This paper describes a radar technique developed in the course of practical experience in cross-channel ferries, high-speed very manœuvrable craft. The principles of the parallel index system of radar navigation are treated in the Admiralty Manual of Navigation, Vol. I.

A practical radio operator does not have to translate consciously the sounds that he hears into morse symbols and then into letters of the alphabet. In the same way an experienced radar operator can become adept at interpreting a relative radar display without actually drawing the vector triangle; the saving of time is important in multitarget situations. The emphasis in the system described here is upon the relative motion which is directly observed on the radar screen.