United States v. Pembrook

Nathaniel Pembrook, David Briley, Shaeed Calhoun, and Orlando Johnson (defendants) were charged with crimes relating to the robbery and attempted robbery of two Michigan jewelry stores on April 22, 2014. The United States’ (plaintiff’s) case was based on data from cell towers. When a cell phone receives a call, sends a text message, or downloads a webpage, the phone searches for a signal from nearby cell towers. When the phone finds a tower, it sends the tower its unique registration information so that communications involving the phone can be routed through the tower. Cell phones usually register with the closest tower geographically, but factors such as physical obstructions and network traffic may impact the tower with which a phone registers. Cell-service providers store information about cell-phone communications with their towers in cell-site-data logs. At trial, the government sought to present testimony from Christopher Hess, a cell-site-data expert, to explain how investigators used the cell-site data from the logs to determine the defendants’ locations. Hess planned to testify that if a cell-site-data log shows a phone connected to a tower at a specific time, it can be inferred that the phone was in the tower’s coverage area at that time. The cell-site data here showed that between April 21 and April 23, 2014, Calhoun, Briley, and Pembrook traveled from Philadelphia, Pennsylvania, to the location of the jewelry stores in Michigan, and then back to Philadelphia. The cell-site data placed the defendants in a general geographic area but did not provide their precise location. Hess also planned to testify about how phones select new towers as they move, about the location of cell towers, and about division of a cell tower’s 360-degree service area into three 120-degree sectors that can be used to narrow down location. Calhoun moved to exclude Hess’s testimony, arguing that the testimony was based on an untested granulization theory that involved determining a defendant’s specific location based on the areas of overlap between two cell-phone towers’ coverage sectors.