How it Works
Your little sat nav unit is a receiver linked to a network of satellites orbiting the Earth. The satellites help the receiver to pinpoint its precise position on the surface of the Earth, to within a few metres. Add maps and some computer technology, and your sat nav can guide you anywhere you want to go.
The Global Positioning System (GPS) was developed by the United States Department of Defense as a military tool for surveying and mapping the Earth's surface. The first satellite was launched in 1978, and there are now 29 in orbit, covering the entire surface of the Earth 24 hours a day. They emit constant signals relating to their position and the time. A GPS receiver on Earth uses these signals to determine the precise location of at least three of the satellites at any time. From this information, it can calculate its own latitude, longitude and altitude on the surface of the Earth, using essentially the old surveying technique of triangulation. With in-car sat nav systems, GPS is usually accurate to within about 10 metres, but some enhanced GPS receivers (used, for example, for mapping) are accurate to 10 centimetres or less. The GPS satellites are operated by the United States Air Force, but it allows free access to the system for civilian use - even though the system costs an estimated $400 million a year to maintain.
Satellite navigation (sat nav) depends on GPS signals to establish the current position of the receiver. Your receiver (your in-car 'sat nav unit') also contains a set of road maps. Using GPS and the maps together, it can plan a route between your current position and any selected destination. It can also plot your progress along the route, by constantly updating your position through signals from the GPS satellites.
Fixed and portable sat navs
A sat nav unit may be built into a new car by the car-manufacturer (often sold as an option). Such 'hard-install' units are usually top-of-the-range in terms of technology and function, and relatively expensive. The alternative is a portable sat nav unit, which is fixed - by means of a suction pad - to a mount (or 'bracket', or 'dock', or 'cradle') on the windscreen or dashboard, and powered by a cable attached to the car cigarette lighter. A portable unit may also have battery power, which means it can be used outside the vehicle, for instance when walking to reach a destination in a town.
Inputting your destination
Before you begin your journey, you have to program your sat nav unit, to tell it where you want to go. You do this by identifying your destination, by keying in the address, usually by pointing to the keypad on the touch-screen. The more precise the address, the more precise the directions, and the best the way to identify an address is to use the full seven-digit postcode, if your sat nav unit will accept this. Once it knows your destination, the sat nav will calculate to quickest route to get there - but if you prefer, you can always choose another route by inputting various 'waypoints' that the route should take you to, or by identifying roads that you wish to avoid ('road exclusion').
When you get going, your sat nav unit will anticipate the manoeuvres that you have to make by talking to you: 'Take the next left', 'After three hundred yards, at the roundabout, take the third exit', and so on.
Meanwhile, the screen will show your changing position on a map of the immediate locality. This might be a 2D (two-dimensional) map, which gives you an overhead view, like a traditional paper map. But most sat nav units now use '3D mapping', which provides a schematic, angled view of the route ahead of you, as if seen from the air; this makes it easier to visualise your next move along the way. The screen will also constantly update written data about your journey, such as your next instruction, your speed, speed limits in the area, your estimated time of arrival, the name of the street you are currently in, and so on.
Dynamic route recalculation
If you take the wrong turning, or choose to take another route, many of the newer sat nav models will simply recalculate your route automatically, and revise all subsequent instructions accordingly. This is also known as 'dynamic re-routing'.
If you have a series of destinations to visit on your journey (for instance, if you are delivering goods to various addresses), you can input all the destinations in advance.
Handheld GPS navigators
Walkers, cross-country runners and mountain-bikers can also benefit from GPS technology. The receivers designed specially for their use are similar to in-car sat nav units, except that they are built for battery operation, contain Ordnance Survey maps (or equivalents) rather than road maps, and do not have voice instruction. With the help of land-based WAAS and EGNOS technology to augment GPS readings, they are accurate to within 7.6 metres (i.e. rather more precise than in-car sat navs). In a similar vein, some PDAs (Personal Digital Assistants) and smartphones can be upgraded with GPS software to provide rudimentary handheld navigation systems.