Radio altimeters measure the distance of an aircraft above the ground rather than above sea level, they are used in automatic navigation and Blind-landing systems.
无线电高度表测量的是飞机离地面而不是离海平面的距离,用于自动导航及仪表着陆系统上。
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That's because drivers must know where to place their cars on the road before they take off over blind crests to ensure a safe landing.