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?
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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.
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$\begingroup$ VS data from what @ymb1? VS data can be calculated from the radar altimeter. $\endgroup$– GdDJul 4 '19 at 16:28
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$\begingroup$ For an airliner, from the air data computers by way of the static ports; the same source that displays the V/S. $\endgroup$– ymb1Jul 4 '19 at 17:57
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$\begingroup$ It depends on how accurate you want to be @ymb1, air data would be okay for approach, but when you're 20 feet off the runway small errors can make for firm landings. $\endgroup$– GdDJul 4 '19 at 18:23
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$\begingroup$ The problem is static ports misread in ground effect, so rad alt is more useful close to the ground. $\endgroup$ Jul 4 '19 at 20:51