When the Supreme Court ruled in 2012 that affixing GPS devices to vehicles to track their every move without court warrants was an unconstitutional trespass, the outcome was seen as one of the biggest high court decisions in the digital age.

That precedent, which paved the way for the disabling of thousands of GPS devices clandestinely tacked onto vehicles by the authorities, is now being invoked to question the involuntary placement of GPS devices onto human beings.

The Supreme Court was considering that issue at its private conference Friday, and could announce as early as Monday whether it will review a petition from a 36-year-old North Carolina sex offender. After serving three years for "indecent liberties with a child," Torrey Grady was ordered to wear a GPS anklet for life, an order that came down in 2013, four years after his 2009 prison release.

Grady has a tough legal road ahead of him. The Supreme Court in 1997 ruled that sexual predators could be confined against their will to a rehabilitation setting after their criminal prison terms expire under what is known as an "involuntary civil commitment"—the same type of procedure used to lock up the mentally ill outside the criminal justice system.

But the case pending before the justices has broader implications beyond the tens of thousands of sex offenders and other criminals ordered to wear GPS devices. It begs the question of what legal requirements, even for regular citizens, must be met to demand somebody wear a device that tracks one's every movement, 24 hours a day.

Grady—who is labeled a "recidivist" offender because he was also convicted of a sex crime in 1997—was required to wear the monitor after a brief hearing in a civil, non-criminal court that found his two convictions necessitated the ongoing surveillance. There was no consideration of the high court's GPS decision, known in legal parlance as Jones. The device was ordered to be worn solely because of Grady's priors.

"It was a 20-minute hearing," Mark Hayes, one of two of Grady's attorneys from North Carolina, said in a telephone interview.

Tracking humans, not cars

Hayes and co-counsel, Lewis Everett, told the Supreme Court in their petition (PDF) that the authorities must first consider whether the Jones precedent applies before the state can even consider the GPS requirement. If Jones applies, then the government would be required to show how the device furthers a legitimate government interest, such as stopping recidivism. And studies suggest they don't, they claim.

"This is even more intrusive because you're talking about literally affixing a device to a person," Hayes told Ars. In their brief, the defense lawyers wrote that Grady "must charge his bracelet daily, which requires him to be plugged into a wall outlet at least once a day for four to six hours at a time."

In Jones, the Supreme Court essentially said that attaching a GPS device to a vehicle amounted to a search "within the meaning of the Fourth Amendment." Because the Federal Bureau of Investigation did not get a warrant, the conviction and life sentence of an accused drug dealer named Antoine Jones was tossed.

State prosecutors are urging the high court to reject Grady's challenge to North Carolina's Satellite-Based-Monitoring (SBM) program. Joseph Finarelli, special deputy attorney general, told the justices in a filing (PDF) that Grady's attorneys have resorted "to hyperbole to characterize the severity and offensiveness of the 'trespass' resulting from the monitoring inherent in the SBM program."

What's more, Finarelli said, North Carolina's legal system concluded that no Jones analysis was required because the orders for GPS monitoring were handled in the non-criminal courts such as the ones allowing the confinement of sexual predators. The Jones case, on the other hand, was tried in a criminal court.

That distinction matters because the Fourth Amendment doesn't carry as much weight in non-criminal proceedings, he said.

"Petitioner virtually ignores this Court’s Fourth Amendment jurisprudence deriving from civil settings other than to say that the Fourth Amendment applies there," Finarelli told the justices.

This issue hasn't moved this far up the legal chain in other states. But most of states conduct post-prison hearings in civil court that often lead to sexual offenders being confined or having to wear GPS monitors.

Finarelli concluded his brief to the justices: "There is no case from this Court supporting Petitioner’s assertion that GPS monitoring equipment worn by a convicted sexual offender and monitored by a state for the purpose of preventing harm to the public constitutes an unreasonable search under the Fourth Amendment."

What if Jones applies?

Yet even assuming that the Supreme Court accepts Grady's petition and issues a decision, a ruling saying that Jones applies isn't an automatic victory for Grady. And it's not an automatic win for North Carolina, either.

If the justices say that the Fourth Amendment applies when attaching the device to a human, the states could continue the GPS requirement if it was deemed reasonable and performed in the name of a "legitimate governmental interest," Grady's attorneys told the justices.

"A balancing must still be done between the search intrusion onto his individual rights and the State's interests. The state and its courts have refused to do so," they wrote in the petition.

Whether a so-called balancing test is required when it comes to Fourth Amendment issues in the digital age was decided by the Supreme Court as recently as last year in a case known as Riley. California prosecutors claimed that they had the right to search the mobile phones of anybody arrested. The Supreme Court rejected that argument, saying the government's proffered reason for searching—officer safety—was not substantiated by the facts to allow such intrusive, warrantless searches into a mobile phone.

"Modern cell phones, as a category, implicate privacy concerns far beyond those implicated by the search of a cigarette pack, a wallet, or a purse. A conclusion that inspecting the contents of an arrestee’s pockets works no substantial additional intrusion on privacy beyond the arrest itself may make sense as applied to physical items, but any extension of that reasoning to digital data has to rest on its own bottom," the Supreme Court ruled.

Grady's lawyers contend that there is "little evidence" supporting North Carolina's contention that monitoring sex offenders' every movement furthers a "legitimate governmental interest of protecting the public."

They cited a 2010 North Carolina Supreme Court ruling in which a dissenting justice wrote that there was "no evidence" acknowledging that the GPS monitoring program "operates to prevent actual harm to our state's children."

Grady's attorneys also told the Supreme Court about an October study (PDF) of more than 9,000 male sex offenders in California who were ordered to wear GPS devices. The study found that sex offenders released from prison had a lower overall re-arrest rate for any type of crime.

None of the examples hold much water, Finarelli said.