Much is written about the use and misuse of radar, but little has been said of what the mariner really needs to help him in his use of radar as an aid to avoiding collision in fog. In considering this, one must have particularly in mind the bridge on which the radar observer is also the watch officer and navigator.

It is generally accepted that he needs to be sure which targets are potential hazards and this must include those with whom the collision risk will be increased by alterations of his own course and/or speed, made to avoid others or in the course of navigation. In the commonly encountered multi-ship situation, there will, therefore, be several ships, maybe six or eight, about which detailed and up-to-date information is highly desirable.

This detailed information is of two kinds, relative to own ship and true. The relative data gives the degree of risk of collision of each target, in terms of the C.P.A., or closest distance of approach on present courses and speeds, and the time interval before this position would be reached. As a means of assessing priorities, the time is just as important as the distance. The bearing of the C.P.A. is less important, except in so far as it indicates on which side the other ship would pass.

The true information required comprises the courses and speeds of the other ships for which the degree of risk is significant.