Global Positioning System

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GPS Units

The Global Positioning System (abrievated to GPS) is a satellite location system that, in conjuction with a special receiver, can locate the user’s position and altitude as well as provide accurate clock, sunrise and sunset times anywhere on earth. The system consists of 24 satellites in orbit around the globe. Each satellite circles the Earth twice per day at an altitude of 20,200 km.

The GPS system was designed, built and is run by the United States Department of Defense and is available free of charge although it costs the US tax payer some $400 million a year to run. The system was started in 1978 and satellites have to be replaced on regular intervals. The satellites carry atomic clocks and broadcast a continuous time signal. These signals can be used by a GPS receiver to calculate its position by trilateration. The receiver needs to be able to see four satellites in order to make this calculation and have a reasonably accurate internal clock. The time taken to calculate an initial position after switch-on depends on how far the GPS has moved since it was last used and how much the clock has drifted. Typical times are between 45 seconds and 5 minutes.

GPS are typically accurate to around 10 meters. Some GPS support the Wide Area Augmentation System (WAAS). This uses additional information supplied some GPS satellites to improve accuracy. Currently WAAS only works for the North America coastal regions. WAAS can improve accuracy down to 1 meter. GPS are less accurate in calculating altitude and users may still wish to carry an altimeter?. They are also heavy on battery consumption.

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