The U.S. Supreme Court, moving cautiously in an era of rapidly changing technology, ruled on Monday that law enforcement’s warrantless installation and use of a GPS device to track a suspect’s vehicle violated privacy rights protected by the Fourth Amendment.

“It is important to be clear about what occurred in this case: The government physically occupied private property for the purpose of obtaining information,” wrote Justice Antonin Scalia for the unanimous Court. “We have no doubt that such a physical intrusion would have been considered a ‘search’ within the meaning of the Fourth Amendment when it was adopted.”

The decision in U.S. v. Jones was a significant defeat for the government, which did not win one vote for its arguments even as the justices divided 5-4 on the rationale for the judgment.

The Jones decision affirmed a ruling by the U.S. Court of Appeals for the District of Columbia, which had reversed the conspiracy conviction of drug trafficker Antoine Jones. The lower court found that the warrantless use of a GPS device to monitor Jones’ whereabouts for 28 days violated the Fourth Amendment.

In his opinion Monday, Scalia explained that for most of the nation’s history, the Fourth Amendment had been understood to have a special concern for government trespass upon the areas stated in its text: “persons, houses, papers and effects.” The vehicle in this case was an “effect,” he said.

“Whatever new methods of investigation may be devised, our task, at a minimum, is to decide whether the action in question would have constituted a ‘search’ within the original meaning of the Fourth Amendment,” he wrote. “Where, as here, the government obtains information by physically intruding on a constitutionally protected area, such a search has undoubtedly occurred. Trespass alone does not qualify, but there must be conjoined with that what was present here: an attempt to find something or to obtain information.”

The Court rejected the government’s argument that no search had occurred because Jones had no “reasonable expectation of privacy” in the area of the Jeep accessed by government agents — its underbody — and in the locations of the Jeep on public roads. The Court declined to address the government’s alternative argument that even if a search occurred, it was lawful because the officers had reasonable suspicion, even probable cause, to believe that Jones was a leader in a large-scale cocaine distribution conspiracy. The government failed to make that argument in the lower court and thus forfeited it, Scalia said.

Justice Samuel Alito Jr., joined by justices Ruth Bader Ginsburg, Stephen Breyer and Elena Kagan, concurred in the judgment. Alito said he would analyze the question in the case, not by relying on common-law trespass, but by asking whether Jones’ reasonable expectations of privacy were violated by the long-term monitoring of the movements of the vehicle he drove.

Explaining his problem with the Scalia approach, Alito said, “If long-term monitoring can be accomplished without committing a technical trespass — suppose, for example, that the fed¬eral government required or persuaded auto manufactur¬ers to include a GPS tracking device in every car — the Court’s theory would provide no protection.”

And, he added, the Court’s approach leads to incongruous results. “If the police attach a GPS device to a car and use the device to follow the car for even a brief time, under the Court’s theory, the Fourth Amendment applies. But if the police follow the same car for a much longer period using unmarked cars and aerial assistance, this tracking is not subject to any Fourth Amendment constraints.”

The best solution to privacy concerns in cases involving “dramatic technological change,” wrote Alito, may be legislative. But until legislatures act, he added, the best approach is to apply the “reasonable person” standard. Under that standard, short-term monitoring on public streets is reasonable, he wrote, but long-term monitoring in most investigations violates privacy expectations.

Justice Sonia Sotomayor, in a separate concurrence, sympathized with both approaches but voiced the broadest approach to and concern for privacy in today’s technological era. She suggested “It may be necessary to reconsider the premise that an individual has no reasonable expecta¬tion of privacy in information voluntarily disclosed to third parties. This approach is ill suited to the digital age, in which people reveal a great deal of information about themselves to third parties in the course of carrying out mundane tasks.”

Jones’ high court counsel, Stephen Leckar of Washington, D.C.’s Shainis & Peltzman, said the decision reveals “the emergence of two powerful new voices in Fourth Amendment and privacy cases: Justices Alito and Sotomayor.” Acknowledging that “big issues remain,” Leckar said, “The ultimate decision rested on the narrowest grounds but Justice Alito’s concurrence suggests four, if not five, justices also have grave concerns when [police] actions venture into areas most people regard as verboten.”

Andrew Pincus of Mayer Brown, who filed an amicus brief supporting Jones on behalf of the Center for Democracy and Technology, said this decision and the justices’ decision two terms ago in a search case involving text messages on police pagers, show the Court balking at broad, new rules pushed by the law enforcement community. “It will tread pretty cautiously in this area,” he said. “It is not going to adopt narrow, narrow constructions of the Fourth Amendment.”

Marcia Coyle can be contacted at mcoyle@alm.com.