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High data rate space transmissions through visible lightcommunication.



 
 
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  #1  
Old June 28th 09, 07:54 AM posted to sci.astro,rec.radio.amateur.space,rec.radio.amateur.antenna,sci.astro.seti,sci.physics
Robert Clark
external usenet poster
 
Posts: 1,150
Default High data rate space transmissions through visible lightcommunication.

I had been thinking about methods of high data rate transmission in
regards to getting *video* transmissions from Mars orbiter missions. I
was irritated by the spotty coverage of the Mars surface at the best
resolutions so I wanted to send real-time *continuous* imaging back to
Earth receiving stations at the highest imaging resolutions. This
would require very high transmission rates, much higher than what is
currently used.
The idea would be to use light transmissions but only of the on-off
variety. You would use a large surface, many meters across, capable of
being alternatively lit up and darkened. There are computer chips of
course capable of operating at Ghz rates. This would determine if the
large surface was lit up or not electrically, possibly by using a
material whose reflective properties can be changed electrically.
I was worried though about the twinkling seen in point sources, which
this would appear to be, such as with stars due to atmospheric
effects. So this might require the telescope(s) to be in Earth orbit.
The question I had though was whether the atmospheric distortion would
cause an "on" signal to appear "off" and vice versa? My understanding
of atmospheric distortion is that it causes the point source to be
constantly apparently undergoing small shifts in position. But this
wouldn't be a problem if what you want to determine is whether it is
on or off. If that is the case then ground based telescopes would
work.
In the large reflecting surface, I actually wanted to use separate,
say, squares on the reflecting surface that could be put separately in
the on-off position to increase the information transmission rate. But
that would require being able to distinguish the squares from Earth
millions of kilometers away. This is why I wanted to use light rather
than radio for this since the larger wavelengths in radio would make
the reflecting surface impractically large for diffraction limited
resolution.
Even with light you couldn't do this with a single telescope. They
would have to be widely separated. Combining the signals from widely
separated scopes is common in radio astronomy but is not nearly as
successful in optical astronomy. That is because the light wavelengths
are so much smaller and you would have to have nanoscale accuracy in
positioning the widely separate mirrors in relationship to each other.
However, in the case of just detecting an on-off signal this shouldn't
be as big of a problem as you're not trying to form a usable image,
but only trying to see if a particular location is on or off. You
would need though highly accurate timing synchrony between the
separate scopes, within nanoseconds, to be sure they are detecting the
same on-off square. Note also here that the shifting in the image due
to atmospheric distortion very definitely would be bad for using
ground based scopes.

It occurred to me this might be a means of acquiring advertising
support for a Google Lunar X Prize entrant. I had also been trying to
come up with a method of having an illuminated image either on the
Moon or in lunar orbit that would be visible to the naked eye on
Earth. Such an idea was discussed he

moon advertising.
put a billboard on the moon.
http://www.halfbakery.com/idea/moon_20advertising

I wouldn't be in favor of doing this in a way that would actually
advertise a product. But I was thinking about it as a way of sending a
message in favor of, for example, world peace. In this case you could
still have advertisers who could say in TV commercials for example
they supplied funding to support the mission and the message.
BTW, I would be in favor of advertisers who could pay to have
advertising signs set up at the rover landing site so that if anyone
who wanted to log on to the the rover transmissions or who watched a
TV program on the rover transmissions would see the ads. This to me is
something different than an ad that someone would be forced to see
just by looking up at the Moon.
In any case you would need something large enough so that with naked
eye resolution at the lunar distance it would still be
distinguishable. This page gives the naked eye resolution at the lunar
distance:

Purpose of Building Telescopes.
http://www.astronomy.org/astronomy-survival/telepur.htm

According to this page the resolution of the human eye at the lunar
distance would be about 22 miles. One single object clearly couldn't
do this. However, if you had separate illuminated landers or orbiters
at this large distance apart they could be used to send a message
visible to the naked eye on Earth.
It could work with orbiters by the example set of satellite formation
flying by the Cluster mission:

Cluster mission.
http://en.wikipedia.org/wiki/Cluster_mission

I also needed to find how large a brightly illuminated surface needed
to be at the lunar surface to be visible by the naked eye on Earth. I
thought of the example of the "Iridium flares":

Satellite flare.
http://en.wikipedia.org/wiki/Satellite_flare

The Iridium satellites have 3 antennas that happen to be also
reflective in visible light, totaling 4.8 m^2 in area. According to
the Wikipedia page, the flares can be up to -8 in apparent magnitude,
though typically at +6 magnitude, and are produced by an individual
antenna, so by one of area 1.6 m^2.
I'll assume the brightest flares are produced just by the orientation
the antennas happen to be in so we could make our reflective surfaces
be oriented with respect to the Sun to get the greatest brightness.
For the same size surface, the brightness would be lessened by the
greater distance to the Moon. The Iridium satellites are at about 780
km altitude so the Moon is about 500 times further. This would lower
the brightness by a factor of 500^2 = 250,000.
This page gives the apparent brightness commonly visible by the naked
eye in urban areas as +3:

