The Supreme Court ruled Monday that collecting phone location data from a geographic area is a Fourth Amendment search, in a decision that both privacy advocates and critics of the ruling say will have vast implications for tech privacy.

The 6-3 ruling in Chatrie v. The United States is a “major win” for privacy under the Fourth Amendment, said one law professor who studies surveillance. And it “will send seismic waves through our Fourth Amendment doctrine” with ramifications “for the foreseeable future,” the dissenting justices wrote. The ruling didn’t fall along some of the traditional lines of justices selected by Republican or Democratic presidents.

Okello Chatrie challenged police’s collection of cell phone data from Google in his bank robbery conviction under a so-called geofence warrant that gleaned insights about his location around the time of the crime. While the Supreme Court punted on the question of whether the specific warrant in his case was proper, it held that the Fourth Amendment’s protections apply to this kind of data collection — and potentially other, future kinds as well.

Among the issues the court debated was whether a generalized collection constitutes a search as defined by the Fourth Amendment’s rights against “unreasonable searches and seizures.” That included questions of whether someone who willingly gives their data to a company like Google retains Fourth Amendment projections for that information, under the “third-party doctrine.”

The majority found that cell location data is substantially similar to cell-site location information addressed in Carpenter v. United States (2018),  where the court similarly held that the government’s collection of this data constitutes a Fourth Amendment search.

In the new opinion, Justice Elena Kagan, writing for the majority, used sweeping language about how the Fourth Amendment might apply as technology advances.

“A new technology should not transform what individuals had reasonably thought they could withhold from the Government,” Kagan wrote for the majority with Justices John Roberts, Sonia Sotomayor, Brett Kavanaugh, and Ketanji Brown Jackson. “An individual has a reasonable expectation of privacy in records about his cell phone’s location, and police intrude on that constitutionally protected interest when they demand the information — even though for only a limited time, and from a third-party tech company.”

Justice Neil Gorsuch wrote a concurring opinion, saying that he differed from Kagan’s opinion only in how it arrived at its conclusions, citing the Fourth Amendment’s language about “papers” and “effects”: “As I see it, Mr. Chatrie’s Location History data qualifies as his personal property.”

Justice Samuel Alito wrote for dissenting justices that the court had gone too far in extrapolating protections specified under the Carpenter decision, saying it “will send seismic waves through our Fourth Amendment doctrine” despite not affecting Chatrie’s case.

“As the majority works its way through the question in this case, it makes sweeping proclamations with implications far beyond the specific procedure that the police used here,” Alito said, adding that the decision “all but guarantees that we will be cleaning up debris for the foreseeable future.”

Andrew Ferguson, a law professor at George Washington University and author of a book about how police use of data threatens personal freedom, said the ruling was big even if it will still be easy for law enforcement to obtain warrants in other ways.

“Chatrie is a major win for Fourth Amendment privacy,” he told CyberScoop. “The Supreme Court did take a significant step today to update the Fourth Amendment in a digital age, and we should be thankful that they did.”

The American Civil Liberties Union also celebrated the ruling.

“The Court’s decision provides critical protection against invasive and overbroad government searches of our personal information,” Brett Max Kaufman, senior counsel with ACLU’s Center for Democracy who helped write a friend of the court briefing on Chatrie’s side, said in a statement to CyberScoop. While Google has changed its system in a way that practically cuts off government requests for future location data, “similar kinds of reverse searches of sensitive data held by other companies will continue to be a threat to privacy. Law enforcement and courts are on notice that new technology does not open up surveillance loopholes, and strict adherence to the Fourth Amendment’s protections is required.”