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New for September 2008!
What kind of
a nerd would put that plate on his automobile? ...don't answer that!
Doppler shift happens when a
signal either originates or is bounced off of a "target"
that is moving with respect to the observer (the radar). The Doppler
shift is what allows a radar to determine the velocity of the target.
The equation below shows the
relationship between the observed frequency and wavelength compared
to the "original" frequency and wavelength. Velocity is
the component radial to the observer (as opposed to orthogonal),
and is positive by convention if the object is moving away from
the observer. Thus the frequency is shifted higher if the object
is approaching, and lower if it is receding (think about the sound
that an ambulance makes when it drives toward you, then away from
This equation can be applied
to sound waves as well as electromagnetic waves. In radar, we are
talking about EM waves, so the velocity of the object is compared
to the velocity of light. Notice that the speed of light can be
easily 10,000,000,000 times faster than the object. It is quite
remarkable that a radar can perform the comparison of the incident
and returned frequencies to such a degree that you can resolve such
a slight difference. The most important thing is to use a stable
source, meaning that its frequency does not change much at all over
time. Then a simple mixer arrangement can be used to determine the