Definition
An aircraft instrument that measures height above the terrain directly below the aircraft by transmitting a radio signal toward the ground and timing how long it takes for the reflected signal to return. It provides absolute altitude (height above ground) rather than altitude above sea level, and is most accurate at low altitudes, typically up to 2,500 feet AGL.
Plain English
An instrument that bounces a radio signal off the ground to tell the pilot exactly how high the aircraft is above the terrain right beneath it.
Context Anchor
Seen in low-altitude operations, instrument approaches, landing callouts, terrain warning systems, and some automatic landing or flight control functions.
Derivation
‘Radio’ refers to the radio waves used to make the measurement, and ‘altimeter’ comes from the Latin altus (‘high’) plus the Greek metron (‘measure’) — literally a ‘height measurer.’ The name signals that this device measures height by radio, distinguishing it from a barometric altimeter, which measures height by air pressure.
Why Pilots Care
Gives precise terrain clearance to prevent controlled flight into terrain, especially at night or in low visibility.
Analogy
It works like judging distance by an echo: send a signal out, wait for it to come back, and use that time to determine how far away the surface is.
Grounding Statement
Radio waves travel straight down, reflect off the ground, and the brief return delay is converted directly into feet above the surface.
Intuition Check
Do not read “altimeter” here as the normal pressure altimeter in the panel. A radio altimeter measures height above the surface directly below the aircraft, not altitude above sea level.
Example Sentence 1
On the Category II approach, the pilot monitored the radio altimeter closely as the aircraft descended through 200 feet above the runway.
Example Sentence 2
During the night hover over the unprepared site, the pilot kept the radio altimeter at 20 feet to maintain safe clearance.