Sea and Space / Navigation / GPS

## What is GPS?

The Global Positioning System (GPS) has been developed in order to allow accurate determination of geographical locations by military and civil users. It is based on the use of satellites in Earth orbit that transmit information which allow to measure the distance between the satellites and the user. If the signals from three or more satellites are received, simple triangulation will make it possible to determine unambiguously the location of the user.

This may seem quite simple and the physical principles behind GPS are not difficult to understand. However, as is often the case, there is a long way from theory to practice.

The present GPS is based on a development programme that began in the early 1970's at the US Department of Defense. It has several components, each of which represents impressive use of current, advanced technology and mathematics.

The three main components are the GPS satellites, the GPS receivers, and the complex computer software needed to decode the signals and compute the geographical position of the user.

The GPS Satellites (Courtesy Hans Toft)

Up to 30 GPS satellites fly, mostly in highly inclined (polar) orbits, at altitudes around 20,000 km. This means that there will be between four and eight of them resonably high in the sky above any site on the Earth at any time.

They continuously emit coded high-frequency radio signals which may be received by special GPS receivers. These signals contain information about the exact orbits of the satellites and the time of atomic clocks onboard the satellites. When comparing the arrival times, the time delay between emission and receipt is measured and from the speed of light, just below 300,000 km/sec, the distance between the satellite and the receiver is computed.

When signals from three or more satellites are received, the GPS receiver will compute the best possible location of the user, that is, that point (in space) that best reproduces the measured time delays.

The achievable accuracy depends on the status of the user. For military purposes (and some specific civil ones), one metre or better in all three coordinates (longitude, latitude, altitude) can be reached. For common civil users, the full accuracy of the coded satellite signal cannot be exploited, but it is still possible to reach an accuracy of about 15 metres in the best cases.

A good GPS receiver is now cheaper than a mobile telephone. While it is great fun to use a GPS receiver during hiking trips etc., there are some applications where GPS has become of vital importance.

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Navigating across the North Sea with GPS.

Virtually all civil aircraft are now equipped with GPS. This allows the pilot to know the position of the aircraft and by continuously plotting this position on a computer map, to follow the progress of the flight along the chosen route. The same is true for ships - no sailor would ever attempt an ocean crossing without guidance from a GPS device.

Even more recently, GPS has come into use in connection with automatic car guidance. With suitable computer equipment, the driver of a car with such a system can be guided by voice through a city unknown to him/her, arriving safely at the destination along the shortest route. It is also used by some truck companies to know where their trucks are located at any time and some taxi companies have begun to test it, thereby expecting to improve the allocation of taxicabs to incoming requests.

There is no doubt that GPS will come to play an increasingly important role in our lives. It may safely be predicted that most citizens will come to depend on it in one way or another in due time.

The technically interested citizen will learn to use it, much like we now use mobile telephones. Even though you may first come to fully appreciate its usefulness if you would become lost in the wilderness, there are many other advantages of GPS for the citizen of a technologically developed society!

#### Exercise:

Discuss GPS and its various current uses. Can you think of other future uses which may be beneficial to society? Find out if there are any regulations for the use of GPS in your country!

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