RDP, ‘… perhaps visibility.’
Perhaps, but remember that the paper was written from a research viewpoint; also that the means of measuring RVR (equipment) and determining SVR for DH calculation were the focus in the early years - RVR was the scientific metric.
The initial driver for ‘Cat 1’ was the electronic principle and accuracy of ILS - post WW 2, which was flown manually. Thus the visual aids supported the need for significant manoeuvre in the vertical and lateral flight paths; the ‘Calvert’ lighting pattern aiding both.
Initial autopilots’ accuracy were little better than manual flight, but workload arguments enabled reduced visibility operation. Thence the focus was on improved accuracy and decision making, what features - visual scene were required to determine the relative flight path and enable manoeuvre.
Latter research expanded the DH - RVR envelope, ILS accuracy, and particularly auto-flight capability including auto-land, which reduced the need for manoeuvre, and has subtlety changed the nature of the decision in each category.
These aspects must be considered from a safety viewpoint, which reflects the complicated regulatory approach for DH - RVR required today (JAR and thence EASA). There are some ‘corruptions’ of these principles, unsupported by research, which depend on ‘unquantifiable’ human performance - what the pilot actually sees, features used, and accuracy of manual flight.