Skywise wrote in
:
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
I agree with your statement of limits except that 'tens of watts' seems to
imply CW, and moonbounces and satellite bounces would probably use strong
short pulses. I think satellite operators would take a dim view of lots of
people firing YAG rangefinder pulses at their satellites though. There might
be an optimum range of power and frequency though, so if a few lasers
operated within that range but at different frequencies, the pulse rate can
be used to distinguish between them and effectively improve SNR. Their
wavelengths will help that too, if different. Would still be lucky to get 75
baud out of it though.
Reliable point to point comms actually get a lot of money thrown at them, the
military often needs to be sure they only work well between two points. The
odds are that a satellite bounce capable of being picked up by field
equipment at intended point would have plenty of scatter and spill that can
be picked up by a sensitive fixed installation almost anywhere else that can
see the satellite.
If Phil Hobbs is around, I suspect he might have a decisive answer to this, I
think his interests cover pretty much all the technical aspects of it.
The one thing that bothers me is: given that radio signals seem to manage
just fine, why would a laser need so much power, and so little get back? I
guess it's purely due to reflection, rather that hitching a ride on the
satellites ability to add gain and retransmit a signal, which is only
designed for UHF radio.