The decision imposes Fourth Amendment restrictions on the ability to conduct surveillance without probable cause.

Police will no longer be able to access cell phone location. That’s a ruling for the privacy of the advocates. The 5-4 decision (.pdf) brings mobile phone location within the Constitution’s Fourth Amendment protection against unreasonable search and seizure.

The underlying case involved a man named Timothy Carpenter, who had been convicted of armed robbery. At trial, attorneys for Carpenter argued the location-evidence should be barred. In denying the motion to suppress the evidence, “Carpenter lacked a reasonable expectation of privacy in the location of information, collected by the FBI because it had shared that information with its wireless carriers.”

Illegally obtained evidence is supposed to be excluded in criminal trials, regardless of the actual guilt of the defendant. The reason is to create structural disincentives for police to violate people’s rights.

Ginsburg, Breyer, Sotomayor and Kagan. The remaining conservatives all dissented. Roberts said that “requests for cell-site records lie at the intersection of the two lines of cases.” One of those pertained to an individual’s “expectation of privacy in his physical location and movements.”

The court held that a person has an expectation of privacy in the public sphere. This was true despite the fact that Carpenter unknowingly shared his location with his cell phone carrier (by default). The Court said:

We decline to grant the state unrestricted access to a wireless carrier’s database of physical location information. In light of the deeply revealing nature of, the depth, breadth, and comprehensive reach, and the inescapable and automatic nature of its collection, the fact that such information is collected by a third party does not make it any less deserving of Fourth Amendment protection . The Government’s acquisition of the cell site.

The right-leaning members of the court argued that sharing information with a wireless carrier was not different from sharing a third party such as a bank or other commercial enterprise, which did not trigger Fourth Amendment protection.

The decision is very consequential and pushes back against the encroachment of “the surveillance state.” However, the Court added, there could be exceptions in cases of emergency (e.g., active shooters) where warrantless searches of cellphone location.