The Global Positioning System uses a fairly basic mathematical principle which goes by the exciting name of Trilateration and is sometimes called triangulation by those who think they know more than they do. It works by using snippets of known information to pinpoint a location, if you got lost in lets say Essex and asked a passing policeman where you were and he said 100 miles from London you may get a bit annoyed but this is a useful piece of information. You now know you are within a 100 mile radius of London and could draw this circle on a map.

If you asked the same question of another passing good spirited individual who told you were 50 miles from Southend you could draw a circle around Southend, these two circles will intersect at two points but you don't know at which one you are standing. So not good if you plan to navigate using this position and much worse if you are planning to use it to guide a cruise missile.

So with a third location "10 miles from Chelmsford" you can pinpoint exactly where you are, simple! Well that's in 2D the GPS system does all this Trilateration in 3D using a Network of 27 US military satellites, 24 of which are used to make calculations and 3 which are spares.

So this is the bit where you start to struggle to visualise how GPS works but imagine if you will the circles around the satellites being 3D spheres which overlap between satellite 1 and 2, 3 and so on..

As long as you get a lock on 3 satellites you will be presented with 2 possible positions, one of which cannot be on the surface of the earth and so it is discarded, the more satellites the GPS receiver can lock on to the higher the accuracy, its not uncommon for a good mutlichannel GPS system to lock onto 8 satellites or more.

The last component we need for an accurate position is to know how far we are from each satellite, the GPS system does this by sending a pseudo-random code which starts every night at exactly midnight, the receiver compares this to its own internal quartz clock and can determine how delayed the radio signal was when it received it and hence the distance between it and the satellite.

Of course a quartz clock is no match for the atomic clock inside the satellite but by using a minimum of 4 satellites the error can be nulled out as each reading should be incorrect by the same amount. We now have the three pieces of information required to navigate Longitude, Latitude and Altitude, when combined with an electronic map running on a navigation device you can pinpoint your position down to 15 meters.

Now there is an urban myth that the US Military has a much more accurate system, well that's true enough they can pinpoint to under 3 meters using WAAS  (Wide Area Augmentation System) and Differential GPS, both systems use additional ground based transmissions to improve accuracy, but that only works in North America and selected areas of the world (excluding most war zones!)

The Myth actually comes from a clause within the GPS system agreement that allows the US to impose the Selective Availability (SA) program, this would mean overnight that the accuracy of the GPS system would decrease from 15 meters to 100 meters.

There are many applications for GPS the most common is for in vehicle navigation systems like Tom Tom and Navman use the GPS signal to feed into PDA or PC based systems where it is used to overlay your current position onto a map and plot journeys. For walkers many products exist that will give the current location to aid map navigation or on some of the high end models you can run maps of the local terrain although these tend to be more limited and of less detail.

For more GPS gadgets check out our GPS and Navigation Section

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