Is GPS technology used by police an attack on civil liberties?

What once was science fiction, is now science fact.

Police can now monitor a criminal suspect's every movement, from the comfort of an office cubicle.

Earlier Tuesday, the U.S. Supreme Court heard oral arguments in a case that will decide how and when police can use GPS tracking devices.

Now, a stalking victim is talking about how this GPS technology saved her life.

No one doubts the power of satellite technology to help police track the movements of a criminal suspect, but what if police wanted to track you for no reason at all?

Under current Wisconsin law, they can.

Now, the nation's highest court is about to settle the epic battle between public safety and personal liberty.

Every time you leave your house, walk into a story, buy groceries, fill up with gas, or pay for parking, you are leaving behind what amounts to an electronic trail of breadcrumbs, that can tell someone else every place you've been.

It is the kind of technology you likely encounter every day, without even thinking twice.

Jamie Johnson knows better than anyone, the sickening feeling that someone else is watching your every move.

Johnson's ex-boyfriend Michael Sveum stalked her the old-fashioned way. He parked in view of her house, and compiled minute-by-minute logs of her daily activities, and kept her in a constant state of fear.

"When I would go out on dates all my dates cars were being damaged and then they would be harassed after that. He said, 'who knows, I may be hiding behind a bush, and blow your head off,'" Johnson said.

In 1996, Sveum was convicted of stalking, but court records show Sveum continued to harass Johnson from prison, and when he got out after six years, the stalking continued.

This time, Sveum was more careful, calling from pay phones and never identifying himself. He had learned how to intimidate his victim, without ever getting caught.

"When I answered it, it would hang up, and the person would call back and hang up again, and this happened every single time, pretty much, when I came home," Johnson said.

In 2002, a Madison police detective was about to make Wisconsin history by using a GPS tracking device to stalk the stalker. The detective attached a GPS device under Sveum's car, and weeks later, police retrieved the data and used the recorded data to place Sveum at the very pay phones that were being used to harass Johnson.

That GPS data helped put Sveum back in prison.

"It potentially saved my life," Johnson said.

Since 2002, GPS tracking has become a popular tool for police to catch stalkers, armed robbers, serial burglars, and even murderers. It is both more powerful, and far cheaper than relying on old-fashioned surveillance.

Todd Morris is the CEO of Brickhouse Security, a national vendor of GPS systems with hundreds of law enforcement clients across the country.

"If you have three undercover police cars with two detectives in each one following one person for 24 or 48 hours, the overtime costs and just the basic costs are astronomical, compared to having one car with two detectives three blocks away following them," Morris said.

"There are not resources for law enforcement to follow people everywhere, and the GPS kind of helps compensate for that," Johnson said.

In the Sveum case, police had to wait until they retrieved the device to find out where he'd been. Today's tracking devices offer real-time feedback, that police can monitor from a computer screen or smartphone, right in their squad car.

Jody Wormet is a special agent for the Wisconsin Division of Criminal Investigation, which supplies GPS devices to local police. When FOX6 asked Milwaukee police for records of how many GPS devices they have, and how or when they use them, they denied the request, saying disclosure "would seriously handicap the department."

When FOX6 asked for an interview regarding GPS tracking, a Milwaukee Police Department spokeswoman said "we're going to take a pass."

"I am reluctant to talk about how many we have, and where they are located, or how often we use them, but I can say we do have those assets located around the state," Wormet said.

The more police utilize GPS tracking devices, the more civil libertarians like Larry Dupuis, who is an attorney with the ACLU of Wisconsin, shift in their seats.

"The police could just go and plop these GPS units on anybody's vehicle and anybody in the neighborhood," Dupuis said.

Dupuis believes police should be required to get a search warrant before placing a GPS device on a suspect's vehicle.

"Why does getting a search warrant interfere with their ability to investigate crimes?  If they are investigating crime, it shouldn't!" Dupuis said.

Morris argues there are times when police can't afford to wait.

"If you are standing on a corner and you see illegal activity going on, and you have two cars driving away, you don't have time to get a warrant, you only have time to act," Morris said.

In 2009, the Wisconsin Court of Appeals ruled that placing a GPS device on Sveum's vehicle did not constitute a search, and therefore, did not require a search warrant.

The State Supreme Court later sidestepped the issue, giving police free reign to place a GPS device on anyone's car for virtually any reason, as long as the vehicle is in a public place.

"They park in open spaces that are not considered private property and we try to use that to our advantage," Wormet said.

Federal courts across the country have come to a similar conclusion, but not without reservations. One federal judge wrote that the potential for GPS to be used for mass surveillance makes "the system depicted by George Orwell seem clumsy by comparison." Another judge lamented that "1984" is "here at last."

Last year, a federal court in Washington D.C. ruled against police, saying that detectives use of GPS to track a pair of drug dealers was a search. Also, earlier this year, the U.S. Supreme Court agreed to take the case, setting the stage for one of the most important Fourth Amendment showdowns in generations.

That could have a major impact on how police use technology to catch criminals like Sveum.

"It proved what he was doing.  I think the ability for law enforcement to use that in any suspected criminal activity is very important," Johnson said.

"Assuming they get a warrant, I have no objection to police using GPS units," Dupuis said.

During oral arguments Tuesday morning, U.S. Supreme Court Justice Stephen Breyer told a government lawyer "if you win this case, there is nothing to prevent the police from monitoring 24 hours a day, the public movement of every citizen of the United States."

The Department of Justice attorney replied that Congress, or state legislators, should be the ones to place limits on GPS tracking, not the courts.

Later, Justice Sonia Sotomayor asked how many GPS devices are in use by law enforcement today, and the government's lawyer said he could only speak for federal agencies, but said it's in the "low thousands." He said it's not a massive, universal technique - at least, not yet.

In most cases, the High Court will take two to four months after hearing oral arguments to issue an opinion.