Most new cell phones are equipped with a Global Positioning System receiver allowing them to ascertain and track their position by receiving and analyzing the radio signals broadcast by GPS satellites. Software in the cell phone uses a technique called trilateration to calculate its position using an active tracking system.
The GPS is a constellation of 24 satellites managed and maintained by the U.S. Department of Defense and positioned uniformly around the earth to form a global encompassing net. Each satellite broadcasts three types of signals on two different frequencies.
The frequencies are called L1 and L2. The L1 signal is broadcast at 1575.42 megahertz and is available for commercial use. The types of data embedded in the signal are almanac data, ephemeris information and pseudorandom code. The ephemeris information is what the cell phone receiver uses to calculate position. The cell phone collects positioning information from three satellites and then draws a cylinder around each one. The cylinders intersect at two locations; one is in space and the other is where the phone is located. Because cell phones are mobile devices, they use active tracking to update their position continuously.
Basic cell phones use this information to transmit this information as a Public Safety Answering Point when a 911 emergency call is made so dispatchers can direct the closest responders. Smartphones have software applications available for numerous uses, including turn-by-turn directions and tracking the movements of people.