Law enforcement officials must obtain a court-authorized warrant before using a GPS device to track the movements of a criminal suspect’s vehicle, the US Supreme Court ruled on Monday, eliciting praise from privacy advocates.

In a unanimous decision, the high court said Fourth Amendment guarantees against unreasonable searches prohibit police or federal agents from affixing a Global-Positioning-System (GPS) tracking device to a private vehicle and then recording the vehicle’s every movement 24 hours a day for weeks or months without prior court approval.

The decision is a setback for the Obama Justice Department, which had argued that the Constitution did not hinder its use of tracking technology to monitor vehicles traveling on public streets.

The opinion in US v. Jones (10-1259) is important because it upholds a broad right to be free from unreasonable searches. But it also highlights an emerging struggle within the high court to establish a consistent method of analysis that properly balances law enforcement objectives with privacy concerns.

As the government deploys an increasingly sophisticated and intrusive repertoire of surveillance technologies, privacy advocates warn that zones of privacy are fast shrinking in America.

“We have entered a new and frightening age when advancing technology is erasing the Fourth Amendment,” said John Whitehead, president of the Virginia-based non-profit Rutherford Institute, in a statement.

“Thankfully,” he said, “the US Supreme Court has sent a resounding message to government officials – especially law enforcement officials – that there are limits to their powers.”

Although all nine justices agreed that the government’s use of a GPS tracking device amounted to a search, the justices split 5 to 4 on how to properly analyze the issue.

Writing for the court, Justice Antonin Scalia tied the question to Fourth Amendment protections of the sanctity of private property against government trespass. He said the government needed a warrant not just to attach the GPS device to a piece of private property, but to attach the device with the intent of obtaining information.

“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,” Justice Scalia wrote in an opinion joined in full by four other justices.

Joining Scalia’s decision were Chief Justice John Roberts, and Justices Anthony Kennedy, Clarence Thomas, and Sonia Sotomayor.

Justice Samuel Alito agreed with the general outcome of the case, but wrote separately to suggest the court should have based its decision on an examination of whether the owner of the monitored vehicle had a reasonable expectation of privacy in the long-term movements of his car.

Scalia’s property-based approach is “unwise,” Justice Alito wrote. “It strains the language of the Fourth Amendment; it has little if any support in current Fourth Amendment case law; and it is highly artificial.”

He questioned how it would apply in cases in which the government tied its surveillance to factory-installed GPS devices in vehicles or to smart phones equipped with GPS.

Three justices joined Alito’s concurrence. They were Justices Ruth Bader Ginsburg, Stephen Breyer, and Elena Kagan.

In past decisions in recent years, the court has examined whether a subject had a “reasonable expectation of privacy” at the time of the particular government intrusion. Scalia’s private-property approach had fallen out of favor through disuse by the court, but according to Scalia, had never been overruled.

“What we apply is an 18th-century guarantee against unreasonable searches, which we believe must provide at a minimum the degree of protection it afforded when it was adopted,” Scalia wrote.

“The concurrence [by Alito] does not share that belief. It would apply exclusively [the] reasonable-expectation-of-privacy test, even when that eliminates rights that previously existed,” he said.

Scalia said under his approach, Fourth Amendment rights would be protected under both the private-property and the reasonable-expectation rationales.

The high court appeal stems from the investigation of nightclub owner and suspected drug trafficker Antoine Jones in Washington, D.C., and Maryland.

Police and federal agents suspected Mr. Jones was involved in selling significant quantities of cocaine. They maintained surveillance of Jones and his nightclub. They installed a video camera near the club and tapped his cell phone.

Investigators suspected Jones maintained a stash house where he kept his drugs. In September 2005, they obtained court-authorization to place a GPS unit on Jones’s car.

The warrant required that the device be installed within 10 days of the judge’s approval. It also required that the device be installed in Washington. For reasons that are unexplained, police did not install the device until eleven days after the authorization and they did it while the car was in Maryland.

By violating the terms set by the judge, the warrant was no longer valid.

Nonetheless, investigators used the device to track the vehicle’s movements for a month. The assembled data revealed patterns in Jones’s daily routine, and regular visits to a particular house in Fort Washington, Maryland.

After agents overheard Jones discussing a possible large cocaine shipment, they used the GPS data to identify the suspected stash house. A raid on that location recovered 97 kilograms of cocaine, nearly a kilo of crack cocaine, and $850,000 in cash.

At trial, Jones was convicted of running a drug trafficking conspiracy. He was sentenced to life in prison.

But the case wasn’t over. His lawyer appealed, arguing that the investigators violated Jones’s Fourth Amendment rights by conducting the GPS surveillance without a valid warrant.

Lawyers for the government justified the warrantless use of the GPS tracking device by comparing it to less sophisticated beeper technology that allowed agents to follow a suspect package by using a concealed radio transmitter.

In 1983, the Supreme Court ruled that agents did not have to obtain a warrant before installing and using the transmitter to track a package. The court said there was no reasonable expectation of privacy in the public movement of a package from one location to another.

The beeper technology required agents to physically follow the package and to remain close enough to it to receive the transmitted signal. In contrast, the GPS tracker requires no real time monitoring by any human agent. The device automatically transmits its location to a computer database.

The appeals court in the Jones case ruled that a radio transmitter is different than a GPS tracking device. The GPS device allowed agents to observe Jones’s movements around the clock without any physical presence or surveillance by a human agent.

The appeals court said GPS technology enables the government to discover substantial private details of a person’s life from his or her movements over time.

“A person who knows all of another’s travels can deduce whether he is a weekly church-goer, a heavy drinker, a regular at the gym, an unfaithful husband, an outpatient receiving medical treatment, an associate of particular individuals or political groups – and not just one such fact about a person but all such facts,” the appeals court said.

In upholding the appeals court, Justice Scalia said the high court’s decision did not clash with the earlier decision in the beeper case. He said the beeper had been physically installed in a package with the knowledge of the owner of the package and, thus, no warrant was necessary to insert the device and track it to its destination.

In contrast, Jones was in possession of his car at all times when the GPS device was physically installed. That intrusion against the owner, Scalia said, was unreasonable without a warrant.