Radar Altimeter/Radio Altimeter – Canadian Flight Trainers

A radar altimeter (also called a radio altimeter) is an instrument that measures the aircraft’s height above the terrain or surface directly below, using radio waves reflected from the ground. Unlike a barometric altimeter, the radar altimeter measures true height above the surface (AGL), not above sea level.

How It Works

The radar altimeter transmits a radio pulse toward the ground and measures the time for the reflected return. Height = (time × speed of light) / 2. Radar altimeters operate accurately from 0 to approximately 2,500 ft AGL, with accuracy of ±1–2 ft at low altitude.

Aviation Use

CAT II and CAT III ILS approaches require a radar altimeter for DH confirmation
On long-range aircraft, GPWS/TAWS systems use radar altimeter height for terrain alerts
Helicopter operations use radar altimetry for low-level terrain avoidance
Provides accurate height for cockpit checklists (e.g., “100 above, minimums”)

Related Terms

See also: Decision Height, Altimeter, ILS.