Apparent magnitude.
http://en.wikipedia.org/wiki/Apparent_magnitude

The 250,000 times lesser brightness at the lunar distance for an
Iridium sized reflective surface would give it a +13.5 higher apparent
magnitude so up to +5.5 in apparent magnitude. To make our reflective
surface be at +3 apparent magnitude we could make the area be 10 times
larger, so at 16 m^2 area, or a square 4 meters across.
We would need a method for a flat reflecting surface of unfolding it
to this size. It might be easier instead to have the reflecting
surface be a balloon inflated by stored gas. Since this would be in a
vacuum, you wouldn't need much gas pressure or mass to accomplish
this.
Another consideration is that because of the brightness of the Moon
it could swamp out our illuminated surface. For the orbiter, this
could probably be alleviated by having the orbiter have a highly
elliptical orbit, (this also would be beneficial in minimizing the
required delta-v and fuel load) then it would be visible at the higher
distances from the Moon in its orbit. For the landers it might work
for them to land in the dark lunar maria.

To communicate the message though we would need a method to turn on
and off the reflecting surface. One possibility would be to have the
reflecting surface consist of very many small squares that could be
rotated to reflect toward the Earth or away. Another possibility might
be to have it covered with LCD's. Whichever method it would have to be
both lightweight and low power.
For our first attempts we probably would not want to send so many
orbiter or landers at once to form a naked-eye visible image. We would
first send just a single one to test it out. Note that this method
with a single vehicle could still be used to send high definition
video by having our single reflective surface be turned on and off at
the required rate, about 256,000 times per sec with compression.


Bob Clark

On Jun 16, 6:57*pm, Robert Clark wrote:
*On another forum there was debate about whether the requirement of
"near real time" high definition video transmissions was achievable
for a such a low-cost mission.
*It would certainly be doable if the receiving antennas on Earth were
the large radio antennas used for space communications with
interplanetary probes or those radio antennas used for radio
astronomy. This is evidenced by the fact that the Kaguya(Selene) lunar
orbiter mission was able to send high definition video to a large
receiving dish radio antenna. And also by the fact that DirecTV sends
high definition video to only 2 foot size antennas from geosynchronous
orbit; so 10 times larger antennas would be able to receive such
signals from a 10 times larger distance at the Moon.
*However, I was wondering if it would be possible to detect this using
amateur sized equipment at such a large distance. Usually for
receiving high data rates you used transmissions at very high
frequencies, as higher frequencies can carry more data. For instance
both Kaguya and DirecTV transmit the high def video at gigahertz
frequencies.
*However, for the system I'm imaging I'm thinking of using much lower
frequencies, and necessarily longer wavelengths. What I wanted to do
is transmit at decametric wavelengths. High data transmissions rates
would be achieved by making it be pulsed in an on-off fashion at high
intensity but at a rapid rate.
*On that other forum the data rate required for high def TV was given
as 256,000 bits per second. So I wanted to make these transmissions be
pulsed at this rapid rate at wavelengths of a few tens's of meters.
*My decametric wavelength requirement was because of the fact that
high schools and universities have programs for detecting radio
emissions from Jupiter at these wavelengths:

NASA's Radio JOVE Project.http://radiojove.gsfc.nasa.gov/

The Discovery of Jupiter's Radio Emissions.
How a chance discovery opened up the field of Jovian radio studies.
by Dr. Leonard N. Garciahttp://radiojove.gsfc.nasa.gov/library/sci_briefs/discovery.html

*These school and university receiving antennas on Earth consist of
dozens to hundreds of vertical dipoles of lengths at the meters scale
to correspond to the radio wavelengths. Some questions I had: how
intense would the pulse have to be on the Moon to be detectable from
the Moon above background noise for a detector on Earth of say a few
dozen dipoles? Could this be done for the transmitter of power of say
a few hundred watts for a low cost, low weight lander mission? Could
the transmitter antenna on the moon be only a few meters size for the
low weight requirement?
*A secondary purpose I had in mind was a pet project of mine involving
linking these many school receivers to form a global telescope at
decametric wavelengths:

From: (Robert Clark)
Date: 23 May 2001 11:15:06 -0700
Subject: Will amateur radio astronomers be the first to directly
detect extrasolar planets?
Newsgroups: rec.radio.amateur.space, rec.radio.amateur.antenna,
sci.astro, sci.astro.seti, sci.space.policyhttp://groups.google.com/group/sci.astro.seti/browse_frm/thread/c0018...

*The long wavelengths should make the requirements for accurate
distance information and timing synchrony between the separate
detectors easy to manage even for amateur systems. Using this method
might make the detection achievable even if the power or transmitting
antenna size requirements are not practical for a low cost, low weight
lander *on the Moon for an individual detector on Earth.
*The recent achievement of real-time very long baseline interferometry
should make it possible to integrate these separate detector signals
in real-time as well:

Astronomers Demonstrate a Global Internet Telescope.
Date Released: Friday, October 08, 2004
Source: Jodrell Bank Observatoryhttp://www.spaceref.com/news/viewpr.html?pid=15251

* * *Bob Clark


  #2  
Old June 28th 09, 08:14 AM posted to sci.astro,rec.radio.amateur.space,rec.radio.amateur.antenna,sci.astro.seti,sci.physics
Peter Webb[_2_]
external usenet poster
 
Posts: 927
Default High data rate space transmissions through visible light communication.

