In December 1949 Adams and Uttley presented to a joint meeting of the Institute and the Royal Aeronautical Society a paper surveying the field of navigation systems for air use, in which they set out in some detail the principles, possibilities and limitations of the various aids which the scientist has to offer toward the solution of the navigation problem. In their conclusions they rightly pointed out that all these devices fall short of the ideal to a greater or lesser extent, so that the potential user is faced with the task of choosing from a bewildering and diverse array of imperfect systems those which are most likely to fulfil his needs.