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This paper deals with airborne Doppler velocity measuring systems. In particular, it deals with the character and limitations of the velocity data provided by such systems. The Doppler effect is discussed and quantitative expressions for the frequency shift are derived. The relationship between the Doppler shift and aircraft velocity is such as to show that the fundamental capability of a Doppler system is the measurement, in the coordinate system of the aircraft, of one component of aircraft velocity for each beam of radiation employed. Certain effects of beam geometry on the measured velocity components are demonstrated quantitatively. The earth's surface as a medium for producing a radar return signal can be regarded as being constituted of a multitude of randomly situated scattering centers. The consequences of this as it bears on the nature and properties of the return signal are discussed. It is shown that the signal is noiselike and has a spectrum which is centered at a Doppler shifted frequency and which has an appreciable bandwidth. Outlines of theoretical methods for calculating the signal spectrum from a knowledge of antenna directional characteristics and other system and operational parameters are presented. It is shown that the frequency of the Doppler signal cannot be measured without error in a finite time.