Nice idea, but ...

Illuminating an area of Mars or the moon and relying on this secondary
reflection will actually produce less photons returning to earth than aiming
the light source directly at the earth.

Furthermore, the radiation from a reflected area is isotropic - goes in all
directions - and hence very little is directed towards the earth.

If you were using the light source directly, rather than having it
illuminate an area of the ground, you could also use lenses or mirrors to
focus it back on the earth, giving thousands or millions of times the signal
strength on earth, the same technique as is used for radio comms.



"Robert Clark" wrote in message
...
I had been thinking about methods of high data rate transmission in
regards to getting *video* transmissions from Mars orbiter missions. I
was irritated by the spotty coverage of the Mars surface at the best
resolutions so I wanted to send real-time *continuous* imaging back to
Earth receiving stations at the highest imaging resolutions. This
would require very high transmission rates, much higher than what is
currently used.
The idea would be to use light transmissions but only of the on-off
variety. You would use a large surface, many meters across, capable of
being alternatively lit up and darkened. There are computer chips of
course capable of operating at Ghz rates. This would determine if the
large surface was lit up or not electrically, possibly by using a
material whose reflective properties can be changed electrically.
I was worried though about the twinkling seen in point sources, which
this would appear to be, such as with stars due to atmospheric
effects. So this might require the telescope(s) to be in Earth orbit.
The question I had though was whether the atmospheric distortion would
cause an "on" signal to appear "off" and vice versa? My understanding
of atmospheric distortion is that it causes the point source to be
constantly apparently undergoing small shifts in position. But this
wouldn't be a problem if what you want to determine is whether it is
on or off. If that is the case then ground based telescopes would
work.
In the large reflecting surface, I actually wanted to use separate,
say, squares on the reflecting surface that could be put separately in
the on-off position to increase the information transmission rate. But
that would require being able to distinguish the squares from Earth
millions of kilometers away. This is why I wanted to use light rather
than radio for this since the larger wavelengths in radio would make
the reflecting surface impractically large for diffraction limited
resolution.
Even with light you couldn't do this with a single telescope. They
would have to be widely separated. Combining the signals from widely
separated scopes is common in radio astronomy but is not nearly as
successful in optical astronomy. That is because the light wavelengths
are so much smaller and you would have to have nanoscale accuracy in
positioning the widely separate mirrors in relationship to each other.
However, in the case of just detecting an on-off signal this shouldn't
be as big of a problem as you're not trying to form a usable image,
but only trying to see if a particular location is on or off. You
would need though highly accurate timing synchrony between the
separate scopes, within nanoseconds, to be sure they are detecting the
same on-off square. Note also here that the shifting in the image due
to atmospheric distortion very definitely would be bad for using
ground based scopes.

It occurred to me this might be a means of acquiring advertising
support for a Google Lunar X Prize entrant. I had also been trying to
come up with a method of having an illuminated image either on the
Moon or in lunar orbit that would be visible to the naked eye on
Earth. Such an idea was discussed he

moon advertising.
put a billboard on the moon.
http://www.halfbakery.com/idea/moon_20advertising

I wouldn't be in favor of doing this in a way that would actually
advertise a product. But I was thinking about it as a way of sending a
message in favor of, for example, world peace. In this case you could
still have advertisers who could say in TV commercials for example
they supplied funding to support the mission and the message.
BTW, I would be in favor of advertisers who could pay to have
advertising signs set up at the rover landing site so that if anyone
who wanted to log on to the the rover transmissions or who watched a
TV program on the rover transmissions would see the ads. This to me is
something different than an ad that someone would be forced to see
just by looking up at the Moon.
In any case you would need something large enough so that with naked
eye resolution at the lunar distance it would still be
distinguishable. This page gives the naked eye resolution at the lunar
distance:

Purpose of Building Telescopes.
http://www.astronomy.org/astronomy-survival/telepur.htm

According to this page the resolution of the human eye at the lunar
distance would be about 22 miles. One single object clearly couldn't
do this. However, if you had separate illuminated landers or orbiters
at this large distance apart they could be used to send a message
visible to the naked eye on Earth.
It could work with orbiters by the example set of satellite formation
flying by the Cluster mission:

Cluster mission.
http://en.wikipedia.org/wiki/Cluster_mission

I also needed to find how large a brightly illuminated surface needed
to be at the lunar surface to be visible by the naked eye on Earth. I
thought of the example of the "Iridium flares":

Satellite flare.
http://en.wikipedia.org/wiki/Satellite_flare

The Iridium satellites have 3 antennas that happen to be also
reflective in visible light, totaling 4.8 m^2 in area. According to
the Wikipedia page, the flares can be up to -8 in apparent magnitude,
though typically at +6 magnitude, and are produced by an individual
antenna, so by one of area 1.6 m^2.
I'll assume the brightest flares are produced just by the orientation
the antennas happen to be in so we could make our reflective surfaces
be oriented with respect to the Sun to get the greatest brightness.
For the same size surface, the brightness would be lessened by the
greater distance to the Moon. The Iridium satellites are at about 780
km altitude so the Moon is about 500 times further. This would lower
the brightness by a factor of 500^2 = 250,000.
This page gives the apparent brightness commonly visible by the naked
eye in urban areas as +3:

