>> How does weather radar obtain the component of wind (or
>> droplet) velocity perpendicular to the direction of radar beam
>> propagation?
>
> It doesn't.

I think that it can try if the target is larger than a single echo. The
angle of the general droplet movement is different for each successive
echo from a larger target so a computer could figure the speed and the
vector from several echo's doppler content. I think that normal stationary
weather radar does this already but I am not sure.

Peter

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