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Yeah, that's what i was thinking too, that's why i said 'presumed'.

That doppler is an odd thing. I don't know if it works with doppler, because it also works thru about .5" of glass.

If it is working with the doppler, it is creating it's own frequency, and it uses the doppler effect to measure. So it works in two directions.

audible wavelength used to be a great hobby of mine. I was an enthusiastic builder/tweaker. if you know it, i also have got a mlsa pulse measuring system.