The US Supreme Court on Monday ruled that people have a reasonable expectation of privacy with regard to mobile phone geolocation data, a decision privacy advocates have sought for years.

The Court's ruling in Chatrie vs. United States [PDF] concluded, "Police officers conducted a Fourth Amendment search when they acquired Chatrie’s location data from Google because an individual has a reasonable expectation of privacy in his cell-phone location information."

Police did so through a so-called geofence warrant. The warrant required Google to provide mobile phone Location History collected from within a 150-meter radius of a credit union during the hour around when it was robbed.

The Fourth Amendment protects against unreasonable searches and seizures by requiring authorities to obtain a warrant based upon probable cause. 

The Court's conclusion does not resolve Chatrie's case, which has been remanded to the US Court of Appeals to assess the disputed warrant's validity. But it does make clear that Location History data requires a warrant. And it amplifies the impact of Carpenter v. United States [PDF], a 2018 ruling that limited warrantless searches of cell-site location information (CSLI).

In a social media post, Stanford Law School professor Orin Kerr expressed surprise that Justice Kavanaugh joined the majority in the 6-3 decision.

"If you're a privacy advocate, Chatrie is just about the best possible outcome you could have expected," said Kerr, who in 2024 argued Chatrie had no Fourth Amendment right to his location data because Chatrie had opted in to Google's Location History.

Privacy advocates, who have been asking US courts for at least two decades to affirm that the Fourth Amendment protects location data, are thrilled with the decision.

EFF Surveillance Litigation Director Andrew Crocker, who co-authored the EFF amicus brief in the case, said, "We applaud the Supreme Court's decision in Chatrie vs. United States. The Court reaffirmed that you have an expectation of privacy in location data that reveals your movements in the physical world, and that even short-term surveillance of these movements is a search subject to the Fourth Amendment."

Crocker said that in recent years police have come to rely on geofence warrants and have violated the privacy of many innocent bystanders. He said while the Court stopped short of disallowing geofence warrants entirely, the EFF intends to push for their elimination in lower courts.

The US government argued that the Fourth Amendment didn't apply because Chatrie had opted into sharing his Location History with Google. But the Court found that argument "meritless."

"That argument ignores how and why Google users turn on Location History: Google repeatedly prompts users to turn on the service, often warning that devices will not 'work correctly' otherwise … while not disclosing in that prompt how frequently users’ location information would be recorded, how precise it would be, or how it might be given to the government," the Court's majority said.

Eden Heilman, legal director of the ACLU of Virginia, told The Register in a statement that the Court's decision confirms that law enforcement cannot use new technology to conduct warrantless surveillance.

"We do not lose our right to privacy simply because we use a cellphone," said Heilman.

Google in 2021 reported that geofence warrants began taking off around 2018 and by 2020 represented about 25 percent of all US warrants it received. Two years later, in an effort to reduce its role as a law enforcement data dispenser, the search biz announced changes to its handling of location history data by storing the data on-device instead of on its servers.

Alas, there's an entire data broker industry that has been selling notionally private data to customers, whether that's the government or businesses. Purchase histories, browsing history data, chatbot logs, and the like continue to be bought and sold.

In March, a bipartisan set of US Senators attempted to close the so-called data broker loophole with the introduction of a bill called the Government Surveillance Reform Act. It was designed to reauthorize Section 702 of the Foreign Intelligence Surveillance Act (FISA) "with necessary Fourth Amendment protections to block the federal government from buying Americans’ private data from shady sources."

Yet events have overtaken the bill. In June, Section 702 of FISA, a law that has allowed the warrantless collection of information, was allowed to expire – a decision that can be read as a vote of no confidence in the current administration. 

There's now an opportunity to rethink how personal data is collected, stored, and sold. Come August in California, data brokers will be required to process opt-out requests from the state's Data Request and Opt-out Platform (DROP). That's the way forward for privacy: Minimize data collection and storage, and provide a privacy enforcement mechanism. ®