ABA: Carpenter v. United States: Building a Property Based Fourth Amendment Approach to Digital Data by Melody J. Brannon, 33 Criminal Justice No. 4 at 20 (Winter 2019):
In the United States, there are more cell phones than people. These cell phones can track our every movement, continuously and with increasing precision, even when we are not using them. That is powerful information. Until Carpenter v. United States, 138 S. Ct. 2206 (2018), was decided, the police could get this information without a warrant, and they did so frequently. In the first six months of 2018, one phone service provider reported over 32,000 requests for historical cell site location information.
Carpenter asked “how to apply the Fourth Amendment to a new phenomenon: the ability to chronicle a person’s past movements through a record of his cell phone signals.” The Court, by a 5-4 majority, held that cell site location information (CSLI), that is automatically, continuously, and comprehensively generated by a third party is protected by the Fourth Amendment. The Court recognized that we reasonably expect privacy in “the whole of our movement.” That the information is compiled by a third-party phone company “does not make it any less deserving of Fourth Amendment protection.” (Id. at 2223.)