well spotted peg.. .the minimum is based on either, but for low vis the rad alt is close enough to the runway (or flat surface) to be useable. at points further out (like cat 1) you can not use the rad alt because of the terrain.you therefore use a DA for cat 1 (by which point the decision has been made) and,if you go around, you fly so as not to violate the associated MDA.of course, our mcdu does not have a field for 'DA' so we use the 'MDA' one. it is,in fact a DA we are inserting.it follows that any DA is higher than its associated MDA because of the sink which occurs on the go around. <img src="tongue.gif" border="0">