Finding¹ the way home 

　　Hundreds of years ago, travellers found their way by looking at the stars. They built towers that could be seen from far, made maps and invented the compass. 

　　Nowadays, almost every explorer and mountain climber uses the Global Positioning System (GPS) to make their journeys safer. There are even plans to put GPS receivers on all the taxis in Beijing in time for the 2008 Olympics. Finding your way has never been so easy. 

　　Technology has changed our lives: for less than 500 yuan, you can get a pocket-sized GPS gadget² that will tell you exactly where you are on Earth at any moment. 

　　When people talk about a GPS, they usually mean a GPS receiver. The GPS is actually a set of 27 Earth-orbiting satellites-24 in operation and three extras in case one fails. 

　　The US army developed this satellite network as a military navigation³ system in the 1970s, but opened it up to business use in the 1990s. 

　　A GPS receiver locates these satellites, records the distance to each, and uses the information to calculate⁴ the receiver’s exact position on Earth. 

　　数百年前，旅行的人通过观察天上的星斗来识路。他们建造从远处就能看得见的灯塔，绘制地图，并发明了指南针。 

　　现在，几乎每个探险家和登山者都使用全球定位系统（GPS）以确保旅行更安全。为了2008年奥运会，北京甚至计划在所有的出租车上安装GPS接收器。位置识别从来没有象现在这样容易。 

　　技术改变了我们的生活。你只需花费不到500元钱，就能买到一个口袋儿大小的GPS部件，可以精确地告诉你某个时刻到底处于什么位置。 

　　人们在谈到GPS时，指的通常是GPS接收器。实际上，GPS是一组由27个环地球轨道运行的卫星组成的卫星网络，其中包括24个处于运行状态的卫星和3个额外的备用卫星。 

　　该卫星网络是美国军队于20世纪70年代开发出来作为军事导航系统的，在90年代开放作商业用途。
 
　　GPS接收器对这些卫星进行定位，记录下到每个卫星的距离，并使用这些信息来计算接收器所处的具体方位。

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