Radar altimeter is used for low-visibility approach collision avoidance and for triggering events in last few meters on AUTOLAND-equipped planes (this is rarer

Radar altimeter is used for low-visibility approach collision avoidance and for triggering events in last few meters on AUTOLAND-equipped planes (this is rarer in USA but common for example in UK airports). The main glide slope is managed with barometric altimeter, sure, but that one lacks the detailed resolution of the radar altimeter and can't detect ground - it can only follow the difference from specific pressure according to standard model atmosphere.