What is the GPS - Global Positioning System ?
The Global Positioning System gives you accurate
position and velocity information anywhere in the world.History of the GPS
In the early 60s, the US Department of Defense decided
it needed a global, all-weather, accurate, positioning and
navigation system.
The Navy and Air Force began investigating the possibility
of using radio signals transmitted from satellites. These
studies eventually evolved into the GPS, which is no longer
solely for the US military but is used in a myriad of
pplications around the world.The first GPS satellite was launched in 1978. The first 10
were development satellites, called Block 1. From 1989 to
1994, 24 new satellites - Block II - were launched,
completing the satellite coverage we have today.
How the GPS Works
Before we venture into 3 dimensions, let us start with a
2-dimensional example of the principles involved. Suppose
you don't know where you are, but you know you are
267 km from Dublin, 605 km from Paris and 493 km from
Amsterdam. You draw a circle on a map of Europe with
its centre in Dublin and a radius of 267 km. You now
know you are somewhere on the circumference of this
circle.
You draw another circle with its centre in Paris and radius
of 605 km. As you know you are on the circumference of
each circle, you must be on one of the two places where
they intersect. You now draw a third circle, around
Amsterdam. Assuming the distances were accurate, this
circle will pass through one of the two possible locations
and pin-point your position.The GPS follows these principles. However we're now using
3, not 2 dimensions and so are dealing with spheres. This
also means we need 4 position references, not 3.The GPS satellites broadcast radio signals containing their
position and time, which your GPS receiver picks up. The
receiver knows exactly where in the sky the satellite is,
it just doesn't know exactly where on earth it is, until it
determines the distance from the satellite.It does this by calculating the time it took for the signal
to reach it. It knows that the radio waves should travel
at the speed of light, and it thinks it knows the elapsed
time since the signal set off. It calculates its distances
from 4 or more satellites and checks whether the spheres
intersect at one point. If they do not it assumes its
clock in inaccurate (satellites have atomic clocks,
receivers quartz clocks) and adjusts the clock to find
the exact location.So, why do GPS receivers sometimes tell you you've
arrived at your destination when instead you know
very well you are in a muddy field with some bad
tempered animals? The two major sources of error
are the US government, who can degrade the signal
if they choose, and the speed of the radio signals.
The signals may be delayed by particles in the
atmosphere, mountain ranges, tall buildings, etc, as
they bounce off all these obstructions.
Connecting GPS Receivers to PCs
When connected to computers, GPS receivers become
very versatile tools. You need some software to read
data from the receiver and save and present it on screen.
The free software from Windmill will do this for you. This
is useful in all sorts of situations. We briefly mentioned
one in the train monitoring story above. In a completely
different application, a marine biology survey is using
Windmill and the GPS as part of a project to map and
monitor marine habitats. The portable system (an old
laptop running the software inside a plastic box) can
quickly be transferred to the inflatable boat they use
for mapping small areas. A similar system could be used
in SCUBA diving, coral reef conservation, salvage and
marine archeology.
___________________________________________________For more information, please contact :-
Windmill Software
Website: www.windmill.co.uk
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