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The paper discusses the design of a self-contained navigational aid for aircraft. There are many techniques and variations upon which such a system may be based. The one chosen for discussion is a hybrid system which combines an inertial north reference, a Doppler navigational radar, and an airspeed indicator. A velocity triangle computer relates these three sources of input data, so that the airspeed indicator may fill in when there is no radar signal. The output of these instruments, track angle and distance, is fed to a positional computer and thence to a steering computer, so that both positional and steering information may be supplied to the pilot. Part I introduces the general problem of a self-contained navigational aid, the possible sources of input data, the usual frames of reference within which aircraft position may be stated, and the forms of route to a destination which may be derived. Part II describes an FM Doppler radar as part of an over-all navigational system, and Part III the circuit implementation of the radar, based on transistors. Part IV describes a converter for connecting the radar to an analog positional computer, and some velocity vector triangle computers. Part V discusses a digital form of positional computer and some of the necessary digital processes. The paper concludes with Part VI, which presents the problem of steering computation and some ideas on the implementation of a complete system.