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

["Dr Thomas A Clark (W3IWI)" <clark@xxxxxxxxxxxxxxxxxxxx>]

Bob Bruninga, WB4APR wrote:
> 
> The Navy Research Satellite STARSHINE will fly on STS-96 and might have
> some potential as a laser reflector for optical comms.  It is a 19 inch
> diameter sphere covered with 800 1" flat mirrors.  It will be in a 51
> degree orbit so most everyone will see it.  Rotating at about 1 RPM it
> will flash sunlight at about a 1 second rate.  Visible at twilight.   At
> only 205 miles high, it will only last a few months.
> 
>     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.

Actually, it's more like microseconds! 

Your first problem is on the uplink. For a small laser (like a blackboard
pointer), the beam size is ~1 arc minute = 1/60 degree = 1/3400 radian.
Therefore at ~300 miles = 500 km range, your beam spot is ~500/3400 or 
about 150 meters in diameter. The satellite is moving at an orbital
velocity ~7 km/sec so it zips thru the laser's beam in a scant 
~(0.15/7)sec= 20 msec. In order to hit the satellite at all with your
uplink, you will need a precision tracking mount and a good way to
direct the "transmitter".

Let's assume that each of the 1"=25mm mirrors is polished so that it
produces a diffraction limited "glint" reflection. For a red laser
at ~0.6 micron wavelength, there are ~40,000 wavelengths of the red
lite in the apreture. Therefore you will have a return beam that has
a size ~1/40000 radians = 5 arc seconds. On the ground, the "spot
size" will be about 10 meters in diameter.

If the satellite is spinning at ~1 RPM, it will spinning at an angular
rate of (3600 arcsec/degree)*(360 degrees/rev)/(60 sec/rev) =
21600 arcsec/sec, so the 5 arcseconds "glint" is swept out by the 
mirror in 5/21600 = 230 microseconds.

Although there are many, many mirrors on this satellite, you will see
the brief glint from only one mirror at a time -- you can try a
simplified version of this by watching the mirror glints from a
a "mirror ball" at your local discotecque. Shine a laser pointer
at the ball and try to have a "red light QSO" with a single point-
source transmitter sending bits to yourself or anyone else in the 
dance hall. Then add the difficulty of pointing the transmitter
to the equation and you'll see why I predict failure.

> One possibilty might be to use conventional AX.25 packet at 9600 baud.
> The shortest possible packet is a few sync bytes, the FROM and TO address
> plus a few bytes of data totaling about 30 bytes.  This would take about
> 30 milliseconds.  

Nope -- using your numbers -- 30 bytes = 240 bits during the ~230 usec
"glint" calculated above translates into a megabit/sec minimum -- Sorry.

73, Tom

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