Doppler Frequency Shift

Moving Radar Tx & Rx		Moving Target
	Fixed Radar Tx & Rx

Doppler shift is an apparent change in frequency (or wavelength) due to the relative motion of two objects. Either one or both of the objects may be moving with respect to the ground. Radar systems exploit the Doppler shift to provide an indication of relative speed. When the two objects are approaching each other (closing), the Doppler shift causes a shortening of wavelength - or increase in frequency. When the two objects are receding from each other (opening), the Doppler shift causes a lengthening of wavelength - or decrease in frequency.

For a Doppler radar system to measure speed, an accurate sample of the original phase of the transmitted signal must be maintained for comparison against the reflected signal.

Note that the angle shown (θ) is for elevation differences only; if there is also an azmuthal angle, it must be factored into the equation as cos (α), where A is the azimuth angle relative to the radar antenna boresight direction.

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Fixed Transmitter & Receiver with Moving Target   ,  where all units must be kept constant (c=speed of light)    +V when moving away, -V when moving toward

Moving Transmitter & Receiver with Moving Target   ,  where all units must be kept constant (c=speed of light)    +V when moving away, -V when moving toward

You might also want to check out the Doppler Shift section of the Electronic Warfare and Radar Systems Engineering Handbook.