Radar Altimeter

Unlike a pressure altimeter, which measures height above mean sea level, a radar altimeter bounces radio waves off the surface directly below the aircraft and measures the return time to compute true height above terrain. It reads from 0 to typically 2,500 ft AGL and is most useful below 500 ft, where approach minimums and decision heights are relevant.

Radar altimeters are required for Category II and Category III ILS approaches, where decision heights drop to 100 ft or lower. They are uncommon in basic piston trainers but appear in more advanced aircraft used for instrument currency and commercial training. If you are pursuing a career path that includes Cat II or Cat III operations, knowing how a radar altimeter integrates with the autopilot and flight director is worth understanding early.