Apparent magnitude.
http://en.wikipedia.org/wiki/Apparent_magnitude

The 250,000 times lesser brightness at the lunar distance for an
Iridium sized reflective surface would give it a +13.5 higher apparent
magnitude so up to +5.5 in apparent magnitude. To make our reflective
surface be at +3 apparent magnitude we could make the area be 10 times
larger, so at 16 m^2 area, or a square 4 meters across.
We would need a method for a flat reflecting surface of unfolding it
to this size. It might be easier instead to have the reflecting
surface be a balloon inflated by stored gas. Since this would be in a
vacuum, you wouldn't need much gas pressure or mass to accomplish
this.
Another consideration is that because of the brightness of the Moon
it could swamp out our illuminated surface. For the orbiter, this
could probably be alleviated by having the orbiter have a highly
elliptical orbit, (this also would be beneficial in minimizing the
required delta-v and fuel load) then it would be visible at the higher
distances from the Moon in its orbit. For the landers it might work
for them to land in the dark lunar maria.

To communicate the message though we would need a method to turn on
and off the reflecting surface. One possibility would be to have the
reflecting surface consist of very many small squares that could be
rotated to reflect toward the Earth or away. Another possibility might
be to have it covered with LCD's. Whichever method it would have to be
both lightweight and low power.
For our first attempts we probably would not want to send so many
orbiter or landers at once to form a naked-eye visible image. We would
first send just a single one to test it out. Note that this method
with a single vehicle could still be used to send high definition
video by having our single reflective surface be turned on and off at
the required rate, about 256,000 times per sec with compression.


Bob Clark

On Jun 16, 6:57 pm, Robert Clark wrote:
On another forum there was debate about whether the requirement of
"near real time" high definition video transmissions was achievable
for a such a low-cost mission.
It would certainly be doable if the receiving antennas on Earth were
the large radio antennas used for space communications with
interplanetary probes or those radio antennas used for radio
astronomy. This is evidenced by the fact that the Kaguya(Selene) lunar
orbiter mission was able to send high definition video to a large
receiving dish radio antenna. And also by the fact that DirecTV sends
high definition video to only 2 foot size antennas from geosynchronous
orbit; so 10 times larger antennas would be able to receive such
signals from a 10 times larger distance at the Moon.
However, I was wondering if it would be possible to detect this using
amateur sized equipment at such a large distance. Usually for
receiving high data rates you used transmissions at very high
frequencies, as higher frequencies can carry more data. For instance
both Kaguya and DirecTV transmit the high def video at gigahertz
frequencies.
However, for the system I'm imaging I'm thinking of using much lower
frequencies, and necessarily longer wavelengths. What I wanted to do
is transmit at decametric wavelengths. High data transmissions rates
would be achieved by making it be pulsed in an on-off fashion at high
intensity but at a rapid rate.
On that other forum the data rate required for high def TV was given
as 256,000 bits per second. So I wanted to make these transmissions be
pulsed at this rapid rate at wavelengths of a few tens's of meters.
My decametric wavelength requirement was because of the fact that
high schools and universities have programs for detecting radio
emissions from Jupiter at these wavelengths:

NASA's Radio JOVE Project.http://radiojove.gsfc.nasa.gov/

The Discovery of Jupiter's Radio Emissions.
How a chance discovery opened up the field of Jovian radio studies.
by Dr. Leonard N.
Garciahttp://radiojove.gsfc.nasa.gov/library/sci_briefs/discovery.html

These school and university receiving antennas on Earth consist of
dozens to hundreds of vertical dipoles of lengths at the meters scale
to correspond to the radio wavelengths. Some questions I had: how
intense would the pulse have to be on the Moon to be detectable from
the Moon above background noise for a detector on Earth of say a few
dozen dipoles? Could this be done for the transmitter of power of say
a few hundred watts for a low cost, low weight lander mission? Could
the transmitter antenna on the moon be only a few meters size for the
low weight requirement?
A secondary purpose I had in mind was a pet project of mine involving
linking these many school receivers to form a global telescope at
decametric wavelengths:

From: (Robert Clark)
Date: 23 May 2001 11:15:06 -0700
Subject: Will amateur radio astronomers be the first to directly
detect extrasolar planets?
Newsgroups: rec.radio.amateur.space, rec.radio.amateur.antenna,
sci.astro, sci.astro.seti,
sci.space.policyhttp://groups.google.com/group/sci.astro.seti/browse_frm/thread/c0018...

The long wavelengths should make the requirements for accurate
distance information and timing synchrony between the separate
detectors easy to manage even for amateur systems. Using this method
might make the detection achievable even if the power or transmitting
antenna size requirements are not practical for a low cost, low weight
lander on the Moon for an individual detector on Earth.
The recent achievement of real-time very long baseline interferometry
should make it possible to integrate these separate detector signals
in real-time as well:

Astronomers Demonstrate a Global Internet Telescope.
Date Released: Friday, October 08, 2004
Source: Jodrell Bank
Observatoryhttp://www.spaceref.com/news/viewpr.html?pid=15251

Bob Clark


  #3  
Old June 28th 09, 06:41 PM posted to sci.astro,rec.radio.amateur.space,rec.radio.amateur.antenna,sci.astro.seti,sci.physics
Richard Clark
external usenet poster
 
Posts: 6
Default High data rate space transmissions through visible light communication.

