Plain English Meaning

An instrument that bounces a radio signal off the ground below to tell you exactly how high you are above whatever is directly under the aircraft right now.

Full Definition

An aircraft instrument that measures the actual height of the aircraft above the terrain directly beneath it by transmitting a radio signal downward and timing how long the reflection takes to return.

Context Anchor

Seen in aircraft equipment descriptions, terrain warning systems, some autopilot systems, and procedures where exact height above the surface matters.

Derivation

Radar comes from 'RAdio Detection And Ranging.' combines the Latin 'altus' (high) with '' (measure). So a radar altimeter literally measures height using radio signals, rather than as a does.