Robert Ehrlich wrote:
"Lars P. Hansen" wrote:

OK, here we go: Could devices like this not also be used to detect
thermals?
The description in the link below about how the laser "sees" minute
dust
particles in the air seems to be well suited to thermals.


I don't buy the explanation they give in the cited url
(http://www.navysbir.brtrc.com/succes...navsea_p3.html)
They pretend the device measures the speed and direction of dust
particles
from the shift in the frequency of reflected light, this is well
known
as Doppler effect and can only give the radial component (toward or
away
from the sensor) of the speed, not its value and direction. For
thermals
we are interested in the speed component which is nearly
perpendicular
to the measured component, so this would be of little interest. Of
course
whith several such devices on the ground, all the 3 compenents of
airmass
speed could be measured, maybe this in the intended use of the device
as it
is advertised, but in a glider you don't have sufficient vertical
distance
for putting 2 devices which could provide an accurate value for the
vertical
component of the speed.

It does work, but they use a little different technique.
The Doppler is only measured on particles at the focal length of the
optics.
The assumption is that the airmass (at least locally) is all the same,
and that the Doppler measurement is taken far enough away so the
effects of the airplane on the airmass are negligible.
So you send out two beams -- say, one forward at 45 degrees, one aft at
45 degrees.
It turns out that if you sum the signals from the two beams, you get
the vertical component of velocity, and if you difference the two
signals, you get the horizontal component.
Since we're measuring frequency, we can get sum and difference
frequencies from a mixer, though I have no doubt it runs through a DSP
somewhere.

So you only need one sensor head (though it puts out multiple beams).

By sending out two more beams, to each side, you can also pick up
sideslip information.
The clever thing is that they're using components developed for the
communications field, which helps to keep costs down.

Tim Ward