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Re: Where Does Doppler Occur?



> I have been wondering... Doppler shift... where precisely does it occur?

Easiest way to visualize it is simply the "range" or distance from you to
the satellite.  The only thing that causes Doppler is a change in the
range.  WHen it is approaching, the range is changing rapidly shorter
giving a + Doppler.  When it is departing the range is changing rapidly
longer (- DOppler).  When it is at its closest point to you, the range is
hardly changing at all (thus 0 Doppler).

Thus if you plot  range/rate over time, you get an "S" curve.  The
steepness at the center depends on how close the satellite's closest point
is to your observation point.  Stand in the middle of a railroad track,
and the DOppler is exactly +X approaching you and -X going away, and it
switches instantly (assuming you survive).  But if you are a mile from the
track, then the rate of change is much slower...but still starts and stops
near the same values.

You will get LOTS of answers to this one, I am sure..
Enjoy,
Bob


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