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The first GPS: Global Positioning System
The first form of conventional portable GPS (Global Positioning System) was originally 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!
The term "GPS" of often used for referring to any Global Positioning System, but also specifically to the first system which was put into orbit. The term "GPS" has been routinely bandied-about to mean both things, making it difficult to refer to the later Galileo GPS without confusion.
The idea of introducing the error in the early US Military GPS was supposed to be enough to make missiles go a bit off-course. However, this didn't work. Even though the error signal was different every day, so people were inconvenienced on their way to work, it was not very effective against perceived enemies. Quite simply, any fixed structure with a GPS in it could average out the errors and find its real location, and then with each new day and new error signal, it could broadcast the correction factor to anyone within range who wanted to get an accurate position. This was "Differential GPS".
Because of the error signal, and because the whole GPS setup was switched off every now and then, and possibly for other reasons, the United States GPS became such that many countries did not trust US GPS! Countries in Europe also required a higher accuracy system, as well as being a system that wasn't controlled by the USA. Some countries, it is suspected, had a good degree of scepticism in the reliance on a United States system for other reasons. So, a European version of Global Positioning System, known as Galileo, is being put up. This was much more accurate, and did not have any built-in errors. Initially the United States didn't take much notice of the European system, but before the first of the satellites got into orbit, the United States threatened to shoot down the European system if it didn't comply with the US being able to jam it, or at least that's the rumour I heard. So, beware! The European Galileo GPS is still not Pure GPS, and could be cut off at any time to any region of the world, at the whim of political powers that think there's a reason, in other words, for almost no reason at all.
Another thing to beware of about GPS the original Global Positioning System, is that there are various naff US export rules that prohibit export of GPS chips from the USA unless they have various crippled abilities. For example, they are not allowed to read a GPS reading if the unit is higher than 18km and/or travelling at a speed of over 1000 knots (some versions say "and", some say "or". It makes a difference). Also, any piece of space at any time can suddenly be "banned". These rules only apply upon manufacturers making the things in the USA and exporting them. Most of the world's electronics is made outside the USA and distributed worldwide including into the USA. However, these restrictions are something implemented inside the chips. So, you can defeat such restrictions by buying chips that are made outside the USA and are made to be without the limitations. You can also build your own GPS if you have the time and cleverness. There's a build-your-own GPS/Glonass unit at http://hackaday.com/2007/10/04/build-your-own-gps-and-glonass-receiver/ (notice how the plans are mirrored). The future of these things, I expect, is for full-software versions, which are open-source and obviously free of the restrictions.
Inertial navigation is free of such problems.
Curiously, the readings given by GPS are not accurate. You can measure this yourself. The true Prime Meridian by which all longitude is measured is the Prime Meridian at Greenwich. A GPS at that location, where the famous LINE is, doesn't read 0° 0' O" as it should. It reads about 300ft wrong. Therefore, all GPS readings (using the early system) can be considered to be skewed onto a slightly different coordinate system. Source: http://en.wikipedia.org/wiki/File:Greenwichmer1.jpg and http://en.wikipedia.org/wiki/Prime_Meridian . Wikipedia: "By international convention, the modern Prime Meridian passes through the Royal Observatory, Greenwich (at 51°28'38?N 0°00'00?E), in southeast London, United Kingdom". Presumably the GPS readings for the Equator http://en.wikipedia.org/wiki/Equator are similarly inaccurate. The reasons for the inaccuracies of GPS are historical (because the system wasn't calibrated properly in the early days), and also because the Earth is an imperfect sphere.