Definition
A cockpit display that shows the aircraft's height directly above the terrain or surface beneath it, as measured by a radar altimeter. It typically presents the height in feet above ground level (AGL) and includes a pilot-set decision height (DH) bug that triggers a visual or aural alert when the aircraft descends through that height.
Plain English
The instrument in the cockpit that tells the pilot how high the aircraft is above the ground right below it, not above sea level. It also lets the pilot set a height where it will warn them as they pass through it on approach.
Context Anchor
Seen in aircraft equipped with radio altimeters, especially during instrument approaches, low-visibility operations, and other situations where knowing height above the surface is important.
Derivation
Radar comes from 'radio detection and ranging' -- the system bounces a radio signal off the ground and times the return to measure distance. Altimeter combines the Latin 'altus' (high) with 'meter' (measure). So the indicator displays what the radar-based height measurement found.
Why Pilots Care
Provides accurate height-above-ground readings essential for terrain avoidance in low visibility or at night.
Intuition Check
Do not confuse this with the normal pressure altimeter. The radar altimeter indicator shows height above the surface directly below the aircraft; the pressure altimeter shows altitude based on air pressure settings.
Example Sentence 1
Approaching minimums on the ILS, the pilot cross-checked the radar altimeter indicator and saw 200 feet AGL just as the decision height alert sounded.
Example Sentence 2
As the approach continued, the radar altimeter indicator updated the height above ground level in real time.