Global Positioning System aka GPS

The GPS is a network of 24 navigation satellites orbiting the Earth that people can use to pinpoint their position. THe satellites orbit at a height of 20,000 kilometres(12,500 miles). A GPS receiver picks up signals from any of these satellites that are above the horizon. It uses information in each signal to work out how far away it is from the satellite. It can calculate its position from the
Earth's surface when it has information from at least three satellites. A basic GPS receiver shows the latitude and longitude of its position on its screen. More advanced receivers, especially those designed for use in vehicles, show their position on a digital map. These receivers often show extra information, such as the vehicle's speed and the length of the journey. Some receivers warn drivers if they exceed the speed limit for a road and even tell drivers which traffic lane to use at the next junction. Directions are shown on the screen and also spoken by a synthesized voice!