What type of altitude does a radar altimeter indicate?

A radar altimeter measures the distance between the aircraft and the Earth's surface directly beneath it. This is known as altitude above ground level (AGL). Unlike other altimeters that may reference barometric pressure and provide information regarding altitude relative to mean sea level or true altitude, the radar altimeter provides real-time and precise data of the aircraft's height above the ground. This is particularly useful during landing approaches and for low-altitude flying, where knowledge of the aircraft's distance to the ground is critical for safety and navigation. Thus, it is designed specifically to inform pilots about how high they are above the immediate terrain, confirming that the correct understanding is altitude above ground level.