1. Preliminary consideration. In attempting to assess the worth of a navigation system already known, over against the demands of the supersonic transport aircraft, we shall take care above all to make no presumptions whatever regarding the future, and not to count on the developments of the transport aircraft as regards the favour it will find with its users and consequently with the airlines likely to put it into operation. The question is not put here as to whether the 3 to 1 reduction in the actual flying time will appear as a profit paying for the numerous other constraints which it will impose, at once on the constructors, on the operators and finally on the users, without counting the requirements regarding ground equipment and administrative agreements; the question is merely to know whether, given the normal flight and speed conditions of an aircraft, we have the means to navigate it in an accurate, safe and economical manner.