This paper, based on an earlier one presented at the Eastbourne Convention in May 1964, discusses certain aspects of S.S.T. performance and its bearing on the design of the air navigation system and on the air traffic control system. The requirement is established for a navigation system capable of directing the S.S.T. at minimum economic penalty and within the limits imposed by air traffic control and sonic boom considerations. The system should also be able to determine estimated time of arrival, fuel remaining, and altitude for any randomly chosen point along the flight path; to derive the time of arrival at a particular altitude and, for a selected time of arrival at a chosen position and altitude, give the deceleration and other necessary command information.