BROOKE GLADSTONE: From WNYC in New York, this is On the Media. I'm Brooke Gladstone.

BOB GARFIELD: And I'm Bob Garfield. This week a 20-year-old California college student named Yasir Afifi took his car in for an oil change. There mechanics found a strange device attached by magnet to the undercarriage, a device called an Orion Guardian ST820. That's a GPS tracking device that had been placed there by the FBI as part of a terrorism investigation. Agents this week found themselves in the embarrassing position of demanding that Afifi return their super-double secret gizmo. He did. But what about satellite tracking an American citizen's every movement? Is it legal? Do authorities need a warrant? At the moment that depends on where you live. Orin Kerr is a law professor at George Washington University, where he's been following this issue. Orin, welcome to the show.

ORIN KERR: Thank you.

BOB GARFIELD: Let's start with the recent California Circuit decision. What happened there?

ORIN KERR: That was a case in which the government wanted to monitor the location of a person who the government thought was involved in growing marijuana. And so the government installed a GPS device about the size of the bar of soap on the suspect's car in his driveway. The government used that information to show that he was, in fact, near where the ma — marijuana was growing and to show his involvement in the crime.

BOB GARFIELD: Worked like a charm, in fact. But, the defendant appealed on the grounds that the tracking by GPS constituted a warrantless search and a violation of the Fourth Amendment.

ORIN KERR: The Court did not agree that it was an unconstitutional search, but Judge Alex Kozinski, a particularly influential judge on that Court, said that it should amount to a search because GPS surveillance is just so different from any other kind of outside surveillance we've seen before. And there's been a disagreement more broadly among Federal judges and state judges as to how the Fourth Amendment to the Constitution should apply to this new kind of GPS surveillance.

BOB GARFIELD: For example, there was a decision by the District of Columbia that found unequivocally that GPS tracking does constitute a search and does require a warrant. How did that district look at a very similar set of facts and draw such a different conclusion?

ORIN KERR: The D.C. court came up with what's really a pretty novel theory of how to apply the Fourth Amendment. Their view was that you look not at whether a particular act by the government invades privacy, but rather whether a set of acts over a period of time invades privacy. So the D.C. Circuit Federal Court of Appeals said that installing a GPS device for a short period of time is not a search, but once it happens over a long period of time, in the — in that case, one month, eventually you get a point where the government gathers so much information, that that invades someone's privacy. It's really the accumulation of evidence over time.

BOB GARFIELD: Yeah, you had a pretty hi — hilarious line in your piece. You know, you were wondering what miraculous technology the government was able to determine when enough data had been collected as to render it a Fourth Amendment search. You said, what do they have, an aggregotometer?

ORIN KERR: Yes, this is the problem with saying that over time eventually the line gets crossed, is okay, you're a police officer watching somebody, [LAUGHS] when's the line crossed? You need to know that in order to know how to follow the Constitution. And, for that reason, the Supreme Court has emphasized that the rules need to be clear. The real question here is does installing a GPS device amount to a highly invasive search, akin to entering someone's home, or is it more like observing somebody in public? And that's the kind of issue that the courts have really divided on.

BOB GARFIELD: It seems to me that the analog that is most obvious is the difference between a GPS search and having a bunch of policemen in unmarked cars staking out a suspect and just following him all over creation. What makes the GPS search different than just an ordinary tail?

ORIN KERR: The idea is that GPS surveillance is so easy to conduct and is so detailed that it just is in a separate category. So in a pair of Supreme Court cases in the early 1980s, the Supreme Court said that beeper surveillance — in those cases it was installing a radio device that determined location — that in that case there was no search; it didn't trigger the Fourth Amendment, unless the government was obtaining information about where the device was inside a home. As long as the government wants to only monitor the location of a car on a public road, no search. But, the Supreme Court said, there may be circumstances where we're talking about a real dragnet where we might have a different result.

BOB GARFIELD: So I got to tell you, I am so at a loss to figure out the difference between GPS technology, which obviously is now ubiquitous, and a homing device, a beeper that doesn't operate by GPS but by radio waves and accomplishes approximately the same thing. How is that different?

ORIN KERR: Those who say there's a difference make this argument. They say, listen, a radio beeper requires a government officer to be following the car in real time whenever the beepers being used. The location is only a radio signal that has to be intercepted by a police officer following the car. In contrast, if you put a GPS device on somebody's car, you could monitor them for a month, for two months, wherever the car goes. No need for an officer to be there. And as a result, the tool is just that much more dangerous because even though the information that is obtained is the same, there's just a greater risk of abuse.

BOB GARFIELD: You know, is it possible for the Congress to step in and write a very clear statue to set simply a lower threshold of warrant for that kind of search, as opposed to going into my house and rooting through my drawers, leaving it for the courts simply to rule whether it's constitutional or not? It — is that a possibility?

ORIN KERR: It's absolutely a possibility. One big issue here — where is Congress? The way the Constitution works in the Fourth Amendment setting is that there's either a warrant protection or there's no protection; there's not a whole lot of middle ground. In contrast, the Congress can enact all sorts of middle grounds. And there's been some hearings about this question recently before the House Judiciary Committee. They can look at this and maybe require a slightly lower threshold court order than a warrant. The courts don't really have that luxury.

BOB GARFIELD: Orin, thank you so much.

ORIN KERR: Happy to join you.

BOB GARFIELD: Orrin Kerr is a law professor at George Washington University and a blogger at the Volokh Conspiracy.