Brilliant for showing you the fastest route for your journey, Satellite Navigation Systems (Sat Navs)  can save endless hours of hassle, motorway queues, and even help you avoid the school rush.

Easy to use, the only tricky part is choosing the right one - and that's where this guide can help.

Sat Nav uses the GPS satellite network to locate your position and calculate how to get to your destination. It displays your current location on a road map and if you punch in a postcode, landmark or address, it can plot the quickest route to get you there.

Some models speak the directions, so you won't have to take your eyes off the road. You'll be warned about speed cameras, speed limits, traffic congestion. Points of local interest will also be flagged.

Perfect for motoring holidays, Sat Navs have vast international coverage - including Europe, the USA and Canada.

Sat Nav systems vary widely in the way they do the same job. Basic models have monochrome screens and standard button functions, while others have colour touch screen displays and satellite photography of your location. Some key features to compare are:

Screen size: The more expensive the device, the larger the screen - and for in-car use size is an advantage. Screens can measure 3.5in, 4.3in or 5in and larger screens allow you to see more of the map. Touch screens - that allow simple operation via the lightest touch - are especially useful when you're driving.

Maps: software controls the way maps are displayed. Simple devices will show a basic 2D map, while others will have detailed 3D maps with paths, tracks and other information, with the display looking more like the view from your car. Additional maps are available for purchase for some systems.

Maps are constantly updated and improved. For the latest versions, you'll need to subscribe for updates, which usually carry a small monthly charge after an initial free period. Mapping types:

Display: the LCD screen will either be monochrome or full colour and its detail varies. A higher resolution screen has a greater number of pixels that make up the image, giving more detail. A common screen resolution is 320 x 240, which is the number of vertical lines multiplied by the number of horizontal lines.

Control: touch screen controls let you use the map by tapping and moving your finger on the screen. Other devices have voice recognition and voice guidance, for hands-free use.

Battery life: Sat Nav is quite heavy on battery charge, but in-car systems will be able to draw power from the car through the cigarette lighter.

Traffic function: the Traffic Message Channel (TMC) delivers high quality, accurate and relevant travel information via the radio or directly into the Sat Nav. This allows the driver to take an alternative route to avoid traffic incidents and other obstacles. Should there be a traffic jam, the Sat Nav will re-route, revising your route immediately.

Position Plus: in built-up areas you may lose the GPS signal. If this happens Position Plus records where you are, calculating your movements while you are out of range.

Gesture Command: for frequently-visited destinations, just stroke the screen and the Sat Nav will take you there.

Dynamic Route Info or Point of Interest: keeps you updated on nearby facilities, local services, service stations and National Trust sites.