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The nature of Doppler velocity measurement is reviewed briefly. This is followed by a discussion of the basic requirements for obtaining a usable signal for practical systems, which include achieving requisite coherence, fulfilling certain signal-to-noise criteria, and maintaining known functional relationships between measured Doppler frequencies and aircraft velocity. Then, those factors peculiar to over-water operation of Doppler systems are discussed. Techniques suitable for the design of systems that fulfill the various existing theoretical and practical requirements are considered. The various interrelated design considerations are grouped into five main categories: 1) choice of the number of beams to employ, 2) antenna stabilization, 3) type of antenna, 4) transmission-reception techniques, and 5) Doppler frequency measurement techniques. A summary of design considerations and the choices made in the cases of systems now declassified are contained in Tables I and III-VI.