The comments of Captain Maybourn on collision-avoidance systems in his paper ‘The Pay-off from Improved Navigational Aids’ (27, 133) prompt me to make a number of observations. What information, in principle, are we able to extract from a radar display ? This information will, of course, be degraded by the system errors, but that is another matter. Some of the following comments may appear trivial but it seems to me that they are of fundamental importance and escape sufficient emphasis.