On Sun, 28 Jun 2009 17:14:14 +1000, "Peter Webb"
wrote:

Furthermore, the radiation from a reflected area is isotropic - goes in all
directions - and hence very little is directed towards the earth.


Actually, it is lambertian in its distribution, and it would have a
major lobe that was directed in rather typical fashion (at the same,
but negative angle to the norm to the surface). However, as is the
intent of your response, very much less will find its way to the
intended target.

73's
Richard Clark, KB7QHC
  #4  
Old June 28th 09, 07:37 PM posted to sci.astro,rec.radio.amateur.space,rec.radio.amateur.antenna,sci.astro.seti,sci.physics
Richard Clark
external usenet poster
 
Posts: 6
Default High data rate space transmissions through visible light communication.

On Sat, 27 Jun 2009 23:54:14 -0700 (PDT), Robert Clark
wrote:

I had been thinking about methods of high data rate transmission in
regards to getting *video* transmissions from Mars orbiter missions. I
was irritated by the spotty coverage of the Mars surface at the best
resolutions so I wanted to send real-time *continuous* imaging back to
Earth receiving stations at the highest imaging resolutions.


A curious distinction in this "continuous." Direct Current
transmission from Mars? I think not. Anything else is rather
conventional.

This
would require very high transmission rates, much higher than what is
currently used.


"Continuous" is not distinctive to rate except at DC. Grab both sides
of conventional 120VAC from any wall socket, and it will seem
distinctly continuous - boosting it 1 THz wouldn't bring any different
sensation.

The idea would be to use light transmissions but only of the on-off
variety.


Rates, and on-off have departed the realm of "continuous."

You would use a large surface, many meters across, capable of
being alternatively lit up and darkened.


This is entirely unrelated to "continuous" rates or modes of
transmission. In and of itself, in regards to establishing remote
communications at light wavelengths, it is guilding the lily and
painting the rose.

There are computer chips of
course capable of operating at Ghz rates.

How that relates to:
This would determine if the
large surface was lit up or not electrically, possibly by using a
material whose reflective properties can be changed electrically.

is bordering on stream-of-consciousness rambling.

I actually wanted to use separate,
say, squares on the reflecting surface that could be put separately in
the on-off position to increase the information transmission rate.


There is no causal correlation between many surfaces and rate. This
is merely the substitution of complexity for the appearance of deep
consideration (which it is not).

This is why I wanted to use light rather
than radio for this since the larger wavelengths in radio would make
the reflecting surface impractically large for diffraction limited
resolution.


You are simply limited in your perception of what RF and Light means.
If one suffers for wavelength, then they both do.

Even with light you couldn't do this with a single telescope.


Sounds like an artificial objection. Have you tried thinking in terms
of a power budget?

They would have to be widely separated.

Does not come naturally as a solution from the rather diaphonous
problem put forward to this point, and the following is not a reason:
Combining the signals from widely
separated scopes is common in radio astronomy but is not nearly as
successful in optical astronomy. That is because the light wavelengths
are so much smaller and you would have to have nanoscale accuracy in
positioning the widely separate mirrors in relationship to each other.

This is problem of degree, one which you painted yourself into a
corner with. Further, it doesn't necessarily follow one from the
other.

However, in the case of just detecting an on-off signal this shouldn't
be as big of a problem as you're not trying to form a usable image,
but only trying to see if a particular location is on or off. You
would need though highly accurate timing synchrony between the
separate scopes, within nanoseconds, to be sure they are detecting the
same on-off square. Note also here that the shifting in the image due
to atmospheric distortion very definitely would be bad for using
ground based scopes.


This is, based on your own objections, rather whipsawed by the
application of the term "nano." Nanoseconds and nanometers are not on
the balance to the solution of your problem. If you had nanometer
issues optically, they are not solved within nanoseconds simply
because they are not forming an image (which is a poor metaphor
because if fails with its own application).

moon advertising.
put a billboard on the moon.
http://www.halfbakery.com/idea/moon_20advertising


Half backed? It is undercooked by half that again.

Let's consider: To obtain a sufficient contrast ratio, the light
would have to exceed the brilliance of the sun.

Did I mention a power budget?

The rest of this hardly borders on novely so much as fantasy. Keep
that to the appropriate groups.

73's
Richard Clark, KB7QHC
  #5  
Old June 28th 09, 09:48 PM posted to sci.astro,rec.radio.amateur.space,rec.radio.amateur.antenna,sci.astro.seti,sci.physics
BradGuth
external usenet poster
 
Posts: 21,544
Default High data rate space transmissions through visible lightcommunication.

Yes, you are perfectly dead on the spot for using the visual spectrum
on behalf of interplanetary and interstellar communications, as by far
the most extremely narrow of monochromatic or those of FM photons
accomplishing their one-way or ideally two-way energy and technology
efficient alternative. I kid you not.

