The object of any kind of marine radar plot when ships are in fog is to synthetize as much data on the current traffic situation as affects or will affect the safety of own ship and so to produce a plan picture of the area around the ship and the vessels moving on it, more or less as it would appear to the eye in clear weather. The plot will be expected to show, not only the current position of each ‘target’ vessel, but also the expected forward movement of each and the risk of collision which this involves.

The word synthetize is used because it so completely implies the building up of separate elements of information into a whole; in this case, a continually changing whole. The word whole is also worth emphasizing, because only if the situation is so presented can the mariner appreciate the form which its development is taking both between own ship and those target ships which present a direct threat and between target vessels whose mutual evasive manœuvres may indirectly endanger own ship. The needed synthesis cannot be attained unless the predicted future positions of target ships are coordinated in time, so that the relative positions at the future time may be compared with the current relative positions.