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Originally developed by the US military, a network of 24
satellites orbit the Earth at an altitude of just over 20,000Km.
Of the 24 satellites, only 18 are being used at any one time,
the other 6 are active spares which can replace a faulty satellite
at any time.
Each satellite transmits a very accurate time signal. As each
satellite is at a different distance from your car, the signals
arrive at different times. These are collected by your Satellite
Navigation system via a small satellite aerial.
As you can imagine, the time differences are microscopic,
and requires advanced technology.
Car satellite aerials usually pick up around 8 satellites,
but only 4 are required for the system to start working -and
will position the car to within 100 metres.
Even with this accuracy you could still miss a junction so
there are other operations which take place to position the
vehicle to within a few feet.
Gyroscope - Inside your navigation computer there is an electronic
gyroscope which detects any change in direction and it's duration,
enabling the computer to follow all movements of the vehicle.
These movements will be compared to the CD map.
The CD Map - the built in CD map has every street in the UK
on it. By knowing pretty much where it is in the first place
(via the navigation satellites) it compares the movements
sent to it by the gyroscope, and after a few turns and junctions
the system knows exactly where you are.
The distance travelled - Your navigation system compares the
CD map to the distance travelled between known turns and junctions
to the pulses it receives from the vehicles speedo, after
a while it 'knows' exactly how many pulses are required to
go a specific distance.
By combining all this information, your vehicle will know
where it is to within a few feet.
It is possible to specify satellite navigation as an optional
extra to a new car and a few even have it as standard on certain
models. Renault offer it on some vehicles such as the Laguna
Initiale, Nissan's top of the range Primera's also have it
as standard.
As an optional extra you can pay anything from £1000 to more
than £1500 on lower spec vehicles. There are a number of after
market solutions that can be fitted to a vehicle after it
is new. These also vary from a few hundred pounds to over
a thousand.
The specification on Satellite Navigation or GPS (Global Positioning
Satellite) varies widely and largely depends on price. Here
are a few things to look out for:
Does it have a colour or black and white monitor
Some systems play DVDs, is this something you want
Some systems can link to traffic information and re-route
you around trouble spots
What are the output options, the system above has a colour
screen that can display maps, directions or both, it also
has an address book to store locations.
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