> From: Sean Breheny [mailto:shb7@CORNELL.EDU]
......
> Finally, on the subject of weather radar, I'd like to ask the experts
> here something: How does weather radar obtain the component
> of wind (or
> droplet) velocity perpendicular to the direction of radar beam
> propagation? In other words, the doppler freq only tells you
> how fast the
> object is moving toward or away from you. When dealing with
> aircraft, you
> have to track the individual plane in order to get the other component
> (that is, subtract its last position form the current one).
> Its seems to
> me as  though this isn't possible with  WX radar since there
> is no target
> to track, you are  getting an echo from millions of water dropplets
> throughout a whole area.
>
> Sean

This might be a little out of the realm of amateur weather radar, but
there is a technique I know of to measure the speed of silt particles in
seawater using a laser.

You split the laser into two beams that converge on the area of interest
and create an interference pattern.  Now you have alternating light and
dark bands in the water.  As particles move through the bands their
reflected light flickers at a rate indicating speed, though direction is
ambiguous.  By using three beams each modulated at different frequencies
the 2D direction can be found.

The water sample measured is about one meter from the end of the
instrument, and by sliding lenses a small volume can be scanned.  Using
radar beams in place of lasers a larger area might be scanned in the
air.

Sherpa Doug

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