Originally Posted by
wiedehopf
One key point not mentioned: A radio altimeter measures geometric height....
Strictly speaking they measure the distance to the nearest reflecting surface. That's not always straight down!
It's quite possible that the point of reflection for an airliner from above isn't the fuselage, which is nice and round so is not great at returning radio signals, but the tip of the tail fin. There may be a return from the wings and horizontal stabilisers, but they'll be later than the return from the tip of the fin, and are ignored.
EDIT - just seen similar point being made in a previous post!