Contains the full lesson along with a supporting toolkit, including teachers’ notes.
In recent years Portable Navigation Devices (PNDs) have become more common and more affordable and are now used not only by car and boat users but also cyclists and walkers. Integration of PNDs with portable music players and other personal electronic devices is expected to rise dramatically over the next few years and some experts predict that 90% of mobile phones will have satellite navigation capability by 2014.
How Do They Work?
Portable navigation devices do not operate alone; they work in conjunction with a sophisticated global navigation satellite system (GNSS). The best known of these isGPS (Global Positioning System), a US system that was set up for military use but was made available to civil aviation in 1983; the full set of 24 satellites was not in place until 1994 and the system was opened to general use in 2000.
At present GPS is the only fully working satellite navigation system; the most significant alternatives are:
Principle of Operation
Although the various systems do not operate in the same way, theGPS system exemplifies some common features.
The orbits of theGPS satellites are tilted at an angle of 55° to the equator and are set on sixgeometric planes, which are 60° apart, with at least four satellites on each plane.
Most GPSsatellites orbit the Earth at an average altitude of about 20,200 km above the Earth’s surface or 26,560 km from the centre of the Earth. At that height the orbitalperiod is 12 hours relative to the stars, or 11 hours 58 minutes relative to the Sun-Earth system; each satellite returns to the same position relative to the surface of the Earth about four minutes earlier each day.
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