A GPS unit consists of a space segment, a control segment, and a user segment. The space segment is a constellation of two dozen satellites orbiting the earth twice every 24 hours, at approximately 10,900 nautical miles above the earth's surface. These satellites are funded and controlled by the U.S. Department of Defense. The control segment is a series of monitoring stations located at different sites on earth. These stations update and correct errors in the navigational message of the satellites. The user segment is a receiver that receives radio waves from the satellites in orbit. It can determine how far away it is from each satellite by keeping track of the time it takes for a radio wave to travel from the satellite to the receiver
A monitor in the car shows the relevant portion of the map. The driver can enter the target location, and the computer will calculate the optimal route and display it instantly. It can respond to user preferences and map a route that avoids highways or avoids local roads. If the map is detailed enough, it will also provide the locations of the nearest gas station, supermarket, restaurant, hotel, and ATM machine. Some GPS units can issue auditory directions (i.e., "Turn left,") to guide drivers as they travel.
GPS can aid in the recovery of a stolen car. Integrated with the automobile security system, GPS can notify the car owner by phone or e-mail when the car alarm is triggered, and indicate the location of the car.
An owner can contact the GPS unit from outside the car using a cell phone or via the Internet, and can start the car engine remotely and unlock the door, solving the problem of frozen locks.
GPS also monitors the car condition, and issues warnings when the battery is low or when it is time for an oil change.
The Ford automobile company has developed a new telematic system through GPS on the Ford Focus, that will alert emergency services when an airbag deploys. The emergency services can locate the vehicle quickly and provide assistance to an injured driver or passengers.
In addition to its applications in private vehicles, GPS is being used by commercial shipping companies to speed the delivery of cargo. It allows companies to track their fleets, record the movement of their trucks, and control route planning. Car rental companies can equip their cars with GPS technology to find stolen rental cars and charge customers for dangerous conduct as speeding .