When you are landing and close to the ground, you don’t want to worry about hectopascals and millibars and inches of mercury or arsenic or boron or whatever and altimeter settings and all that $hit.
You just want to know at every instant what is your ground clearance, and soon enough, how high you are above the runway.
Radio altimeters can be a good altitude-awareness device near the ground, and they are an essential part of all reduced-minimum Category II and III approaches. The radio altimeter measures actual height above the terrain in a way that is similar to radar. By timing the trip of a radio signal traveling from the airplane to the ground and back, the radio altimeter can measure height to within a few feet.
The radio altimeter can alert a pilot that he is closer to the terrain than expected, and thus provide an extra measure of altitude awareness.
Remember not to confuse radio altitude with minimum descent height or decision height on approach because they are not necessarily the same thing. The radio altimeter is measuring height above the terrain directly beneath the airplane. According to the radio altimeter, the airplane appears to climb when it flies over a valley. It also has no way of knowing if a ridge lies just ahead.
Most radio altimeters can measure the height from 2,500 feet agl down, though many do not display radio altitude until you’re nearer to the terrain. The radio altimeter is the heart of the ground proximity warning system, which is required on all airliners. The GPWS sounds its loud “pull up” command if the radio altimeter measures a low altitude and the airplane is not configured for landing with gear and flaps out.
GPWS is to altitude awareness of what the emergency brake is to a car without brakes roaring down a steep hill. When all else fails it might help. But a crew’s fate may be sealed by the time the GPWS measures the seriousness of the situation and gives its instruction.
This is what the radio altimeter provides. It kicks in at 2,500 feet above the ground and read right down to zero and even below zero.
Why below zero? Because on large aircraft, the radio altimeter is set to read Zero Altitude when the wheel touches the ground. So by the time it rests on the ground, with the oleos compressed, it reads in the negative.
The radio altimeter has become so important for maneuvers close to the ground that three of them are fitted on every airplane and a number of automatic systems use them.