May 2002
Beginner to intermediate
448 pages
13h 18m
English
Content preview from Geographic Information Systems: An Introduction, 3rd Edition
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10.2 Satellite Positioning System
Global positioning system
The global positioning system (GPS) is a military satellite-based navigational system developed by the U.S. Department of Defense. The system comprises 29 satellites, with 24 operational in six orbit levels and at an orbit height of 2020 km out in space. In principle, 4 to 6 satellites are available—15° over the horizon—at any time, everywhere on the Earth. Each satellite is equipped with four very accurate atomic clocks. The satellites' clocks and orbits are controlled by five ground stations that transfer data on changes in the orbit parameters and time to the satellites. Continuous signals are transmitted from the satellites. The signals contain data on the updated orbit parameters and the points in time for the signals leaving the satellite.
GPS and electronic chart display and information system
The global positioning system (GPS) may be integrated with an electronic chart display and information system (ECDIS). As discussed in this section, the normal accuracy of GPS is ±25 m. Accuracy better than ±5 m, which may be needed in narrow channels, is available in differential GPS, which requires coastal reference stations. The accuracy provided by GPS and differential GPS is better than that of many older nautical charts. This means that accurate nautical charts are a prerequisite for the sensible use of GPS.
Updating is essential to the successful use of ECDIS, as navigational conditions continually change. National ...
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