There is also a value for a CAT 1 precision approach calculated by the Tower controllers based on the number of runway lights (at max intensity) an observer, normally the airport airside safety guy, can count from the threshold of the runway. This is Runway or Ground Visibility and is a guide to pilots for ILS CAT 1 approaches. CAT 11/111 requires RVR measurement equipment and in Australia currently only SY, ML and CB are equipped.