The reason for this is that, on a Cat I approach (or, indeed, a non-precision approach, there is no guarantee prior to the threshhold that the ground does anything remotely predictable as you close on the runway. You may, therefore, go from 500' Rad to 30' Rad in the blink of an eye.
What? I would hope that isn't the case because the ILS setup would be
HIGHLY suspect! Given that the average GP angle is 3 degrees and that adequate terrain clearance exists at that angle, it stands to reason that the terrain in the vicinity of a Cat. 1 DH can't possibly change by the amount described in the above quote.
I certainly DO use Radalt on a Cat. 1 ILS, but monitor it as a backup for the altimeter indications. I also set
250 FT on a NPA because THIS is the minimum obstacle clearance within 5 NM of the THR (reference Pans Ops), with a fix within that distance. I monitor this to ensure that the MDA doesn't breach the minimum obstacle clearance criteria.
I believe in using everything at my disposal to keep my operation safe.