Originally Posted by
Worcester
That's interesting, so the approach is governed by the radio altimeters? Does that not fluctuate as the aircraft flies over buildings, flyovers, hillocks, raised reservoirs, castles etc?
To a degree, yes. If the obstacle and/or change in terrain is significant enough it'll register.
Hence why at airports which allow full Category III operations (such as that required for an autoland) there are very specific rules in place and obstacle free areas in the immediate approach path. This includes aircraft waiting to take off having to wait at a holding point further from the runway than during normal operations. Having the rad alt suddenly come alive at 2500ft above a ridge when the surrounding terrain is actually 3000ft away isn't a problem. But you don't want it happening in the last 100ft!