Radar altimeter
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A Radar Altimeter measures altitude above the terrain presently beneath an aircraft or spacecraft. This type of altimeter provides the distance between the plane and the ground, as opposed to a barometric altimeter which only provides altitude above sea level. Radar altimeters are critical for flying very low over terrain. Radar altimeters are frequently used by commercial aircraft for approach and landing, especially in low-visibility conditions (see Instrument Flight Rules), as well as by military aircraft flying extremely low over terrain to avoid radar detection.
