In a CAT I, CAT II, CAT IIIA and CATIIIB-with-DH approaches, after we overfly the OM we are allowed continue the approach even if reported RVR gets below minimum. But we can only continue till the DH, at which we go around unless we have the required visual references.
what if there is no DH at all?
In CAT IIIB-with-no-DH there is no DH, only AH. There is nothing to decide, no point at which we have to see any visual references. So I guess we should overshoot as soon as the RVR goes below minimum, at any moment during approach. Or maybe revert to CAT IIIB-with-DH minimum and then decide at that DH, It occurs to me now.
What?
Nooooooo
Right , you have to have the required RVR at the approach ban point, (Outer marker or equivalent normally check your company SOP) after that you can if you wish continue with the approach to your appropriate minima. A Cat3b No DH approach means you can continue through to the ground... the clue is in the no decision height bit.