A radio altimeter is an automatic device that measures altitude above ground using radio waves. Which frequency band does it use, and what altitude range does it typically provide?

• A. 4.2 GHz; from near ground up to about 2,500 feet AGL
• B. 1030 MHz; from sea level to 10,000 feet
• C. 2.0 GHz; from 50 to 30,000 feet
• D. 40 GHz; from surface to 5,000 feet

Answer: (A)

Radio altimeters use a microwave band around 4 GHz, specifically near 4.2 GHz, because this frequency provides practical antenna sizes and reliable ground reflections for measuring height above terrain during the critical approach and landing phase. They work by sending a radio pulse to the ground and timing how long the echo takes to return, giving height above ground level rather than altitude above sea level.

The typical usable range is from the surface up to about 2,500 feet AGL. Beyond that, the echoed signal becomes weaker and ground clutter can overwhelm the reading, so the system is designed to be most accurate near the ground where it’s most needed. Other frequencies listed aren’t standard for a radio altimeter (they’re used by other systems or are impractical for this purpose), so they don’t match the usual altimeter function.