Definition
An electronic instrument that measures the actual height of the aircraft above the terrain directly below it by transmitting a radio signal downward and timing how long it takes to reflect back from the surface. It displays absolute altitude (height above ground), not altitude above sea level, and is typically used at low altitudes during approach and landing.
Plain English
An instrument that bounces a radio signal off the ground beneath the aircraft to measure how high the aircraft is above whatever surface is directly below it right now.
Context Anchor
Seen in instrument flying, low-altitude operations, approach procedures, and aircraft systems that need an exact height above the surface near the aircraft.
Derivation
From 'radio' (using radio waves to measure) and 'altimeter' (height-measuring device). The name describes the method: unlike a pressure altimeter, which infers altitude from air pressure, this one uses radio signals to measure the real distance to the ground.
Why Pilots Care
It supplies accurate height-above-ground information essential for terrain avoidance, especially during night or instrument approaches where visual judgment is unavailable.
Analogy
It works a little like timing an echo. If a sound comes back quickly, the wall is close; if it takes longer, the wall is farther away. A radio altimeter does that with radio signals and the ground.
Grounding Statement
Picture the aircraft sending a signal straight down, receiving the reflection back, and turning that time delay into a height reading.
Intuition Check
A radio altimeter is not the same as the regular pressure-based altimeter. It shows height above the surface directly below the aircraft, not height above sea level.
Example Sentence 1
On final approach, the radio altimeter began calling out heights as the aircraft descended through 500 feet above the runway.
Example Sentence 2
During the non-precision approach the crew monitored the radio altimeter to maintain terrain clearance in the clouds.