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GPS : Global Positioning System
The term "Global Positioning System", often shorted to "GPS", refers to any system which positions the location of the observer anywhere in the world. Such "Where Am I?" systems have been around for a while, and here's how they work:
GPS works by there being a number of satellites in orbit, and as they all know where they are precisely, and they know what time it is precisely, they can transmit that information. Then, because radio waves travel at the speed of light, which is known precisely, it's possible for the GPS unit to work out precisely where it is, and to tell you. (In effect it's advanced radio trigonometry).
A basic GPS box can tell you your exact Latitude and Longitude, and to some extent your Altitude. (Curiously, it can also tell you what time it is!). However, GPS has come a long way since it began, and there are a multitude of GPS add-ons or GPS Plus gadgets and applications which include Sat-Nav (and all the problems that go with that!).
The first form of conventional portable sat-nav that worked globally was the Early GPS Global Positioning System. This was put up by the United States, initially as a military navigation system. However, it soon proved so useful for civilian purposes that it became "The GPS", and had widespread use globally. It had many things going for it, but had an unfortunate built-in error in the location, which it has been speculated was put in so that enemies of the USA would not be able to use the US-made GPS against its forces! Incidentally, there is a Russian GPS which is Glonass.
Following on from the success of the earlier American system, mainly for civilian purposes, people in Europe found that they required a higher accuracy precision Global Positioning System. The first satellites for Galileo went up and went on test. However, this was not well met by the military in the USA, who realised that Galileo could be used by anyone anywhere in the world, and could in theory be used in battle against US forces. So, the USA insisted on NATO having the capability to jam Galileo. (It is rumoured that the USA threatened to shoot it down if they weren't given that power).
With the continuing move towards open standards and freedom of use of technology anywhere without let or hindrance, as per Linux, it is hoped that someone puts up an additional system that can't be jammed by anyone. This would be Pure GPS, a system that everyone can use and everyone can rely on, and without fear of it suddenly being cut off.