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Re: Laser Satellite Comms

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.)

Now, the "disco ball" in space is rotating at 1RPM, or 6 degrees/second.  When a disco ball rotates 6 deg/sec, the spot of light on the disco wall rotates at twice that rate, or 12 degrees/second.  Because the ball is 205mi from earth, the spot on the earth's surface travels 2pi x 205mi / 360 degrees =  3.6 miles for every degree.  12 degrees/second x 3.6mi/degree = 43 miles/second.  (I assumed a particular geometry here.  Sometimes the path is longer, which makes the spot move even faster.  I also ignored the orbital motion.)

Now, we have a 2" diameter spot of light travelling at 43 miles/second on the earth's surface.  How long will I be illuminated by this spot?  

2" / (63360 inch/mile) / (43 miles/second) = 7x10^-7 second.

By this calculation, a flash from a point light source on earth will last less than a microsecond.  That's a damn short flash.  Communication using such flashes seems impossible.

A flash of reflected sunlight will last a lot longer, because the sun is so big.  My back-of-the-envelope calc comes out 40 milliseconds. 

If the source on earth is larger than a point, the reflected spot will also be larger, and thus last longer, but still not enough to be useful.

On top of this, I would guess that there's an awful link budget problem and laser aiming problem.  

Communication just seems completely impossible using amateur equipment.

Even with professional equipment, I don't see any use for this satellite.  The whole project seems like a waste of government money.

If you want to see flashes of sunlight reflected from satellites, you can look at Iridium flares.

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