Just a bump because my attempt to add alt.lasers and sci.optics
failed for some reason

Skywise wrote in news:90NBn.253477$Vh1.226071
@newsfe15.ams2:

David Jonsson wrote in news:f710120c-a8df-
:

Hi

Can anyone help me determine how much data that is physically possible
to transport with a beam of light from Earth reflected on a satellite
back to Earth again with an good reflector?

I made a fast calculation on the moon

I've added alt.lasers and sci.optics to include more people
much more knowledgeable than I, but here's what I know...

It's all about signal-to-noise ratio. It might interest you to
know that astronomers still bounce lasers off the reflectors
that were placed on the Moon by Apollo astronauts. They use
high powered lasers and use telescopes to detect the return
signal. Even though the lasers are 10's of watts or more, the
amount of light detected is measured in number of photons.

With a signal that weak, your data rate is going to be measured
in bits per minute, if that much.

As for mirrors in orbit, all you'll be able to do is send one
signal up and reflect it to one location back on Earth. Further,
the mirror's attitude will have to be controlled in order to aim
the beam back to the right spot. That's a lot of money spent
to allow two and only two points on the planet to communicate.

Even without doing the calulations, I'd venture to say that to get
anywhere near the data rate you envision would require lasers of
such power as to be dangerous.




and found that since the mirror
is distributed over a distance of 0.1 m a difference in time of the
signal of one nanosecond will occur limiting the bitrate to maximum
one gigabit per second which is not worth transporting. On the other
hand maybe 1000 different light frequencies can be used making it
possible to sell the data flow for $ 500 000 per month.

To find out if it is worth doing assume a transport price of 0.5 $ per
megabit per second for one month (approx 600 gigabyte per $).

Is it worth building, place in orbit and maintain such a satellite?

I can imagine a low orbit satellite with a big concave mirror, a plane
mirror in the focal point and another big concave mirror aiming the
reflected beam back to earth in a non diverging beam to another place
on Earth. Precision would be at least 1 000 higher than the Moon
example giving a cash flow of ½ billion $ per month.

How much would the atmosphere distort this signal?

Someone might complain about clouds blocking the signal but it would
anyway be valuable for cloud free moments. Internet operators could
save money whenever the sky is clear.

David

David Jonsson, Sweden, phone callto:+46703000370



Brian



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