The Supreme Court ruled 5-4 in Carpenter v. United States on Friday that law enforcement needs a warrant in order to track a person's location information from cellphone towers over an extended period of time, NBC News reports. Chief Justice John Roberts sided with the liberals on the decision, writing in the opinion that people "compulsively carry cellphones with them at all times," making monitoring that data "near perfect surveillance" akin to attaching "an ankle monitor to the phone's user."
The government had argued unsuccessfully that "cellphone users voluntarily reveal to their providers information about their proximity to cell towers" so customers "cannot reasonably expect that the providers will not reveal that business information to the government."