The problem is that it’s perhaps too darn good and perhaps even too
energy efficient. Signal pointing and tracking errors from a
satellite platform are seriously narrow, though receiving isn’t all
that insurmountable.

~ BG



On Jun 28, 12:14*am, "Peter Webb"
wrote:
Nice idea, but ...

Illuminating an area of Mars or the moon and relying on this secondary
reflection will actually produce less photons returning to earth than aiming
the light source directly at the earth.

Furthermore, the radiation from a reflected area is isotropic - goes in all
directions - and hence very little is directed towards the earth.

If you were using the light source directly, rather than having it
illuminate an area of the ground, you could also use lenses or mirrors to
focus it back on the earth, giving thousands or millions of times the signal
strength on earth, the same technique as is used for radio comms.

"Robert Clark" wrote in message

...
I had been thinking about methods of high data rate transmission in
regards to getting *video* transmissions from Mars orbiter missions. I
was irritated by the spotty coverage of the Mars surface at the best
resolutions so I wanted to send real-time *continuous* imaging back to
Earth receiving stations at the highest imaging resolutions. This
would require very high transmission rates, much higher than what is
currently used.
The idea would be to use light transmissions but only of the on-off
variety. You would use a large surface, many meters across, capable of
being alternatively lit up and darkened. There are computer chips of
course capable of operating at Ghz rates. This would determine if the
large surface was lit up or not electrically, possibly by using a
material whose reflective properties can be changed electrically.
I was worried though about the twinkling seen in point sources, which
this would appear to be, such as with stars due to atmospheric
effects. So this might require the telescope(s) to be in Earth orbit.
The question I had though was whether the atmospheric distortion would
cause an "on" signal to appear "off" and vice versa? My understanding
of atmospheric distortion is that it causes the point source to be
constantly apparently undergoing small shifts in position. But this
wouldn't be a problem if what you want to determine is whether it is
on or off. If that is the case then ground based telescopes would
work.
In the large reflecting surface, I actually wanted to use separate,
say, squares on the reflecting surface that could be put separately in
the on-off position to increase the information transmission rate. But
that would require being able to distinguish the squares from Earth
millions of kilometers away. This is why I wanted to use light rather
than radio for this since the larger wavelengths in radio would make
the reflecting surface impractically large for diffraction limited
resolution.
Even with light you couldn't do this with a single telescope. They
would have to be widely separated. Combining the signals from widely
separated scopes is common in radio astronomy but is not nearly as
successful in optical astronomy. That is because the light wavelengths
are so much smaller and you would have to have nanoscale accuracy in
positioning the widely separate mirrors in relationship to each other.
However, in the case of just detecting an on-off signal this shouldn't
be as big of a problem as you're not trying to form a usable image,
but only trying to see if a particular location is on or off. You
would need though highly accurate timing synchrony between the
separate scopes, within nanoseconds, to be sure they are detecting the
same on-off square. Note also here that the shifting in the image due
to atmospheric distortion very definitely would be bad for using
ground based scopes.

*It occurred to me this might be a means of acquiring advertising
support for a Google Lunar X Prize entrant. I had also been trying to
come up with a method of having an illuminated image either on the
Moon or in lunar orbit that would be visible to the naked eye on
Earth. Such an idea was discussed he

moon advertising.
put a billboard on the moon.http://www.halfbakery.com/idea/moon_20advertising

*I wouldn't be in favor of doing this in a way that would actually
advertise a product. But I was thinking about it as a way of sending a
message in favor of, for example, world peace. In this case you could
still have advertisers who could say in TV commercials for example
they supplied funding to support the mission and the message.
*BTW, I would be in favor of advertisers who could pay to have
advertising signs set up at the rover landing site so that if anyone
who wanted to log on to the the rover transmissions or who watched a
TV program on the rover transmissions would see the ads. This to me is
something different than an ad that someone would be forced to see
just by looking up at the Moon.
*In any case you would need something large enough so that with naked
eye resolution at the lunar distance it would still be
distinguishable. This page gives the naked eye resolution at the lunar
distance:

Purpose of Building Telescopes.http://www.astronomy.org/astronomy-survival/telepur.htm

*According to this page the resolution of the human eye at the lunar
distance would be about 22 miles. One single object clearly couldn't
do this. However, if you had separate illuminated landers or orbiters
at this large distance apart they could be used to send a message
visible to the naked eye on Earth.
*It could work with orbiters by the example set of satellite formation
flying by the Cluster mission:

Cluster mission.http://en.wikipedia.org/wiki/Cluster_mission

*I also needed to find how large a brightly illuminated surface needed
to be at the lunar surface to be visible by the naked eye on Earth. I
thought of the example of the "Iridium flares":

Satellite flare.http://en.wikipedia.org/wiki/Satellite_flare

*The Iridium satellites have 3 antennas that happen to be also
reflective in visible light, totaling 4.8 m^2 in area. According to
the Wikipedia page, the flares can be up to -8 in apparent magnitude,
though typically at +6 magnitude, and are produced by an individual
antenna, so by one of area 1.6 m^2.
*I'll assume the brightest flares are produced just by the orientation
the antennas happen to be in so we could make our reflective surfaces
be oriented with respect to the Sun to get the greatest brightness.
For the same size surface, the brightness would be lessened by the
greater distance to the Moon. The Iridium satellites are at about 780
km altitude so the Moon is about 500 times further. This would lower
the brightness by a factor of 500^2 = 250,000.
*This page gives the apparent brightness commonly visible by the naked
eye in urban areas as +3:

