What does a radio altimeter do?

• A. Indicates barometric altitude
• B. Indicates indicated altitude above sea level
• C. Indicates altitude above airport elevation
• D. Accurately provides the height AGL

Answer: (D)

A radio altimeter measures the airplane’s height above the terrain directly beneath it. It sends a radio pulse to the ground, measures how long it takes for the echo to return, and converts that to a distance. That distance is read as height above ground level (AGL), not above mean sea level or above a fixed point like the airport elevation. This makes it especially useful during low-altitude flight and instrument approaches, where knowing exactly how high you are above the surface matters for obstacle clearance. The other options describe measurements tied to pressure altitude (barometric altitude) or a fixed reference point, which the radio altimeter does not provide.