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Tuesday, 26 March, 2002, 16:38 GMT
Galileo: How will it work?
European transport ministers have given the green light to a network of navigational satellites known as Galileo. BBC News Online looks at the technology behind the project.

The technology relies on a global network of satellites in space that transmit radio signals.

These signals can be picked up on the ground by anyone using a Global Positioning System (GPS) receiver.

When three or more satellites are overhead, the receiver is able to work out exactly where it is on the ground by a technique known as trilateration.

In very simple terms, if you know where you are in relation to three known points, you can work out your location.

To find out where you are, your GPS receiver needs to work out two things:

• The location of three or more satellites above you
• The distance between you and each of the satellites.

Satellites travel in predictable orbits high above the Earth. The GPS receiver stores information telling it exactly where each satellite should be at a given time.

The distance between you and each satellite is measured by calculating the amount of time it takes for the radio signals to travel from the satellite to the receiver.

Radio signals, like all electromagnetic waves travel at the speed of light in a vacuum.

Although the Earth is not a vacuum and its atmosphere slows the signal, a GPS receiver is able to work out the speed of the radio signal using complex mathematical models of a wide range of atmospheric conditions.

It then figures out how long it took for the signals from the satellite to reach you.

Each satellite contains atomic clocks that can measure time, and thus position, very accurately.

Galileo will use clocks that will allow you to find your position anywhere on the Earth's surface to within 45 cm, according to the European Space Agency (ESA).

What are the differences between the two systems?

The technology is the same but there are a few differences:

• Accuracy: GPS gives information that can pinpoint a position to within 30 metres. ESA says Galileo will be accurate to within about a metre
• Availability: Galileo should give better coverage over Northern Europe and in urban areas
• Control: Galileo is a civil system whereas GPS and the Russian system, Glonass, are under military control.

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