Apparent magnitude.http://en.wikipedia.org/wiki/Apparent_magnitude

The 250,000 times lesser brightness at the lunar distance for an
Iridium sized reflective surface would give it a +13.5 higher apparent
magnitude so up to +5.5 in apparent magnitude. To make our reflective
surface be at +3 apparent magnitude we could make the area be 10 times
larger, so at 16 m^2 area, or a square 4 meters across.
*We would need a method for a flat reflecting surface of unfolding it
to this size. It might be easier instead to have the reflecting
surface be a balloon inflated by stored gas. Since this would be in a
vacuum, you wouldn't need much gas pressure or mass to accomplish
this.
*Another consideration is that because of the brightness of the Moon
it could swamp out our illuminated surface. For the orbiter, this
could probably be alleviated by having the orbiter have a highly
elliptical orbit, (this also would be beneficial in minimizing the
required delta-v and fuel load) then it would be visible at the higher
distances from the Moon in its orbit. For the landers it might work
for them to land in the dark lunar maria.

*To communicate the message though we would need a method to turn on
and off the reflecting surface. One possibility would be to have the
reflecting surface consist of very many small squares that could be
rotated to reflect toward the Earth or away. Another possibility might
be to have it covered with LCD's. Whichever method it would have to be
both lightweight and low power.
*For our first attempts we probably would not want to send so many
orbiter or landers at once to form a naked-eye visible image. We would
first send just a single one to test it out. Note that this method
with a single vehicle could still be used to send high definition
video by having our single reflective surface be turned on and off at
the required rate, about 256,000 times per sec with compression.

* * *Bob Clark

On Jun 16, 6:57 pm, Robert Clark wrote: On another forum there was debate about whether the requirement of
"near real time" high definition video transmissions was achievable
for a such a low-cost mission.
It would certainly be doable if the receiving antennas on Earth were
the large radio antennas used for space communications with
interplanetary probes or those radio antennas used for radio
astronomy. This is evidenced by the fact that the Kaguya(Selene) lunar
orbiter mission was able to send high definition video to a large
receiving dish radio antenna. And also by the fact that DirecTV sends
high definition video to only 2 foot size antennas from geosynchronous
orbit; so 10 times larger antennas would be able to receive such
signals from a 10 times larger distance at the Moon.
However, I was wondering if it would be possible to detect this using
amateur sized equipment at such a large distance. Usually for
receiving high data rates you used transmissions at very high
frequencies, as higher frequencies can carry more data. For instance
both Kaguya and DirecTV transmit the high def video at gigahertz
frequencies.
However, for the system I'm imaging I'm thinking of using much lower
frequencies, and necessarily longer wavelengths. What I wanted to do
is transmit at decametric wavelengths. High data transmissions rates
would be achieved by making it be pulsed in an on-off fashion at high
intensity but at a rapid rate.
On that other forum the data rate required for high def TV was given
as 256,000 bits per second. So I wanted to make these transmissions be
pulsed at this rapid rate at wavelengths of a few tens's of meters.
My decametric wavelength requirement was because of the fact that
high schools and universities have programs for detecting radio
emissions from Jupiter at these wavelengths:


NASA's Radio JOVE Project.http://radiojove.gsfc.nasa.gov/


The Discovery of Jupiter's Radio Emissions.
How a chance discovery opened up the field of Jovian radio studies.
by Dr. Leonard N.
Garciahttp://radiojove.gsfc.nasa.gov/library/sci_briefs/discovery.html


These school and university receiving antennas on Earth consist of
dozens to hundreds of vertical dipoles of lengths at the meters scale
to correspond to the radio wavelengths. Some questions I had: how
intense would the pulse have to be on the Moon to be detectable from
the Moon above background noise for a detector on Earth of say a few
dozen dipoles? Could this be done for the transmitter of power of say
a few hundred watts for a low cost, low weight lander mission? Could
the transmitter antenna on the moon be only a few meters size for the
low weight requirement?
A secondary purpose I had in mind was a pet project of mine involving
linking these many school receivers to form a global telescope at
decametric wavelengths:


From: (Robert Clark)
Date: 23 May 2001 11:15:06 -0700
Subject: Will amateur radio astronomers be the first to directly
detect extrasolar planets?
Newsgroups: rec.radio.amateur.space, rec.radio.amateur.antenna,
sci.astro, sci.astro.seti,
sci.space.policyhttp://groups.google.com/group/sci.astro.seti/browse_frm/thread/c0018...


