Jocobrad (devised by K. D. Jones, G. R. Cowap and M. R. Bradshaw) has been devised in the light of research into the reactions and responses of mariners to various computer-based radar plotting and forecasting systems. It is a new concept in computer-based radar plotting systems which allows the operator to ascertain very quickly a safe manoeuvre in any encounter, having regard to every other target in the vicinity and to the time period associated with a contemplated manoeuvre. The system assumes that some means of echo filtering, acquisition and tracking is available and that a simple program calculates the true course and speed of every target which approaches within radar range. Automatic acquisition and plotting of every such target is essential as manual acquisition would detract from the more important features of JOCOBRAD. JOCOBRAD can be operated in one of three separate modes; useful information being conveyed by interswitching between true and relative motion while operating in two of the three.