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Next time someone tells you to "Get lost!" – just smile and tell them calmly that you can't, thanks to your GPS navigation unit. Be sure to watch their face.

Okay, that is probably not the best reason to shell out a few hundred dollars or more for a GPS (Global Positioning System) unit, but there are, in fact, plenty of good reasons to do so – not the least of which is the joy of never again having to interpret an upside-down street directory.

What is a GPS unit and why do you need one?

The GPS (Global Positioning System) is a "constellation" of 24 satellites that orbit the Earth and make it possible for people with ground receivers to pinpoint their geographic location by a system of triangulation. It also gives you a highly accurate time reference thanks to the atomic clock on board each satellite. The GPS system was built and controlled by the United States Department of Defense, but it can be used by anyone with a GPS receiver unit, free of charge. Most general-use GPS receivers can locate a user with an accuracy of better than 10 metres.

Following the launch of the first satellite in 1978, GPS equipment was largely used in military and scientific applications. The primary military purpose of the GPS is to be able to tell exactly where the troops are, and for accurate targeting of smart bombs, cruise missiles, or other weapons. The satellites also carry nuclear detonation detectors – a major part of the United States Nuclear Detonation Detection System.

Scientists, on the other hand, rely on GPS systems to provide accurate data that was never available previously. This has enormous benefits in geological studies such as measuring the movement of the Earth's tectonic plates (seismology) and arctic ice floes, along with volcanic activity.

Outside the scientific arena, everyday commercial applications for GPS receivers are expanding. The system is used as a navigation aid in aeroplanes and ships, plus computer-controlled machinery such as harvesters, mine trucks and other vehicles. In recent years, GPS technology has entered the consumer market, with lower costs enabling everything from boat and car-based systems, down to pocket-sized personal models (an essential item for would-be woodsmen). GPS units are also commonly bundled into mobile phones, and personal GPS units are almost de rigueur for any serious outdoor adventuring, such as bushwalking, cross-country skiing, ballooning, sailing and flying. There are also a range of personal GPS receivers that can turn a regular smart phone into a fully fledged navigational unit.

GPS units are also often standard equipment on many cars, trucks and commercial vehicles. In these conveyances, the features go far beyond replacing the ever-handy street directory. Today's GPS receivers cannot only tell you where you are and direct you where to go, they can also display a range of valuable information like traffic and weather details, "black spots", fixed speed cameras and red light cameras. And avoiding one of those even once can just about pay for the system!