The dispute dates back to a 2011 robbery in Detroit, after which police gathered months of phone location data from Timothy Carpenter's phone provider. They pulled together 12,898 different locations from Carpenter, over 127 days.
Watch this: The Supreme Court decided on a major win for privacy
The legal and privacy concern was that police gathered the four months' worth of Carpenter's digital footprints without a warrant. A Sixth Circuit Court of Appeals judge ruled that cellphone location data isn't protected by the Fourth Amendment, which forbids unreasonable search and seizure, and therefore didn't require a warrant.
In the Supreme Court's ruling, Chief Justice John Roberts wrote the government's searches of Carpenter's phone records were considered a Fourth Amendment search.
"The government's position fails to contend with the seismic shifts in digital technology that made possible the tracking of not only Carpenter's location but also everyone else's, not for a short period but for years and years," he wrote.
Roberts said allowing government access to historical GPS data infringes on Carpenter's Fourth Amendment protections and expectation of privacy, by providing law enforcement with an "all-encompassing record" of his whereabouts. He added that historical GPS data presents an "even greater privacy risk" than real-time GPS monitoring.
Carpenter's attorneys, including lawyers from the American Civil Liberties Union, argued before the Supreme Court that cellphone location data constitutes sensitive digital records and should be protected under the Fourth Amendment.
"This is a groundbreaking victory for Americans' privacy rights in the digital age," ACLU attorney Nathan Freed Wessler, who argued the case, said in a statement. "The Supreme Court has given privacy law an update that it has badly needed for many years, finally bringing it in line with the realities of modern life. The government can no longer claim that the mere act of using technology eliminates the Fourth Amendment's protections."
Phone location data is a hot-button issue for privacy advocates. In May, Sen. Ron Wyden, a Democrat from Oregon, asked phone service providers why they were giving away location data to Securus Technologies, a service that monitors calls to prison inmates, which police could use to track anybody's phone in the US, without a warrant.
The Supreme Court's decision now sets a precedent for every Americans' phone location data being used in criminal investigations, Wessler said.
"It's not just about Carpenter, it's about the rights of Americans who own cellphones, which is in excess of 95 percent of people in the country," Wessler said in a press conference call.
Fighting 'near-perfect surveillance'
Wyden said Friday's ruling was a "welcome step" for privacy and fighting against the expanding power of government surveillance.
"The court's recognition that digital devices can generate 'near-perfect surveillance' of a person's private life is a validation of the vital protections against unreasonable search and seizure provided by our Constitution," the senator said.
The losing argument was that phone companies can provide customers' data to law enforcement because they own those records, not the person. During the trial, US Deputy Solicitor General Michael Dreeben told the Supreme Court that people agree to hand over their information to providers for their service.
"It is asking a business to provide information about the business' own transactions with a customer," Dreeben said in November.
As technology improves, so will surveillance techniques, Ryan Radia, a Center for Technology and Innovation research fellow, said. For example, cell tower location data is going to become much more accurate once 5G rolls out, Radia said.
He said Friday's decision showed that the Supreme Court is willing to continue to weigh privacy implications that technology continues to change.
"For people who are understandably worried about technological evolution enabling tools and mass surveillance, they can take solace in the fact that the court has willingness to limit the government's abilities to surveill," Radia said.
Edward Snowden, the NSA whistleblower, said the decision was a major victory for the US.
The Computer and Communications Industry Association's president, Ed Black, said the decision would have significant impact on how the Fourth Amendment protects your data from government surveillance.
"This decision will provide users with the confidence that the sensitive location data they share with innovative digital devices and services will only be disclosed to law enforcement with a warrant based on probable cause," he said.
You can read the 119-page decision here:
Originally published June 22 at 7:21 a.m. PT. Updated at 7:38 a.m. PT: To include details from the Supreme Court's decision, at 7:54 a.m. PT: To include a statement from Sen. Ron Wyden, at 8:10 a.m. PT: To add reactions to the decision, at 9:40 a.m.PT: To include statements from the ACLU's press conference.
Correction at 11:53 a.m. PT: To adjust a description of LocationSmart.
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