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Imagine the situation. You are driving alone in a desert or on a mountain. You have no idea where you are. You passed the last house two hours ago. Then your car breaks down. It is night and it is cold. Yon have no mobile phone. What do you do? Well, next time take a GPS with you. This invention may be able to help vou. It is a device(装置) which uses satellites(卫星) to find the user's position(位置). It can find your position to within 20 metres. A GPS cannot start your car, but at least you will know where you are.
GPS, which means Global Positioning System, is a small radio receiver. It looks like a mobile phone. You can hold it in your hand, or put in your pocket. It is sometimes put into a watch or a telephone. We also find GPS devices in cars, planes, or boats. Some of these devices have electronic maps, so you know where you are. For example, in a city they can tell you the name of the street.
There are three parts to the Global Positioning System. The first part is the receiver. You can hold it in your hand, or have it fixed into your car, plane, etc. The second part is a group of satellites orbiting the Earth. The receiver contacts at least four of the satellites and calculates(计算) its position. The third part of the system is a network of ground stations.Theyare all over the world. They control the satellites and make sure they are working well.
Some people think that in the future the GPS will be as common as the mobile. They are becoming cheaper and more and more accurate(精确的). There are also new uses for the GPS. Perhaps they will become like watches. Everyone will have one and you will never be lost again.
