Sat nav systems use a Global Positioning System (GPS) satellite receiver to calculate their position on the Earth's surface.
This is overlaid onto a local road map showing your position to within 10 metres. Sat nav software calculates the 'best' route between two points, using average road speed data with other parameters.
All sat nav systems give voice instructions for route guidance, for example, 'turn left in 300 yards,' and the best sat nav systems also use a colour screen to show a schematic illustration of the road ahead.
Some sat navs let you input your destination as a full postcode, which is quicker than entering a full address.
Sat nav screen options
Sat nav screen quality
The sat nav display needs to be big enough to view clearly. It should also be bright enough to be seen in daylight, and, preferably, feature anti-glare protection. Screen clarity has improved significantly with the introduction of widescreen sat nav systems.
Sat nav menus and advice
Sat nav menus follow the same convention as most modern computers and other electronic devices. Even so, some sat nav menus require more concentration than others to find what you're looking for. This can create problems, for example, with some sat nav systems it's difficult to change routes mid-journey.
Map views - plan vs 3D
Whether you find a 'top-down' plan view or an 'angled' three-dimensional (3D) version easiest to read on your sat nav is a personal preference, so look at both to see which you like best.
Sat nav systems detect where your car is using GPS technology. The software built into the sat nav displays your GPS location on its screen in map form.
Updating sat nav maps
The latest TomTom sat nav systems allow you to help correct mistakes in the maps supplied. The user can log problems on the move using the 'Mapshare' system, and next time the sat nav is connected to the Internet (using a PC) this information will be uploaded to the TomTom database.
Once the correction is verified by TomTom it will be made available for all TomTom sat owners to download free of charge.
Setting up your sat nav
Sat nav installation
The most common fixing method for sat nav systems uses a windscreen sucker, but a few sat nav makers also offer a dashboard mount, which can be useful for cars with a steeply raked windscreen, as it means you can fix the sat nav closer to you.
Sat nav systems are usually powered or recharged by your car's cigarette lighter, but some can run off regular batteries as well.
Sat nav set-up
To set up the sat nav itself, simply turn it on and follow the instructions to set up your preferences, such as whether you want imperial or metric guidance.
Next, tell the sat nav where you want to go; enter the address or postcode by selecting onscreen letters and numbers using buttons, a rolling wheel, or touch screen - and off you go.
So which is the best sat nav for you? Which? tests sat navs on every aspect of functionality- from clarity of information on screen to the ability to recalculate alternative routes. Find out which ones earned Best Buy status with a 1 month trial of Which? for just £1.
Copyright Which? Ltd 2009, all rights reserved.