Science & Technology in Action

5th Edition

Satellite Navigation

Ordnance Survey Ireland

A portable navigation device initially calculates its position to the nearest 300 m. It then uses a number of different methods to refine the calculation, account of atmospheric and other errors.
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Lesson excerpt

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:

  • Galileo - a European system that is designed to operate in conjunction with GPS but will be capable of independent operation. It was hoped to have the full set of 30 satellites in place by 2010.
  • GLONASS - a Russian system which was in disrepair from the late 1990s and has recently been made operational again in a cooperative venture by Russia and India.
  • Compass - a Chinese system that is still in development.

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.

True or False?

  1. PND stands for Peripheral Navigation Device. false
  2. GPS is a US Global Navigation Satellite System. true
  3. The European GNSS is called Galileo. true
  4. There are currently just three different satellite navigation systems. false
  5. GNSS satellites transmit radio signals in the microwave part of the electromagnetic spectrum. true
  6. A GNSS satellite calculates the position of a PND and transmits that information to it. false
  7. PNDs maintain accurate time using built-in atomic clocks. false
  8. Electromagnetic waves travel 300 m in one millisecond. false
  9. A PND uses signals from at least three satellites to calculate its position to the nearest 300 m. true
  10. Accuracy can be improved by using signals from local ground-based stations as well as satellite signals. true