Greetings
The radio altimeter is used as a "blind man stick" to consolidate the alitude (true altitude) as the last safety net in case of predictive (TAWS) and to give a height in order to compute is the aircraft is in the envelop in the case of reactive (GPWS)
Man made obstacles are incorporated in some database in the US, the biggest demand is coming from helicopters and Biz Jet.
