But the altimeter does not show how high the plane is above the ground. A plane might be flying at an altitude of 15, 000 feet, but it would be only a thousand feet or so above the ground if the land in that area happened to be 14, 000 feet above sea level.

　　The pilot adjusts his altimeter to the actual sea­level pressure before setting out from the airport and then corrects it in flight according to new information given to him by radio. He flies high enough to be above any mountain he may pass on his course. The barometer altimeter is correct within about 300 feet.

　　Another kind of altimeter, the radio altimeter, makes use of radio reflections. It calculates the height of the flying plane by sending out electrical signals to the surface below and measuring the time required for them to bounce back(反弹). It is correct within 15 feet over water, but is not reliable over land. Big planes usually have both kinds of altimeter.Title: 1. ____________

　　2. ____________ An instrument used in an airplane to tell the pilot 3. ____________ of the craft.

　　4. ____________

　　5. ____________

　　The radio altimeter

　　How it works by 6. ____________ to show the altitude of the airplane, or the height above sea level by giving out 7. ____________to the surface below and measuring the time required for them to bounce back

　　How to

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