The three-dimensional satellite navigational system of Navstar enables a traveller to find out his or her position anywhere on or above the planet. Data transmitted from a Navstar satellite provides the user with the time, the precise orbital position of the satellite and the position of other satellites in the system. The first of this American satellite se ries was launched in 1978. By 1985 there were 10 Navstar satellites in orbit. Right now, there are 24 orbiting Navstar satellites devoted to navigation.

To use a system called the Global Positioning System (GPS), a user can buy a locator, and with that locator can calculate distance by measuring the time it takes for the satellite's radio transmissions, traveling at the speed of light, to reach the receiver. Once the distance from four satellites is known, the position in three dimensions (latitude, longitude, and altitude) can be calculated using mathematical calculations called trigonometry. Velocity in three dimensions can be computed from the principle of doppler effect in the received signal. Of course, the new GPS receivers do all the work for the users. The mathematics are pre-programmed into the locator s, and the information is displayed automatically.

GPS technology has applications other than determining a position on the earth. One innovative application of GPS technology is to determine the Earth's movement after an earthquake. By consulting a network of these sensitive receivers, you can make remarkably accurate measurements of the movement of the Earth's crust. GPS was also used to precisely locate drop points for airlifted Bosnian relief supplies. GPS has also accomplished pure data collection for use in the scientific community. GPS is addressing such problems as volcanic processes, ice dynamics, sea level change, and atmospheric sounding.

All 24 satellites in the Navstar system make up the space segment of the GPS, but the GPS also has two other segments known as the control and the user segments respectively. The satellites are in circular 20,200 km orbits at an inclination angle of 55 degrees. The period of the orbit is 12 hours. Beside the space segment, the GPS also has two other segments known as the control and the user segments.

The control segment of the GPS is made up of five Monitor Stations located at Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs. There are also three Ground Antennas located at Ascension Island, Diego Garcia, and Kwajalein. Finally, the Master Control Station (MCS) is located at Falcon Air Force Base in Colorado. The monitor stations track all satellites in view, accumulating data. This information is processed at the MCS to determine satellite orbits and to update each satellite's navigation message. Updated information is transmitted to each satellite via the Ground Antennas.

The user segment consists of antennas and receiver-processors that provide positioning, velocity, and precise timing to the user.