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.
Plain English
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.
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.' Altimeter combines the Latin 'altus' (high) with 'meter' (measure). So a radar altimeter literally measures height using radio signals, rather than air pressure as a barometric altimeter does.
Why Pilots Care
Gives accurate height-above-ground information critical for terrain avoidance and safe low-level flight when visibility is poor.
Analogy
Think of it like an invisible tape measure pointing straight down from the airplane to the surface below.
Intuition Check
Do not read “altimeter” here as the normal cockpit instrument that shows height based on air pressure. A radar altimeter measures height above the surface directly below the aircraft.
Example Sentence 1
On final approach, the radar altimeter called out '100' as the aircraft crossed the threshold.
Example Sentence 2
The helicopter pilot used the radar altimeter to hold a steady 15-foot hover over the landing pad.