In a long-awaited decision in Carpenter v. United States, the Supreme Court held 5-4 that the Fourth Amendment requires the government to receive a warrant to obtain cell-site location information (CSLI).
In the Court’s majority opinion Chief Justice Roberts provides an explanation of how CSLI works. “Cell phones continuously scan their environment looking for the best signal, which generally comes from the closest cell site. Most modern devices, such as smartphones, tap into the wireless network several times a minute whenever their signal is on, even if the owner is not using one of the phone’s features. Each time the phone connects to a cell site, it generates a time-stamped record known as cell-site location information (CSLI).”