Car navigation capabilities depend on a global positioning system, known as GPS, which uses a network of satellites with radio transmitters. For a car navigation system to work, there have to be at least four GPS satellites near enough for it to pick up a signal. Once the car's GPS calculates its position, it can use a map program to find its way to a new location.
GPS Satellites
GPS satellites each have atomic clocks and radio transmitters on board. The atomic clocks synchronize the satellites to almost exactly the same instant, which the satellites use to send out signals simultaneously. These signals contain information about the time, position and identity of each satellite.
Calculating Location
Because a car is never located exactly the same distance from each satellite, its GPS gets signals from nearer ones slightly before more distant ones. By measuring the slight delay, the GPS receiver can tell how much further it is from one satellite than it is from another, and since the transmission includes information about where each satellite is, the receiver can calculate its total distance from each satellite. It then uses this information to calculate exactly where it is located.
Planning a Trip
The car navigation system also features a sophisticated built-in map, which knows precisely where streets are located on the surface of the earth. When you enter the address of your desired destination the map plots a course along the streets to that location. As you travel there, the map continuously receives GPS transmissions, which it uses to update its location information, tracking your progress to your destination. If you get off course, the GPS system knows and plots a new course.
