In United States v. Carpenter the Supreme Court will decide whether police must obtain warrants per the Fourth Amendment to require wireless carriers to provide cell-site data. State and local governments have an interest in obtaining cell-site data as quickly and easily as possible as it can provide solid evidence a particular person was near the scene of a crime.
Cellphones work by establishing a radio connection with the nearest cell tower. Towers project signals in different directions or “sectors.” In urban areas, cell sites typically cover from between a half-mile to two miles. Wireless companies maintain cell-site information for phone calls.