To help him in his use of radar as an aid to avoiding collision in fog, the watch officer needs to be sure which targets appearing on the radar screen are potential hazards, including those with which the collision risk will be increased by alteration of his own course and/or speed made to avoid other ships or in the course of navigation. In the commonly encountered multi-ship situation there are therefore likely to be at any particular instant several ships, perhaps 10 or more constantly changing, concerning which detailed and up-to-date information is highly desirable. This detailed information is of two kinds, namely information concerning the motions of other ships relative to own ship, and information concerning the true motions of other ships.