What is multipath interference in radar altimetry, and how does it affect measurements near the ground?

What is multipath interference in radar altimetry, and how does it affect measurements near the ground?

• A. Wind gusts or turbulence.
• B. Signal is unaffected by geometry.
• C. Multipath and surface reflectivity degrade accuracy.
• D. Battery voltage fluctuations.

Answer: (C)

Multipath interference happens when the radar pulse reaches the receiver via more than one path, not just the direct line-of-sight route. In radar altimetry, the altitude is found from the time it takes for the pulse to travel to the ground and back. If part of the energy reflects off the surface and travels along another path (perhaps bouncing off the ground, nearby objects, or the aircraft itself) before returning, the receiver sees multiple echoes that overlap in time and phase. Near the ground, these multiple reflections can be of comparable strength to the direct reflection, so the combined waveform is distorted. The result is a biased and noisier height measurement because the system assumes a single, clean return. Surface reflectivity, determined by dielectric properties and roughness, influences how strong those secondary paths are: higher reflectivity surfaces can create more pronounced multipath components, worsening accuracy. So, multipath and surface reflectivity work together to degrade radar altimeter accuracy near the ground.

Multipath interference happens when the radar pulse reaches the receiver via more than one path, not just the direct line-of-sight route. In radar altimetry, the altitude is found from the time it takes for the pulse to travel to the ground and back. If part of the energy reflects off the surface and travels along another path (perhaps bouncing off the ground, nearby objects, or the aircraft itself) before returning, the receiver sees multiple echoes that overlap in time and phase.

Near the ground, these multiple reflections can be of comparable strength to the direct reflection, so the combined waveform is distorted. The result is a biased and noisier height measurement because the system assumes a single, clean return. Surface reflectivity, determined by dielectric properties and roughness, influences how strong those secondary paths are: higher reflectivity surfaces can create more pronounced multipath components, worsening accuracy.

So, multipath and surface reflectivity work together to degrade radar altimeter accuracy near the ground.