Imagine the situation. You are driving alone 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. You have no mobile phone. What do you do? Well, next time take a GPS with you. This device（装置）may be able to help you. It uses satellites to find the users' position. It can find your position within 20 metres. A GPS cannot start your car, but at least you will know where you are.
The GPS (the Global Positioning System) is a small radio receiver. It looks like a mobile phone. You can hold it in your hand, or put it 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, they can tell you the name of the street in a city.
There are three parts of the Global Positioning System. The first part is the receiver. You can hold it in your hand or fix it into your car, plane, etc. The second part is a group of satellites orbiting （绕……轨道运行）the Earth. The third part of the system is a network of ground stations. They are 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 phone. They are becoming cheaper and better. 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.
