On May 7, 10:48*am, Christoph Bollig wrote:
Hi everyone,

Just to add briefly to the communications question:

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?

Mirrors are not really the best option for this. I assume that a
satellite with a detector and an active laser as repeater would work
better. In fact, part of this technology is currently being developed
for data transfer from earth observation satellites to ground. With
advances in earth observation, the amount of data which needs to be
transferred has increased to an extend which makes it difficult to
transfer on radio frequencies. This will increase even further in the
future.

I don't agree. See below.

As for possible bit rate, I would assume that the fundamental limit is
similar to the limits in optical fibres. On top of this, you have the
problems of atmospheric distortions etc. For an order of magnitude
estimation, I would take the data rate of a single fibre for
comparison (not a fibre cable, which is a bundle of many fibres), if
you can overcome atmospheric limits.

These might be of itnerest:http://www.tesat.de/product-lines/op...at-optical-pro....

BTW, there are a lot of links if you do a google search for:
laser communication satelite

Are any of these links from earth to a satellite and back to another
spot AND not reamplifying on the satellite (because of reasons given
below).

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.

Not correct. A single mirror will not effect the bandwidth, since the
final path length is the same for all photons. Otherwise you would not
be at the point it reflects to. The much bigger issue is signal to
noise ratio and diffraction of the laser and the reflected beam.

The moon reflector contains of a lot of smaller mirrors. Sorry I wrote
mirror instead of retroreflector.

I can not see that the signal to noise issue is the problem. The
atmosphere will distort the signal and broaden the spectrum probably
according to the Beer Lambert law:
http://en.wikipedia.org/wiki/Beer%E2...the_atmosphere

Someone mentioned detecting the signal on the satellite and
retransmitting it but that would involve electronics and would cut off
anything above some 10 GHz where electronics become too resistive. The
laser signal has a frequency of hundreds of THz and is thus capable of
10 000 times higher data transfer.

Assuming all internet users have electronics requires a way to pack
optical signals together in an optical way to get the speed increase.
The only known technology I know of is wavelength demultiplexing but
it seems very demanding since it requres separate electronics for each
wavelength.

Dividing the band in 10 000 parts and doing traditional electronics
and amplification on it might not be a problem either when I come to
think of it.

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.

A low orbit satellite would be moving around all the time making a
stable link between two points impossible.

Right, round trip times makes a low earth orbit satellite necessarry
with the telescopes being redirected all of the time. Moving them does
not look like a big issue. However more tan 95% of internet traffic
is not real time and would not suffer from a delay of 200 ms in a
geostationary orbit. The router at the Internet provider could
determine if a packet should be sent on low or high latency lines.

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.

The sky needs to be clear on both ends for it to work. And I doubt
internet operators would accept this...

The way Internet traffic is sold makes even this kind of traffic
valuable. Imagine the high cost of an Atlantic cable.

David