Cecco:
I recently was safety pilot on a citationjet (CAT I certified) and it struck me that it had no radar altimeter. I gather that a radar altimeter is therefore only mandatory for CAT II and CATIII certified aircraft?
Mostly the case although the RAs on an autoland aircraft are required for even a VFR autoland.
Only on (most) CAT II approaches can the RA be used to determine DA instead of the baro altimeters, because where "RA" is stated on the CAT II chart the elevation of the CAT II DA point has been surveyed.