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Re: Laser Satellite Comms



Great!  I just love these little "Fermi" problems and back of the envelope
calculations.  Your starting point of the 2 inch spot back down, and Tom's
calculation of the divergence of a cheap laser, then suggests that all of
the transmitted energy will hit the receiver, but reduced by the capture
area of the 1 inch mirror compared to Tom's 150 meter uplink beam.

I calculate this loss at 75 dB?

So wouldnt a "2 inch" optical receiver capture this 230 usec burst with
only a loss of 75 dB?  Ah!  But now what is the chance of the 2 inch spot
hitting "your" 2 inch receiver?  Well... Real Small!  How small?

First, lets assume the mirrors are not perfect and the spot actually
diverges to a nice 8 inch spot (lose another 12 dB, but it makes a nice
receive lens).

Lets assume there are 60 mirrors around the circumference, thus there are
many "sweeping" beams every 6 degrees across the ground.  From 250 miles
away, that is every 25 miles.  Thus your chances are 8 inches in 25 miles
of seeing the spot at any instant.(.0005 %)  But at 1 RPM, you get a new
sweep every second.  And during a 5 minute pass, you get 300 such chances.
So your chances are 0.15% per pass?  At 2 good passes a day and a 6 month
mission, this results maybe a 50% chance at success in receiving ONE
packet at a megabit per second...

I guess you can send the ACK back on the next mission...

Oh, and assuming you can keep the transmitter and reciver BOTH perfectly
on the satellite at all times... <grin>.

bob


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