The general outline of the International Civil Aviation Organization specification for an instrument approach system was largely based on equipment developed during the last war. Only minor modifications have been incorporated in the specification since then. The paper states the problem of providing a radio guidance system similar to that in use but with an improved accuracy. It is suggested that the accuracy must be improved by a factor of 3 if the system is to be used for landing and not merely approach.Some parameters required of an instrument landing system localizer are established and a line of attack on the aerial problem is described.