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The fundamental concepts of a Doppler navigation system are described. The theory of operation, design considerations, performance characteristics, and limitations of a Doppler radar are discussed along with the basic principles and major characteristics of navigational computers and heading references. The over-all performance characteristics of a Doppler navigation system and their dependence on the various characteristics of its major components are analyzed. It is shown that the over-all system accuracy of a Doppler system is a function of the accuracies of all three major components of the systemÂ¿the Doppler radar, the computer, and the heading reference. Thus, the over-all system accuracy is shown to be no better than that of its weakest link. In most current systems, it is the heading reference error which swamps the other errors in the system. The performance capabilities of typical modern Doppler navigation systems are outlined. Such systems are shown to provide information on ground speed, drift angle, present position, and course and distance to destination. Complete Doppler navigation systems have been or being built which will produce maximum (95 per cent probability) position errors of less than 1.5 per cent of distance traveled over land and average sea state (neglecting unknown water motion effects). The maximum position error over water of any sea state is likely to be less than 2 per cent of distance traveled. Over-all position errors in per cent of distance traveled tend to decrease with distance.