How does a radar altimeter determine height above terrain, and what limits performance near the ground?

How does a radar altimeter determine height above terrain, and what limits performance near the ground?

• A. It uses GPS to compute altitude above sea level.
• B. It measures barometric pressure to estimate height.
• C. It uses AI to infer height from image.
• D. It sends microwave pulses and measures time-of-flight of echoes; near the ground, multipath and surface reflectivity can degrade accuracy.

Answer: (C)

Radar altimeters determine height above terrain by sending microwave pulses toward the ground and measuring the time it takes for the echoes to return. Since the speed of radio waves in air is known, the distance to the ground is calculated from the round-trip time (roughly distance = c × time / 2). The system relies on detecting a clean, direct ground echo to give an accurate height. Near the ground, performance is limited because of multipath and surface reflectivity. Multipath occurs when the signal bounces off nearby surfaces (the ground, wings, landing gear, other objects) and returns multiple echoes, which can confuse which echo represents the true direct path. The ground’s surface roughness and reflectivity vary with terrain, moisture, and angle of incidence, causing the echo to spread or change strength and making it harder to identify the correct return. These factors degrade accuracy as you approach the surface, leading to potential errors in the measured height. Other methods exist for altitude, but they don’t measure height above terrain in the same way: GPS provides altitude above mean sea level, barometric sensors infer height from pressure with weather and calibration sensitivity, and image-based AI is not how radar altimeters operate.

Radar altimeters determine height above terrain by sending microwave pulses toward the ground and measuring the time it takes for the echoes to return. Since the speed of radio waves in air is known, the distance to the ground is calculated from the round-trip time (roughly distance = c × time / 2). The system relies on detecting a clean, direct ground echo to give an accurate height.

Near the ground, performance is limited because of multipath and surface reflectivity. Multipath occurs when the signal bounces off nearby surfaces (the ground, wings, landing gear, other objects) and returns multiple echoes, which can confuse which echo represents the true direct path. The ground’s surface roughness and reflectivity vary with terrain, moisture, and angle of incidence, causing the echo to spread or change strength and making it harder to identify the correct return. These factors degrade accuracy as you approach the surface, leading to potential errors in the measured height.

Other methods exist for altitude, but they don’t measure height above terrain in the same way: GPS provides altitude above mean sea level, barometric sensors infer height from pressure with weather and calibration sensitivity, and image-based AI is not how radar altimeters operate.