How the aircraft sense the sink rate during autoland? Radio altimeter is required for autoland, and the target of autoland flare mode is to reduce the sink rate to an acceptable sink rate at touch down， then how the sink rate is computed by the radio altimeter since radio altimeter just provides the height information rather than the rate?
The radar altimeter doesn't calculate the rate of descent. A radar altimeter only gives height information, but it can take many readings very quickly and feed that data to a computer which will calculate the rate of descent.