A telephone tracker is a software that uses either a smartphone's built-in GPS or radio communications between a handset and cell towers to approximate the location of the phone. Telephone trackers are typically used to locate a lost phone, but they can also lead police or rescue personnel to the location of the handset in an emergency.
The most accurate phone trackers use the phone's own GPS receiver to pinpoint its location. A solid GPS signal allows the phone to be tracked to within a few meters of its current location. Many carriers have built-in phone tracking software that can be activated ahead of time, and triggered if the phone is lost or stolen.
If a phone lacks GPS, or the GPS receiver is not active, it may still be tracked using cell tower triangulation. When a phone communicates with a tower, it leaves a record of that radio connection, as well as a record of the relative strength of the signal. Since a phone may be able to communicate with multiple nearby towers, the tracking software can compare these signals and approximate the distance from each antenna, effectively triangulating the phone's location. This method is more accurate in urban areas, where large numbers of towers in close proximity can provide multiple readings on the phone's location.