Lawfare reports that "a three-judge panel of the Fourth Circuit handed down an important decision in United States v. Graham ," finding that "the warrantless, extended, accessing of the two defendants' cell-site data amounted to an unconstitutional search under the Fourth Amendment. However, the court refused to suppress the collected information because of the Fourth Amendment's 'good faith' exception."
"Judge Davis began his opinion by explaining the technological foundations of CSLI. Cellphone network providers maintain cell sites or 'base stations' that emit signals that cell phones connect to. When a cellphone is used, such as in the case of sending a text or making a call, the phone connects to the station that has the strongest signal, which is usually the closest station. Moreover, phones can jump and connect to other stations as the phone moves in and out of a particular station’s coverage. All of this means that CLSI can 'approximate the whereabouts of the cell phone at the particular points in time in which transmissions are made' based on analyses of which station(s) the phone has connected to."
"Though it placed great weight on the extended nature of the CLSI collection and identification, the majority specifically refused to identify the tipping point for when tracking information becomes unlawfully 'extended.' Rather, the majority simply held only that 'the government engages in a Fourth Amendment search when it seeks to examine historical CSLI pertaining to an extended time period like 14 or 221 days.'"