The government may request that a court issue a 'geofence warrant'. This is a warrant which gives law enforcement authorities the right to obtain data from any cell phones which were in the same vicinity where a crime took place. A reverse location search is performed by a service provider to disclose all device identifiers picked up in a certain geographic radius in a particular time range. Some courts have ruled that these warrants are not constitutional.
Note the difference between global positioning system (GPS) and cell site location information (CSLI). Both can be used to track the position of a mobile device, but they involve different techniques.
A GPS system uses radio waves sent from a satellite to a receiver inside the phone. The transmission need only go from the satellite to the phone, not vice versa. The system needs signals from four satellites to triangulate the phone's position. The GPS system should be able to determine location to within 10 feet.
A CSLI system uses information from cell phone towers in the immediate vicinity to pin down the location of the phone.
A AGPS (assisted global positioning system) uses cellular data to assist the GPS system to ascertain location. AGPS is actually less precise than GPS, but it uses less power.
The Sixth Circuit distinguished between GPS data and CSLI data in United States v. Carpenter, concluding that there was no search and no warrant was required because cell site location information is data used to facilitate communications, and the information could be obtained from a service provider rather than the individual's device itself. See, United States v. Carpenter, 819 F. 3d 880 (6th Cir. 2016).
The Supreme Court reversed the Sixth Circuit finding that, “[I]n fact, historical cell-site records present even greater privacy concerns than the GPS monitoring considered in Jones: They give the Government near perfect surveillance and allow it to travel back in time to retrace a person’s whereabouts, subject only to the five-year retention policies of most wireless carriers. The Government contends that CSLI data is less precise than GPS information, but it thought the data accurate enough here to highlight it during closing argument in Carpenter’s trial. At any rate, the rule the Court adopts ‘must take account of more sophisticated systems that are already in use or in development,’ and the accuracy of CSLI is rapidly approaching GPS-level precision.” United States v. Carpenter, 138 S. Ct. 2206, 2210 (2018) (citation omitted).