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Re: Re: [amsat-bb] Laser Satellite Comms



At 03:38 PM 3/1/99 -0800, Franklin Antonio wrote:
>At 07:24 AM 3/1/99 , Bob Bruninga wrote:
>>The Navy Research Satellite STARSHINE will fly on STS-96 and might have
>>some potential as a laser reflector for optical comms. ...
>>
>>    Although the ability to actually detect a laser at amateur power
>>levels and optics is miniscule, I just thought someone might want
>>to calculate the possibiilty.  Because of the flat mirrors and dynamics of
>>movement, the point-to-point comm possibilities can only exist for
>>milliseconds each second.  Thus a high data rate burst is needed. 
>
>I did some quick calculations, and the results say that this is just
completely impossible.  Perhaps I did something wrong.  I often get such a
calculation wrong on the first try.  Please double check my calcs and let
me know if I screwed up.
>
>When a point light source on the earth reflects off of a 1" mirror and
back to earth, the spot of light on the earth will be 2" in diameter.
(Presuming that the path up and down are equal length -- a reasonable first
order assumption.  Different path geometry will of course produce a
different result.)

You're neglecting the effects of diffraction, which spreads the reflected
beam out considerably, especially at that distance.  The beam spot of the
laser will be more than a foot in diameter at that distance and the
reflected beam will spread even more than that .. top-of-the-brain first
order estimate suggests a burst duration of several milliseconds at least.
Add the effects of atmospheric refraction both ways and longer contacts
than that may be possible.

Now then .. a diode laser probably won't do it - you'd need a HeNe laser
and a KER cell modulator, and probably a fairly high power laser at that ..


Bruce Bostwick  Work-- mailto:llbgb@utxdp.dp.utexas.edu
            Personal-- mailto:lihan@ccwf.cc.utexas.edu
   Ham radio related-- mailto:kd5biv@qsl.net
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