In George Orwell's novel "1984," the unblinking eye of government surveillance is omnipresent and inescapable. Orwell could not have known what technology would one day make his nightmare scenario possible, but he could foresee that whatever it was, the government would misuse it.

This week, the Supreme Court agreed. In a case that for the first time sought to put limits on the government's growing use of digital technology to monitor Americans, the justices came down firmly on the side of privacy. The court ruled unanimously that secretly tracking people's movements by attaching GPS devices to their cars violated the Fourth Amendment's ban on unreasonable searches unless police first get a warrant from a judge.

In throwing out the conviction of a suspected drug dealer sentenced to life imprisonment on the strength of evidence obtained through GPS tracking, the justices rejected the government's claim that it had essentially unlimited power to monitor people's movements. In the case before the court, police had tracked the man's every move for weeks through the electronic signals transmitted by a small GPS device attached to his car's bumper. The surveillance ended when he eventually led them to a stash house where officers found weapons, $850,000 in cash and nearly 100 kilos of cocaine.

The government argued the man had no expectation of privacy when he traveled on public roads and that in any case tracking his movements through the GPS was no different than assigning dozens of police officers to visually shadow his comings and goings. The only difference was the GPS accomplished the same task more efficiently.

But writing for the court's majority, Justice Antonin Scalia dismissed that claim, on the grounds that installing the device on a private car constituted a "search" under the Fourth Amendment and therefore required police to obtain a warrant. Justice Scalia based his reasoning on an obscure provision of 18thcentury tort law that seemed to limit the ruling's application to devices physically attached to a subject's vehicle, but in concurring opinions several of the justices suggested that the requirement for a warrant may also apply to a much broader range of technologies.

For example, the GPS units in many smart phones emit signals that allow cellphone companies to reconstruct the minute-by-minute movements of their customers, as do the navigation systems built into many new cars. Even ordinary cellphones that don't have a GPS unit send out signals that are relayed through a network of transmission towers, allowing a user's position to be triangulated.

Justice Sonia Sotomayor joined JusticeSamuel A. Alito Jr. and two others in expressing concern that government monitoring of these new devices based on digital technology could have a chilling effect on constitutional protections against unwarranted government intrusion into people's private lives. Such questions are bound to be the subject of future litigation, and Congress ought to begin hearings now on comprehensive legislation aimed at clarifying these matters. As Justice Alito noted, this is a matter better addressed by legislators than judges.

For the moment, however, the court has at least established the principle that the government's authority to emulate Orwell's all-seeing Big Brother has limits. The new digital devices, and techniques such as data mining, have opened the way for government to gather unprecedented amounts of information about citizens' private lives, and legal safeguards are essential to prevent its misuse.

The justices in this case stopped short of saying where the line should be drawn in all such cases, but they were certainly right to reaffirm the basic tenet of a free society that says just because people use public roads to get around doesn't mean they forfeit their constitutional rights.