What is the basic measurement principle of a radar altimeter?

What is the basic measurement principle of a radar altimeter?

• A. Doppler shift of the reflected RF energy
• B. Time required for the RF pulse to travel from the aircraft to the Earth's surface and back
• C. Frequency difference between transmitted and received signals
• D. Atmospheric pressure changes

Answer: (B)

Measuring altitude with a radar altimeter relies on the time-of-flight principle. The device emits a brief RF pulse toward the ground and then detects the echo. By precisely timing the round-trip interval and using the fact that electromagnetic waves travel at the speed of light, the altitude is found as roughly half of the product of the travel time and the speed of light (with small corrections for air, if needed). Doppler shift or frequency differences relate to motion along the line of sight rather than height, so they aren’t the method used to determine altitude. Atmospheric pressure changes aren’t the measurement method either; while pressure correlates with altitude in general, the radar altimeter determines height from the reflected signal’s travel time, not from pressure readings.

Measuring altitude with a radar altimeter relies on the time-of-flight principle. The device emits a brief RF pulse toward the ground and then detects the echo. By precisely timing the round-trip interval and using the fact that electromagnetic waves travel at the speed of light, the altitude is found as roughly half of the product of the travel time and the speed of light (with small corrections for air, if needed). Doppler shift or frequency differences relate to motion along the line of sight rather than height, so they aren’t the method used to determine altitude. Atmospheric pressure changes aren’t the measurement method either; while pressure correlates with altitude in general, the radar altimeter determines height from the reflected signal’s travel time, not from pressure readings.