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.


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