WASHINGTON – A number of Supreme Court justices invoked the specter of Big Brother while hearing arguments Tuesday over whether the police may secretly attach GPS devices on Americans' cars without getting a probable-cause warrant.

While many justices said the concept was unsettling, the high court gave no clear indication on how it will rule in what is arguably one of the biggest Fourth Amendment cases in the computer age. The Obama administration maintains that Americans have no privacy rights when it comes to their movements in public.

Justice Stephen Breyer told Deputy Solicitor General Michael Dreeben that, "If you win this case, there is nothing to prevent the police or government from monitoring 24 hours a day every citizen of the United States."

Breyer said that "sounds like 1984."

Chief Justice John Roberts wondered aloud whether the government's position was that it may secretly attach GPS devices to the cars of the nine members of the Supreme Court without a warrant.

"You think they are entitled to do that?" Roberts asked.

"The justices of the Supreme Court?" Dreeben replied.

"So your answer is, 'yes,' you could tomorrow decide that you put a GPS device on every one of our cars, follow us for a month; no problem under the Constitution?" the chief justice continued.

"Well, equally, Mr. Chief Justice, if the FBI wanted to it could put its team of surveillance agents around the clock on any individual and follow that individual's movements as they went around on the public streets ..., Dreeben replied.

Justice Sonia Sotomayor suggested the government's position went too far, especially in the age of "smart phones" that contain GPS tracking devices.

"It would be OK to put a computer chip and put it on somebody's overcoat?" she asked. Dreeben said Sotomayor was off base because her scenario would allow GPS monitoring inside a home. "That is off-limits," he said.

However, "a car parked in the garage," he added, "does not have a reasonable expectation of privacy."

But the justices seemed troubled on whether a warrant was always necessary, and whether they should take into account how long the monitoring continues. "Where do you draw the line?" Justice Samuel Alito asked.

One of the Obama administration’s main arguments in support of warrantless GPS tracking is the high court’s 1983 decision in United States v. Knotts, in which the justices said it was OK for the government to use beepers known as "bird dogs" to track a suspect’s vehicle without a warrant.

Unlike beeper-assisted surveillance, which requires human "visual” surveillance, GPS tracking is a "robotic" process wholly devoid of human observation, said Stephen Leckar, the defense attorney for a District of Columbia drug dealer appealing a life sentence. Federal authorities monitored his client's vehicle's movements with GPS without a court warrant for a month.

Dreeben, however, told the justices that Americans have no right to privacy outside of their homes, so warrants are not required for GPS.

"Technology doesn't make something private that was public," he said.

In response to a question from Justice Sotomayor, Dreeben added that federal authorities employ GPS monitoring "in the low thousands annually." (Wired published a story Tuesday about two federal GPS tracking devices that a California man discovered on his vehicle.)

That 1983 beeper case is among the reasons why the issue is before the justices. The U.S. Court of Appeals for the District of Columbia Circuit ruled last year that Leckar's client, Antoine Jones, had his Fourth Amendment rights violated with the warrantless use of GPS attached underneath his car. The court reversed Jones' conviction.

The appeals court said the beacon in the 1983 case tracked a person, "from one place to another," whereas the GPS device monitored Jones’ "movements 24 hours a day for 28 days."

The appeals court ruled the case "illustrates how the sequence of a person’s movements may reveal more than the individual movements of which it is composed."

Justice Anthony Kennedy, however, wondered aloud whether there was a difference between GPS usage and "30 officers" tailing Leckar's client.

"The use of GPS has grave threats to privacy," Leckar responded.

The justices agreed to hear the case to settle conflicting lower-court decisions – some of which ruled a warrant was necessary, while others found the government had unchecked GPS surveillance powers.

Justice Ruth Bader Ginsburg asked whether warrantless GPS usage is any different than the proliferation of surveillance cameras.

"GPS is like a million cameras," Leckar said.

Justice Breyer suggested that "The real issue here is whether this is reasonable," he said of warrantless GPS use. "It is not."

Moments later, Justice Antonin Scalia said the police "can do a lot of stuff that is unreasonable under the Fourth Amendment."

"Why is this an invasion of privacy?" he asked.

"It's a computer, robotic substitute," Leckar replied. He added that a GPS tracker is "an uninvited stranger."

Scalia quickly replied: "So is a trail."

Justice Alito suggested the court had an escape valve, and could decide the case without answering whether GPS usage needs a warrant.

Federal agents obtained a warrant granting them up to 10 days to install the GPS device on Jones' vehicle. But they did not affix it until the 11th day.

"A violation of the 10-day rule," Alito said, "isn't necessarily a violation of the Fourth Amendment."