Like in ALL autopilots the RadAlt induces a bias for sensitivity ( read microvolts) from acquisition of ground return to the certified minimum of A/P utilization ( indeed this can be ground contact).
As a consequence, and i think it is the case in the A320, the whole logic of flare, derogation, throttle etc..depends on Radalt.
However i do not see why F/D should be off for a cat 1? Is it like mandatory when flying manual to have the F/D OFF ?