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Simply put, satellite navigation is all about knowing where you are or
knowing where you're going to. As mobile creatures, we need to
control our moves. Sometimes it's a matter of fun and amuzement, but
many times it's a matter of life and death.
Since we're talking about satellite navigation, we're obviously talking
about the use of the satellite technology to control our phisical
orientation. Once upon a time the Sun and the pattern of fixed stars
have been our guides. But now, we rely on constellations of manmade
satellites to guide the way.
Although we can still use the fixed stars to determine our position
within a few hundred metres, navigation satellites can tell us where
we are to the nearest 5 to 10 m, whatever the weather conditions.
All we need to take advantage of this sophisticated new technology is
a receiver to pick up signals transmitted by navigational satellites.
And since receivers are now very small components, they can be
incorporated into the electronics of a car, or even a mobile phone.
To find your location accurately, your receiver needs to receive signals
from at least two navigational satellites. The receiver determines your
distance from each of the satellites by measuring the time taken by
the signal to travel from the satellite to your receiver antenna. These
distance measurements tell you that you are situated somewhere on
the circle where two spheres intersect. The spheres have each of the
two satellites at their centres and their radii are the satellite-receiver
distances. If you are standing on the surface of the Earth, you are
standing on the surface of a third sphere. Your position must be one
of two points where the Earth’s surface touches the circle.
If you want to determine your position in an aircraft above the surface
of the Earth, however, you need to have a fourth satellite in view. In
this case, the sphere which has the fourth satellite at its centre and
a radius equal to the distance between that satellite and your
receiver, takes the place of the Earth in the example above. Three
and four satellites are the minimum needed to determine positions on
the surface of the Earth and above it. In general, the more satellites
used the greater the positioning accuracy. Many receivers have
channels for receiving signals from up to 15 satellites.
It is only possible to determine a location on Earth if you know the
location of the navigational satellites very precisely. This is achieved
by placing the satellites in highly stable Medium Earth Orbits (MEOs)
at an altitude of about 20 000 km. MEOs are the orbits of choice for
a number of reasons: their stability enables exact orbit predictions;
the satellites travel relatively slowly and so can be observed over
several hours, like a fixed star; and, the satellites can be arranged in
a constellation so that at least four are visible from any point on the
Earth’s surface at any time.
A satellite navigation system consists of a space segment, typically
24 navigational satellites, a ground or control segment and a user
segment. The satellites are placed in circular orbits at heights of about
20 000 km so that at least four are visible from any point on the
Earth’s surface at any time.
The ground or control segment consists of several tracking stations
placed at different locations over the Earth’s surface, and a central
ground station. The tracking stations monitor the position and health
of the satellites, send data back to the central station for processing
and then relay accurate measurements of each satellite’s position to
the satellite for incorporation into its navigational signal.
This section of our website, together with the sub-GPS menu,
provides you with everything you need to know about satellite