Which instrument uses radio echoes to determine altitude above ground level?

• A. Barometric Altimeter
• B. Airspeed Indicator
• C. Radar Altimeter
• D. Altimeter with magnetic compass

Answer: (C)

Radar altimeter works by sending a microwave radio pulse toward the ground and measuring the time it takes for the echo to return. Because radio waves travel at a known speed, the system converts that round‑trip time into height above the surface. This gives height above ground level, which is especially useful at low altitudes and during approaches where you need precise terrain clearance.

Barometric altimeters, by contrast, estimate altitude from ambient air pressure and reference mean sea level, not the actual ground beneath you. The airspeed indicator uses the pitot‑static system to measure how fast the aircraft is moving through the air, which is a different quantity entirely. An instrument described as an altimeter with a magnetic compass isn’t a separate method for measuring height—the compass provides direction, not altitude.