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#1
April 27th 10, 07:04 PM posted to sci.astro.satellites.visual-observe
 David Jonsson external usenet poster Posts: 9

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 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
#2
April 28th 10, 03:25 AM posted to sci.astro.satellites.visual-observe
 Skywise external usenet poster Posts: 318

#3
April 28th 10, 08:48 PM posted to sci.astro.satellites.visual-observe,alt.lasers,sci.optics
 Skywise external usenet poster Posts: 318

#5
April 29th 10, 05:16 PM posted to sci.astro.satellites.visual-observe,alt.lasers,sci.optics
 AES external usenet poster Posts: 11

In article ,
Skywise wrote:

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.

Any favorite or recommended publications on these lunar ranging
experiments?

(Either seriously technical archival-journal-level articles, or good
quality Scientific American or Popular Mechanics level articles -- but
preferably well-illustrated and available on line.)

--
"For the fact is that much of the financial industry has become
a racket ‹ a game in which a handful of people are lavishly paid
to mislead and exploit consumers and investors. And if we don¹t
lower the boom on these practices, the racket will just go on."
-- Nobel Laureate Paul Krugman, 18 April 2010
#6
April 30th 10, 04:51 AM posted to sci.astro.satellites.visual-observe,alt.lasers,sci.optics
 Skywise external usenet poster Posts: 318

AES wrote in news:siegman-9D530E.09164329042010

In article ,
Skywise wrote:

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.

Any favorite or recommended publications on these lunar ranging
experiments?

(Either seriously technical archival-journal-level articles, or good
quality Scientific American or Popular Mechanics level articles -- but
preferably well-illustrated and available on line.)

After posting I found some links that I had once found before
when I was curious about the lunar ranging lasers.

Wikipedia articles:
http://en.wikipedia.org/wiki/Laser_R...etro-Reflector
http://en.wikipedia.org/wiki/Apache_...y_Lunar_Laser-
ranging_Operation

(aka APOLLO), which also included pictures of the equipment and of
the laser in action:
http://www.physics.ucsd.edu/~tmurphy/apollo/apollo.html

On this site are some PDF documentation. A few noted facts I found
skimming the first one describing the equipment as constructed,
http://www.physics.ucsd.edu/~tmurphy...710.0890v2.pdf

One month after Apollo 11, the 2.7 meter scope at McDonald Observatory
"...used a ruby laser with 4 ns pulse width, firing at a
repetition rate of about 0.3 Hz and ~3 J/pulse. This
station routinely achieved 20 cm range precision, with
a photon return rate as high as 0.2 photons per pulse,
or 0.06 photons per second."

"In the mid 1980’s, the McDonald operation was transferred
to a dedicated 0.76 m telescope (also used for satellite
laser ranging) with a 200 ps Nd:YAG laser operating at
10 Hz and 150 mJ/pulse."

"At about the same time, a new station began operating in
France at the Observatoire de la Cˆote d’Azur (OCA). Using
a 1.5 meter telescope, a 70 ps Nd:YAG laser firing at 10 Hz
and 75 mJ/pulse, this became the premier lunar ranging
station in the world."

Regarding the current APOLLO experiment:

"...combination of a 3.5 meter aperture and 1.1 arcsecond
median image quality near zenith translates to a high
photon return rate. Using a 90 ps FWHM Nd:YAG laser
operating at 20 Hz and 115 mJ/pulse, APOLLO obtains
photon return rates approaching one photon per pulse,
so that the requisite number of photons for one-millimeter
normal points may be collected on few-minute timescales.
To date, the best performance has been approximately 2500
return photons from the Apollo 15 array in a period of
8 minutes. The average photon return rate for this period
is about 0.25 photons per shot, with peak rates of 0.6
photons per pulse. Approximately half of these photons
arrived in multi-photon bundles, the largest containing
eight photons. APOLLO brings LLR solidly into the multi-photon
regime for the first time."

Brian
--
http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism
Seismic FAQ: http://www.skywise711.com/SeismicFAQ/SeismicFAQ.html
Quake "predictions": http://www.skywise711.com/quakes/EQDB/index.html
Sed quis custodiet ipsos Custodes?
#7
May 7th 10, 09:48 AM posted to sci.astro.satellites.visual-observe,alt.lasers,sci.optics
 Christoph Bollig external usenet poster Posts: 2

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
#8
May 7th 10, 09:49 AM posted to sci.astro.satellites.visual-observe,alt.lasers,sci.optics
 Christoph Bollig external usenet poster Posts: 2

Hi again,

Skywise wrote in

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.

Satellite laser ranging (SLR) is done routinely since many years.
There is a network of SLR stations around the world. Some satellites
have retro reflectors mounted to them, exactly for this purpose. This
includes some or all (not sure) of the GPS satellites.

Christoph
#9
May 7th 10, 04:07 PM posted to sci.astro.satellites.visual-observe,alt.lasers,sci.optics
 AES external usenet poster Posts: 11

In article ,
Christoph Bollig wrote:

Satellite laser ranging (SLR) is done routinely since many years.
There is a network of SLR stations around the world. Some satellites
have retro reflectors mounted to them, exactly for this purpose. This
includes some or all (not sure) of the GPS satellites.

I believe that continental drifts can be studied by doing sufficiently
accurate laser ranging on a given satellite from SLR stations located on
two (or more) continents. Do enough averaging over time and you can
figure out both the satellite's orbit and the relative motions of the
stations themselves with the precision that's needed.
#10
May 15th 10, 02:18 PM posted to sci.astro.satellites.visual-observe,alt.lasers,sci.optics
 David Jonsson external usenet poster Posts: 9

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

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

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