七选五
Radar is an electronic device that detects planes, ships, coastlines, landmarks, and even storm clouds. The name radar comes from the first letters of the words radio detection and ranging. As the human eye uses light waves to see, radar "sees" with radio waves. Without radar, planes could not land safely in bad weather and ships could not move safely in thick fog. In the 1800s it was discovered that radio waves could be reflected from objects. But scientists did not make great advances in radar research until the 1930s, when the world was threatened by war. It has been widely used in almost every conflict since then.
Echoes make it possible for radar to work. When a beam of radio waves is sent out, it strikes an object and returns an echo. This echo is picked up by radar and recorded on a screen similar to a television screen. On the screen an observer sees a flash, or a blip that shows the direction of the object and distance.
Airport control towers use radar to guide planes in for safe landings. Almost all large ships depend on radar to prevent collisions with icebergs or other ships. Weather forecasters "see" the size, direction, and speed of storms on radar screens. Astronomers can even use radar to accurately measure the distance to the moon. Thus, the device has generated major advancements.
Huge radar antennas have been installed all over the United States. They guard us against possible enemy attacks. We have ballistic missiles (弹道导弹) warning systems that can alert us to attacks when missiles are more than 1,000 miles away. Other radar systems warn us against enemy aircraft and even spying space satellites.
| A. Police track speeders in radar-equipped cars. B. These measurements are now highly accurate. C. The device can be used in a wide range of fields. D. During World War II many countries used radar effectively. E. It is therefore especially valuable to shippers and travelers. F. Today radar is used by commercial airliners and weather forecasters. G. Strengthening national defense is one of radar's most important functions. |