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Please note: this information was current as of November 2006 but is still a useful guide to today's market. For more recent information, see our 2012 Car GPS review.
A car navigation system consists of:
You enter into the system where you want to go. Using the map data, it then plots a route (you can usually choose between the fastest or the shortest route — which aren’t necessarily the same — or specifically exclude toll roads or highways), calculates the travel distance and estimated time of arrival, and displays the route on a map.
Using satellite signals, it keeps track of your position and guides you along the plotted route with travel instructions on the display and verbally via a computer-generated voice.
Portable units sit in a cradle with a suction cap that can be attached to the windscreen. They plug into the cigarette lighter, but also have a battery that provides a few hours of operation. They’re installed within moments, and can therefore easily be moved from car to car.
Integrated systems are usually connected to the car’s electronics, and can overcome some of the limitations of portable units. For example, they can use speed information to keep calculating your position when there’s no satellite signal (for example, in a tunnel).
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Video: What to look for - GPS
Want a new GPS for the car? Here's what you should look for.
GPS stands for Global Positioning System. It was developed by the US military, but is available free of charge to the general public. It has many commercial uses, from land, sea and air navigation to land surveying and map-making. GPS consists of 24 satellites that orbit Earth exactly twice a day at an altitude of about 20,200 km. The orbits are aligned so that at least four satellites are ‘visible’ at any time from most places on Earth. The satellites carry highly accurate atomic clocks and constantly send coded time signals to Earth. A GPS receiver can read these signals and use the time delay between the send and receive times to calculate its distance from the satellite (assuming the signal travels at the speed of light). A receiver uses a method called three-dimensional trilateration to then calculate its position on Earth:
If you want to read more about GPS, there's lots of information on the web - for example: