The U.S. Supreme Court Monday said the federal government needs a warrant to place a GPS tracking device on a suspect's car in a case involving the monthlong surveillance of an alleged Washington drug trafficker.

The case was closely watched to see how the justices would weigh the privacy concerns of citizens against police armed with sophisticated GPS technology.

Justice Antonin Scalia, who often looks to the original intent of the Constitution's Framers when deciding cases, based his ruling on common-law trespassing, instead of relying on the familiar reasonable expectation of privacy test to determine Fourth Amendment violations.

For Justice Samuel Alito, Scalia's reasoning may have been a little too old-school. Alito -- writing a concurring opinion for Justices Stephen Breyer, Ruth Bader Ginsburg and Elena Kagan -- said the case concerns 21st-century surveillance technology, yet ironically, the court has chosen to decide this case based on 18th-century tort law.

The majority's reasoning largely disregards what is really important (the use of a GPS for the purpose of long-term tracking), he wrote.

That was on the minds of privacy and civil rights advocates when the court was considering the case, U.S. v. Jones. With the proliferation of smartphones and GPS devices pre-installed on some cars, there was an Orwellian fear that a ruling for the government could give the FBI unchecked authority to track anyone cheaply and quickly. Breyer, during oral arguments in November, said a win for the government would produce what sounds like 1984.

If long-term monitoring can be accomplished without committing a technical trespass -- suppose for example, that the federal government required or persuaded auto manufacturers to include a GPS tracking device in every car -- the court's theory would provide no protection, Alito wrote.

For Privacy Advocates, a Landmark Case

Critics of the government's position on GPS monitoring are not grousing about the decision because it was based on government trespassing of private property, which Scalia said would be considered a search at the time the Fourth Amendment was drafted. The decision was a clear rebuke of the government's position that law enforcement can conduct warrantless GPS surveillance of a suspect's public movement without running afoul of the Fourth Amendment.

Not one justice agreed with that, said Gregory Nojeim, director of the Freedom, Security and Technology project at the Center for Democracy and Technology. When you look at the implication of the case for the elephant in the room, which is tracking of cell phones that a vast majority of adult and teenage Americans now carry, it appears there are five votes for requiring a warrant: [Justice Sonia] Sotomayor, plus the Alito four.

Sotomayor -- who sided with Scalia that the Fourth Amendment protects, at a minimum, against warrantless trespassing on private property -- detailed her concerns over privacy rights and GPS monitoring in a concurring opinion.

She even broached the idea of reconsidering the premise of an individual losing their reasonable expectation of privacy for giving up information to a third party in a digital age where disseminating data is part of an American's most mundane tasks.

She and the folks who joined Alito's decision are clearly concerned to the degree of which GPS tracking and contemporary forms of surveillance threaten privacy, said Catherine Crump, a staff attorney at the American Civil Liberties Union. It is one of the court's first attempts to grapple how the huge technological revolution we've all seen in the last 15 years will implicate constitutional values.

Congress' Role in Privacy

In Alito's opinion, there is no clear line the government can cross to determine whether it is conducting long-term surveillance that impedes a person's privacy expectations. After all, he notes, a judge's expectation of privacy can be much different than an average citizen's.

Alito suggests that Congress and states are better suited to understand what kind of privacy Americans can expect in an age where their cellphone can pinpoint their location, similar to how federal lawmakers began regulating wiretaps after the Supreme Court wrestled with the issue in 1967.

Maybe it would be good for Congress to grapple with this and set some rules for the road, Andrew Pincus, an attorney with Mayer Brown, said in reference to Alito's opinion. The court is proceeding cautiously and it is not going to adopt the invitation of the government to basically say, as technology advances, the Fourth Amendment retreats.