GPS (Global Positioning Satellites) is a network of twenty four working satellites which orbit the planet and were originally launched in the mid eighties by the United States department of defence. It was meant for military use but civilian access was granted and allowed for the development of satnav as we know it.

The existing twenty seven satellites (i think they must have a few spare ones in orbit) are powered by the means of solar cells and require replacing about every 10 years.This is done by regular launches of space missions that allow the US department of defence to keep the system up and running and a regular supply of new satellites

GPS works by having a receiver that picks up the signal from the satellites and works out the position by how long the signal takes to reach the unit from each of the the different satellites in range, for a 2D position you need a signal from at least 3 satellites and at least 4 satellites for a 3D position. Once this information transfer takes place the position can be calculated by means of Trilateration (comparing the signals from at least three different sources using the geometry of triangles).

Luckily the receiver will calculate all this complicated mathematical information for you so no need to be a maths genius and the calculations are made in times of only milliseconds so no big delays and having to wait around for the position to be worked out.You do however need to have a good clear area to receive the signals so you may encounter problems receiving in heavily built up areas and places like valleys.

With fantastic benefits to be had it should be noted that at anytime the US department of defence could stop civilian access to its GPS signal so best not to throw away all your paper based maps just yet.There was a scare when last year President Bush hinted that the GPS system could be shut down in times of national crisis to prevent its use by terrorists,this in itself would have a dire effect on all systems that use GPS to function.

It should be noted that the European union is in the process of launching its range of Galileo satellites( to offer an alternative to the US department of defences GPS system.This program is underway as we speak with four satellites due to be launched this year.It is expected to be operational this year with completion by 2010.This is an exciting time for this fantastic technology and who knows what we will be able to achieve as we are offered more alternatives and improved performance.

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