This paper first considers the characteristics of the supersonic transport aircraft (S.S.T.) as established by various international authorities and, on the basis of these characteristics, endeavours to deduce the operational requirements for a navigation system. An indication is then given of the extent to which low frequency radio aids (such as Decca and Harco) can meet these requirements in the subsonic, transition and supersonic flight regimes, whilst discussing the relationship of these ground-based systems to self-contained aids. The paper was presented, in a fuller version, at the meeting of the Deutsche Gesselschaft fiir Ortung und Navigation held in Berlin, 22–7 April 1963.