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.

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/optical-products
http://www.tesat.de/component/docman...tical-products

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

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.

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.


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...

Christoph