# Global Positioning System

Every 12 hours, 24 main satellites in 6 orbits circle the Earth.

A GPS receiver operated by a user on Earth measures the time it takes radio signals to travel from four or more satellites to its location, calculates the distance to each satellite, and uses this calculation to determine the user’s longitude, latitude, and altitude.

The NaV star constellation was originally developed for military use by the United States Department of Defense, but a less precise version of the service is now available free of charge to civilian users all over the world. The basic civilian service will locate a receiver within 10 meters (33 feet) of its true location, though various augmentation techniques can be used to pinpoint the location to within 1 cm (0.4 inch)

Because of its accuracy and ubiquity, GPS has evolved far beyond its original military purpose, causing a revolution in personal and commercial navigation.

GPS signals are used to determine the positions and velocities of battlefield missiles and artillery projectiles, but so are the US space shuttle and the International Space Station, as well as commercial jetliners and private planes.

How does GPS function?

1. At 10,600 miles above the Earth, 21 GPS satellites and three spare satellites are in orbit. The satellites are arranged in such a way that four of them will be visible above the horizon from any point on Earth.
2. Each satellite is equipped with a computer, an atomic clock, and a radio. It constantly broadcasts its changing position and time because it understands its orbit and the clock.
3. Each satellite double-checks its sense of time and position with a ground station once a day and makes any minor corrections.
4. Any GPS receiver on the ground contains a computer that “triangulates” its position by receiving bearings from three satellites. The end result is a geographic position (longitude and latitude) to within 100 meters of most receivers.
5. The position can also be displayed if the receiver has a display screen and a map.
6. If a fourth satellite is detected, the receiver/computer can calculate the altitude as well as the geographic position.
7. If you’re on the move, your receiver may calculate your speed and direction of travel and provide you with estimated arrival times to specific destinations.