Elements of GPS

GPS has three parts: the space segment, the user segment, and the control segment. The space segment consists of a constellation of 24 satellites (and about six "spares"), each in its own orbit 11,000 nautical miles above Earth. The user segment consists of receivers, which you can hold in your hand or mount in a vehicle, like your car. The control segment consists of ground stations (six of them, located around the world) that make sure the satellites are working properly. The master control station at Schriever Air Force Base, near Colorado Springs, Colorado, runs the system.

To help you understand GPS let’s discuss the three parts of the system—the satellites, the receivers, and the ground stations—and then look more closely at how GPS works.

A Constellation of Satellites

An orbit is one trip in space around Earth. GPS satellites each take 12 hours to orbit Earth. Each satellite is equipped with an atomic clock so accurate that it keeps time to within three nanoseconds—that’s 0.000000003, or three-billionths, of a second—to let it broadcast signals that are synchronized with those from other satellites.

The signal travels to the ground at the speed of light. Even at this speed, the signal takes a measurable amount of time to reach the receiver. The difference between the time when the signal is received and the time when it was sent, multiplied by the speed of light, enables the receiver to calculate the distance to the satellite. To make this measurement as accurate as possible, the GPS navigation signals are specially designed to make it easy for GPS receivers to measure the time of arrival and to allow all the satellites to operate on the same frequency without interfering with each other. To calculate its precise latitude, longitude, and altitude, the receiver measures the distance to four separate GPS satellites. By using four satellites, the receiver calculates both its position and the time and doesn't need an expensive atomic clock like those on the satellites.

Receivers

GPS receivers can be carried in your hand or be installed on aircraft, ships, tanks, submarines, cars, and trucks. These receivers detect, decode, and process GPS satellite signals. More than 100 different receiver models are already in use. The typical hand-held receiver is about the size of a cellular telephone, and the newer models are even smaller and fit in a wristwatch or a Personal Data Assistant. The commercial hand-held units distributed to U.S. armed forces personnel during the Persian Gulf War weighed only 28 ounces (less than two pounds). Since then, basic receiver functions have been miniaturized onto integrated circuits that weigh about one ounce.

Ground Stations

The GPS control segment consists of several ground stations located around the world.

A master control station at Schriever Air Force Base in Colorado

Six unstaffed monitoring stations: Hawaii and Kwajalein in the Pacific Ocean; Diego Garcia in the Indian Ocean; Ascension Island in the Atlantic Ocean; Cape Canaveral, Florida and Colorado Springs, Colorado
Four large ground-antenna stations that send commands and data up to the satellites and collect telemetry back from them.

The monitor stations track the navigation signals and send their data back to the master control station. There, the controllers determine any adjustments or updates to the navigation signals needed to maintain precise navigation and update the satellites via the ground antennas. To further improve system accuracy, in 2005, the master control station added data from six monitor stations operated by the National Geospatial-Intelligence Agency to the six GPS monitor stations.