have to decide whether to extend the warrant requirement to phone-location records. Stored data have generally enjoyed less protection than real-time tracking.
It would also have to grapple with the different kinds of location data. Greg Nojeim, senior counsel at the Center for Democracy and Technology, says law enforcement often sees technological distinctions that the average person isn't even aware of.
GREG NOJEIM: The Justice Department manual tells personnel that they should get a warrant for GPS tracking of a cellphone. It doesn't say they should get a warrant for cell tower tracking location information.
KASTE: Distinctions like that don't make sense to privacy activists. And they hope the minority opinions in yesterday's ruling will pave the way to broad privacy protection for location data in general, regardless of whether it's generated by GPS, cell towers, Wi-Fi, or some other - as yet un-invented - form of personal triangulation.
Martin Kaste, NPR News.