[Laser] Freespace Laser Surplus Device Source

[Pat Barthelow]

Yes Yves,

You have it right.  A lot of the posts on the web and elsewhere erroneously 
describe the measurement technique as being measuring the time required for 
a signal to travel and return along the optical path.   This technique may 
be used, in some kinds of equipment, but, it would require a very precise 
time reference within the instrument.
By modulating the light wave carrier, and then simply summing the phase of 
the return signal with the outgoing phase,  then, either mechanically or 
electronically "moving" one end or the other of the line end points, you can 
very precisely create a measured line that the incoming modulation perfectly 
cancels the outgoing signal, thereby placing yourself exactly at a 
wavelength node, or anti node. Add or subtract the "movement" whether done 
electronically or mechanically,  and you know what fraction of a wavelength 
you are located, but don't know the total integral number of wavelengths.  
Do this for 3 carefully chosen frequencies, and you can determine the actual 
line length.

In the olden days,  precision was obtained largely by oven heated precise 
crystal modulation oscillators, precision delay lines,  and, if needed, a 
precise measurement of the index of refraction of the air in the path.   The 
geodimeters I used, had 3 or 4 different modulation frequencies, all in the 
20 to 30 mhz region.   Way back then, surveying instruments had errors of 
measurement of typically +- 1-3 ppm, plus an undeterminable constant error 
of maybe 1 centimeter.

Sincerely, Pat Barthelow     aa6eg at hotmail.com
http://www.jamesburgdish.org
Jamesburg Earth Station  Moon Bounce Team


http://www.cq-vhf.com/This%20Month's%20Issue%20Spring07.html








>From: F1AVYopto at aol.com
>Reply-To: Free Space LASER Communications <laser at mailman.qth.net>
>To: laser at mailman.qth.net
>Subject: Re: [Laser] Freespace Laser Surplus Device Source
>Date: Thu, 17 May 2007 10:48:18 EDT
>
>
>Dans un e-mail daté du 17/05/2007 15:59:16 Paris, Madrid (heure d'été),
>aa6eg at hotmail.com a écrit :
>
>http://ocw.mit.edu/NR/rdonlyres/Earth--Atmospheric--and-Planetary-Sciences/12-
>215Modern-NavigationFall2002/32B4C612-FDB4-48F0-8429-15FB54F7C633/0/12215lec15
>.pdf
>
>
>Pat,
>Very interesting informations about the ranging measurement  principles.
>If I well understand small and accurate frequency changings give  phase 
>beats
>between the Tx beam and the  bounced signal.
>The range is deducted from the frequencies values for each null  phase.
>Tnx for info
>73 Yves F1AVY
>
>
>
>
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