How does a Radio Altimeter measure height?

A Radio Altimeter measures height by utilizing the principle of radar technology. Specifically, it emits a radio wave signal towards the ground and then detects the echo of that signal as it reflects off the surface below. The time it takes for the signal to travel to the ground and back is measured, and this information allows the altimeter to calculate the distance from the aircraft to the ground.

The choice that highlights measuring the frequency difference between sent and received signals refers to the Doppler effect and how a radio altimeter operates. The change in frequency of the returned signal, along with the timing of the transmission and reception, enables the radio altimeter to provide accurate height information above ground level, irrespective of variations in atmospheric conditions or ground composition.

Other options involve methods that don't provide the same level of accuracy or are not relevant to the functioning of a Radio Altimeter. For instance, GPS data can show position but doesn't measure height above ground with the precision needed for low-altitude flight operations. Barometric pressure relies on atmospheric conditions that can change and thus may introduce errors in height measurements. Descent rates from airspeed relate to changes in altitude but do not directly measure the distance to the ground.