Back-up system 就是指这种后备的设施。我们来看个例子：航天飞机的内部有好多后备的设备来保护宇航员，

         On the second day of the flight, the space shuttle's main air-conditioning system broke down. But the astronauts switched on the back-up system and were able to continue the mission for another three days and land safely at Edwards Air Force Base.

        在航天飞机起飞后的第二天，飞机里的空调设备坏了。宇航员于是开动了后备的空调设备，这样他们才能再持续工作三天,然后安全地在爱德华兹空军基地降落。

        在当前高技术极为普遍的时代，如果没有后备的设施，有的时候非但会影响工作，而且还会造成很大的危险。下面就是一个例子：

         This airline pilot talked about getting ready to land that famous day when all the lights in New York went out. He said he was lucky the airport had a back-up system of its own generators -- otherwise he would have to try to land in total darkness.

        大家都知道纽约发生停电，全城陷入黑暗的那一天。这个飞行员谈到当天他准备降落的情景。他说，幸运的是，机场自己有后备的发电设备。否则，他得设法在黑暗里降落。"