Re: detection of wind speed aloft

From:         "Pete B." <P.J.Bunyon@newcastle.ac.uk>
Date:         15 Jun 95 14:25:29 
References:   1
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On 15 Jun 1995, Andy Tompson wrote:

> finding good altitudes where the headwinds were low. I got to thinking how
> headwinds can be measured from the plane. Now, I guess there are pitot tubes on
> the plane that can be used to the wind speed relative to the plane. Do they
> just subtract groundspeed to get the headwinds? If so, how do they measure the
> groundspeed? Is there some type of correlation as a function of density,
> temperature, and throttle setting (specific to the plane)? Is there some type
> of differential measurement made with data from different parts of the airframe
> to measure headwinds? Maybe this is all relatively simple, so maybe someone can
> clear the air, as it were. Thanks in advance.

	Hi Andy. I think I'm correct in saying that commercial aircraft
use a doppler system to detect groundspeed. Basically, this uses 3 or 4
beams which are sent out in different directions around the aircraft. By
measuring the Doppler shift from each of the returned beams, the
groundspeed can be determined. Then, by knowing the airspeed and heading,
a vector triangle can be constructed from which the wind speed and
direction can be determined.
	The Doppler system is also an input to the flight management
system and is used in conjunction with other input devices to determine
the aircraft's position. Obviously, if you know the velocities in
different directions over a period of time, it is simple to determine
position.

	Hope this helps.

	Cheers, Pete.