If you have ever used map and compass, you will understand a little about how GPS works. In order to find your position on a map, you need to have three points of reference. The intersecting line from reference points is where you are. Map and compass work uses triangulation (bearings), GPS uses trilateration (distances) to calculate location. Satellites orbiting earth emit unique signals that can be received by a GPS. The GPS software interprets signal, identifying satellite that it came from, where it was located, and time that it took for signal to reach system. Once receiver has both time and distance it begins to determine position.

Three satellites provide intersection point and fourth is used to check that positioning is accurate. Accuracy depends upon synchronization of atomic clocks in satellites with clock in GPS system. Although clock in GPS is not atomic, utilizing fourth satellite gives it that functionality as internal clock adjusts itself to correct any discrepancy discovered.

GPS has gone far beyond its initial military application. Drivers can find their way through city streets, long distance trekkers use technology to cross unfamiliar terrain, mariners and pilots use GPS enhanced data to cross seas and skies.

In--vehicle GPS can be integrated into car entertainment system or can be installed as a removable device. These systems need to tell driver where he/she is and how to reach their destination. The information includes road directions plus relevant features along way such as rest stops, gas stations, points of interest, etc. Auto GPS uses voice commands so that driver can concentrate on road.