How does a radar altimeter work and what are its applications?

Radar altimeter measures absolute height above terrain by transmitting radio waves downward and measuring return time. Types: FMCW (Frequency Modulated Continuous Wave) - measures frequency difference between transmitted and returned signal, dominant technology; and Pulse - measures round-trip time. Operating frequency typically 4.2-4.4 GHz. Accuracy ~1% or 1-2 ft at low altitude. Applications: Ground proximity warning (EGPWS inputs), Autoland (flare guidance), Decision height callouts, and Terrain clearance for helicopter operations. Range typically 0-2500 ft. Critical for low-visibility approaches. Interference concerns with new 5G networks near airports prompted protective measures. Dual-redundant systems for critical applications.