When two ships encounter one another at sea, there is a possibility that they will collide. Whether there is actual danger or not depends on the physical facts of their respective positions and motions. If danger does exist, the motions must be changed. The purposes of Dr. Morrel's paper are (1) to inquire what facts must be known, both to distinguish the cases of actual danger and to choose effective remedial action; (2) to determine when and how well these facts must be known; and (3) to consider how these facts may be obtained and how they may be best utilized.

The paper attempts to construct an integrated treatment of the whole problem, in terms of the fundamentals that define its nature and shape its solution. The problem is large and complex. Consequently, a compact treatment of it must perforce omit most of the details; both of the evidence and analysis by which the principles are established and of the technology by which they may be exploited.

Much of the treatment uses mathematics; but emphasis is placed on the implications rather than on procedures.

The paper was presented at the meeting of the Ausschuss für Funkortung held at Kiel on 10, 11, 12 September 1960 and is printed with that body's kind permission.