When harbour surveillance radars were first fitted in the United Kingdom in 1948 it became apparent that under conditions of high traffic density difficulty might arise in identifying the ship being assisted into harbour with a particular echo on the radar screen. The group at the Admiralty Signal and Radar Establishment (A.S.R.E.) working on navigational aids for the Ministry of Transport were set the task of examining the problem and suggesting a solution, and this paper will describe the results of this work.