Definition
An aircraft instrument that measures the actual height of the aircraft above the terrain directly below it by transmitting a radio or radar signal downward and timing how long the signal takes to reflect back from the surface. The reading shows true height above the ground, not height above sea level.
Plain English
An instrument that bounces a radio signal off the ground and uses the return time to tell the pilot exactly how high the aircraft is above whatever is directly underneath it.
Context Anchor
Seen in instrument flying, low-altitude operations, approach systems, and aircraft warning systems that need the airplane’s actual height above the surface.
Derivation
Altimeter comes from the Latin altus, meaning high, and the Greek metron, meaning measure -- so simply 'a height-measuring device.' The 'radio' or 'radar' part tells you how it measures: by sending out a radio signal and listening for the echo, rather than by sensing air pressure as a normal altimeter does.
Why Pilots Care
It supplies actual height above the ground rather than pressure-derived altitude, which is critical for terrain avoidance during low-visibility flight.
Grounding Statement
The instrument sends a signal down, waits for the bounce back, and turns that time delay into height above the surface.
Intuition Check
A radio or radar altimeter is not the same as the normal pressure altimeter. The pressure altimeter shows altitude based on air pressure; the radio or radar altimeter shows height above the surface directly below.
Example Sentence 1
On final approach, the radio altimeter began calling out the height above the runway as the aircraft descended through 500 feet.
Example Sentence 2
In the mountains the radar altimeter gave immediate height-above-ground readings that helped avoid rising terrain.