A Sat Nav, which means Satellite Navigation, is a small portable radio receiver. The first Sat Nav devices looked like mobile phones. You held them in your pocket. Now we find Sat Nav devices in cars, aeroplanes, and most mobile phones have Sat Nav built into them. Modern Sat Nav devices, for example the popular Garmin Sat Nav systems, have accurate electronic maps, so you know exactly where you are. For example, the can even tell you the names of the streets in a city. We switch them on, and their electronic voice guides us to our destination.