Police cannot put a GPS device onto a vehicle to track its movements without first getting a warrant, the Arizona Supreme Court ruled Wednesday.

In a precedent-setting decision, a majority of the justices said people in vehicles have a “reasonable expectation” of privacy. That includes being able to travel where they want without government monitoring.

And while people drive on public roads, Chief Justice Scott Bales said that is quite different than using technology to track someone, in this case for 31 hours.

But Wednesday’s ruling did not help Emilio Jean who was arrested and convicted on charges of money laundering and transporting marijuana in the truck. A majority of the justices — not including Bales — said that with the state of the law on GPS tracking and searches unsettled until this point, the officers who put the device on the truck without a warrant were acting in good faith and the conviction stands.

The ruling was not unanimous.

Justice John Pelander, writing a dissent for himself and two of his colleagues, said he sees no reason for a hard-and-fast rule against such warrantless tracking. He pointed out that the Department of Public Safety officers used the device only for what they said was a short period of time and only on public roads, “where the truck was plainly visible to anyone.”

“Thus, the majority’s concerns about Orwellian invasions of privacy are unfounded here,” Pelander wrote.

But with four of the seven justices saying there is a right of privacy, that ban on warrantless GPS searches is now the law in Arizona.

The case involves a truck owned by David Velez-Colon that he and Jean were driving from Georgia to Arizona. DPS officers, suspicious of the vehicle, ran a records check which said the truck was stolen.

Suspecting it was being used to transport drugs, the officers installed a GPS tracking device without obtaining a warrant.

Monitored for several days by DPS and federal drug agents, officers stopped the vehicle as it reentered Arizona from California with Velez-Colon driving and Jean, as co-driver, sleeping. A search of the vehicle conducted after dogs “alerted” on the truck revealed drugs.

Velez-Colon made a plea deal, agreeing to testify against Jean.

Brad Bransky, a deputy Coconino County public defender, argued to the high court that the warrantless use of the GPS made the subsequent stopping of the vehicle and the search illegal, meaning the drugs that were found could not be used to convict his client. The majority agreed with his legal theory but said that, at least for Jean, the conviction stands.

Bales, writing the majority opinion, said the nature of GPS tracking technology requires courts to apply special rules — and special limits — that would not otherwise apply to police simply watching a vehicle.

“In this age, vehicles are used to take people to a vast number of places that can reveal preferences, alignments, associations and foibles,” Bales said, citing a similar case out of the state of Washington. “The GPS tracking devices record all of these travels, and thus can provide a detailed picture of one’s life.”

What that means, he said, is that people in a private vehicle — including the passengers — “have a reasonable expectation of privacy that is invaded by the government’s continually tracking the vehicle through a surreptitious GPS tracking device.”

Nor was the majority dissuaded by arguments by dissenting colleagues that the short nature of the tracking of the vehicle made it legally acceptable. Bales said a hard-and-fast prohibition on such tracking is necessary “to provide clear guidance to law enforcement for when a warrant is required.”

Pelander, in his dissent, said the majority ruling misses the point by focusing on the fact that a GPS device was used to track the vehicle. He said the legal test is what police are surveilling and where they are doing that. And in this case, he said, everything occurred on public roads.

“That new technology allows police to surveil more effectively public locations and activities does not change the public nature of the location or constitute a violation of one’s reasonable expectation of privacy,” he wrote.

Bales disagreed.

“GPS tracking is qualitatively different from visual surveillance, even on public roadways, because it can monitor the whole of a person’s progress through the world,” he wrote.

Bales said Wednesday’s ruling should not be seen as a radical departure from existing law but simply a reaffirmation of the protections in the Fourth Amendment against warrantless government surveillance.

“Requiring such searches generally to be supported by a warrant based on probable cause does not unduly burden the government’s interests,” he said.