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How GPS works

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How GPS works


The GPS consists of three main segments: space, control, and user. Satellites, which are launched into specific orbits and monitored by ground station, transmit information in form of radio signals. Ground stations update the information from satellites and send update signals back to satellites. Users with GPS receivers can decode satellite signals to find helpful information such as a location in a desert.

Space Segment

The space segment includes 24 satellites that are launched into six specific orbits to ensure coverage of the whole globe. The six orbits are 20,200 km above the earth. They are 60 degrees apart with 55 degrees inclination with respect to the equatorial plane.

The satellites are equipped with atomic clocks that provide accurate time to within three billionths of a second. The satellites transmit signals on two L-band frequencies (L1 = 1575.42 MHz and L2 = 1227.6 MHz) to equipment on the ground. The satellite signals consist of a Pseudo Random Code (PRC), ephemeris, and almanac data. The Pseudo Random Code is a complicated sequence of pulse signals. This code ensures not only that the same frequency can be used for all satellites, but also that GPS receivers can receive the GPS signals without the need of big satellite dishes. Moreover, Pseudo Random Code also identifies which satellite is sending. When a receiver receives a Pseudo Random Code, the number of the satellite shows up on a screen. The ephemeris data reflects status of satellites, current date and time. The position of a satellite is encoded in almanac data. A receiver decodes almanac data of a satellite and finds out where that satellite is at any time.

Control Segment

As shown in Figure 6, the GPS control segment includes monitor stations, ground antenna stations, and a master control station. Monitor stations are located at five different places around the world. They are Hawaii and Kwajalein in the Pacific Ocean, Diego Garcia in the Indian Ocean, Ascension Island in the Atlantic Ocean, and Colorado Springs in Colorado. The monitor stations receive signals from all satellites and collect data. The master control station, which is located at Schriever Air Force Base in Colorado, processes the information from monitor stations to decode satellite message signals. Then, the master control station updates each satellite's message. The satellites receive updated information through the ground antenna stations and send it to receivers.

User Segment

The user segment consists of numerous kinds of GPS receivers. The GPS receivers can be hand carried or installed on airplane, ships, submarines, and vehicles. The satellite transmits its update data to ...

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