What does the Radar Altimeter indicate on the Navigation Display?

The Radar Altimeter provides information about the aircraft's altitude relative to the ground directly beneath it, which is referred to as ground proximity. This instrument uses radar waves to measure the distance between the aircraft and the terrain, allowing pilots to determine how high they are flying above the surface, regardless of the elevation of the ground below. This is particularly useful during approaches and landings, as it offers a precise altitude reading that assists in maintaining a safe distance from the ground.

The other options pertain to different measurements that are not related to the function of the Radar Altimeter. Altitude Above Sea Level refers to pressure altitude or indicated altitude, which is based on atmospheric pressure and is not the function of a radar altimeter. Vertical Speed measures the rate of ascent or descent of the aircraft, while Engine RPM relates to the speed at which the engines are running, neither of which would be indicated by a Radar Altimeter.