
Doppler Frequency Shift


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 azimuthal angle, it must be factored into the equation as cos (α),
where A is the azimuth angle relative to the radar antenna boresight
direction.
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. 