The long wavelengths should make the requirements for accurate
distance information and timing synchrony between the separate
detectors easy to manage even for amateur systems. Using this method
might make the detection achievable even if the power or transmitting
antenna size requirements are not practical for a low cost, low weight
lander on the Moon for an individual detector on Earth.
The recent achievement of real-time very long baseline interferometry
should make it possible to integrate these separate detector signals
in real-time as well:


Astronomers Demonstrate a Global Internet Telescope.
Date Released: Friday, October 08, 2004
Source: Jodrell Bank
Observatoryhttp://www.spaceref.com/news/viewpr.html?pid=15251


Bob Clark


  #6  
Old June 28th 09, 09:59 PM posted to sci.astro,rec.radio.amateur.space,rec.radio.amateur.antenna,sci.astro.seti
Uncle Al
external usenet poster
 
Posts: 697
Default High data rate space transmissions through visible lightcommunication.

Robert Clark wrote:

I had been thinking about methods of high data rate transmission in
regards to getting *video* transmissions from Mars orbiter missions.


Would modulating a 1 GW continuous laser at Mars be sufficient? The
laser is already in place - and running,
[snip crap]

http://www.mazepath.com/uncleal/race.htm

Hey stooopid - what happened to the Mars surface space face? You
dumped a gigabyte of crap extolling it. Where's the compost?

Even with light you couldn't do this with a single telescope.

[snip more crap]

What, a gigagatt continuous is not enough? Give an idiot the answer
and obtain NASA ratiocinating on how to get to the moon (and
presumably, back).

I had also been trying to
come up with a method of having an illuminated image either on the
Moon or in lunar orbit that would be visible to the naked eye on
Earth. Such an idea was discussed he

[snip rest of crap]

http://www.geocities.com/SouthBeach/1380/crmoon.html

idiot

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2
  #7  
Old June 29th 09, 01:31 AM posted to sci.astro,rec.radio.amateur.space,rec.radio.amateur.antenna,sci.astro.seti,sci.physics
Robert Clark
external usenet poster
 
Posts: 1,150
Default High data rate space transmissions through visible lightcommunication.

On Jun 28, 1:41 pm, Richard Clark wrote:
On Sun, 28 Jun 2009 17:14:14 +1000, "Peter Webb"

wrote:
Furthermore, the radiation from a reflected area is isotropic - goes in all
directions - and hence very little is directed towards the earth.


Actually, it is lambertian in its distribution, and it would have a
major lobe that was directed in rather typical fashion (at the same,
but negative angle to the norm to the surface). However, as is the
intent of your response, very much less will find its way to the
intended target.

73's
Richard Clark, KB7QHC


This describes the reflection from the Iridium antennas as specular
where most of the reflected light is concentrated in a single
direction:

SeeSat-L Apr-98: Method for predicting flare.
http://satobs.org/seesat/Apr-1998/0175.html

About specular reflection:

Specular reflection.
http://en.wikipedia.org/wiki/Specular_reflection

We could get even higher concentration of the image by using
parabolic mirror reflectors.


Bob Clark
  #8  
Old June 29th 09, 05:03 AM posted to sci.astro,rec.radio.amateur.space,rec.radio.amateur.antenna,sci.astro.seti,sci.physics
Craig Markwardt[_2_]
external usenet poster
 
Posts: 137
Default High data rate space transmissions through visible lightcommunication.

On Jun 28, 8:31*pm, Robert Clark wrote:
On Jun 28, 1:41 pm, Richard Clark wrote:

On Sun, 28 Jun 2009 17:14:14 +1000, "Peter Webb"


wrote:
Furthermore, the radiation from a reflected area is isotropic - goes in all
directions - and hence very little is directed towards the earth.


Actually, it is lambertian in its distribution, and it would have a
major lobe that was directed in rather typical fashion (at the same,
but negative angle to the norm to the surface). *However, as is the
intent of your response, very much less will find its way to the
intended target.


73's
Richard Clark, KB7QHC


*This describes the reflection from the Iridium antennas as specular
where most of the reflected light is concentrated in a single
direction:

SeeSat-L Apr-98: Method for predicting flare.http://satobs.org/seesat/Apr-1998/0175.html

*About specular reflection:

Specular reflection.http://en.wikipedia.org/wiki/Specular_reflection

*We could get even higher concentration of the image by using
parabolic mirror reflectors.


So basically you discovered the parabolic antenna. Congratulations.

CM
  #9  
Old June 29th 09, 06:03 AM posted to sci.astro,rec.radio.amateur.space,rec.radio.amateur.antenna,sci.astro.seti,sci.physics
BradGuth
external usenet poster
 
Posts: 21,544
Default High data rate space transmissions through visible lightcommunication.

On Jun 28, 5:31*pm, Robert Clark wrote:
On Jun 28, 1:41 pm, Richard Clark wrote:

On Sun, 28 Jun 2009 17:14:14 +1000, "Peter Webb"


wrote:
Furthermore, the radiation from a reflected area is isotropic - goes in all
directions - and hence very little is directed towards the earth.


Actually, it is lambertian in its distribution, and it would have a
major lobe that was directed in rather typical fashion (at the same,
but negative angle to the norm to the surface). *However, as is the
intent of your response, very much less will find its way to the
intended target.


73's
Richard Clark, KB7QHC


*This describes the reflection from the Iridium antennas as specular
where most of the reflected light is concentrated in a single
direction:

SeeSat-L Apr-98: Method for predicting flare.http://satobs.org/seesat/Apr-1998/0175.html

*About specular reflection:

Specular reflection.http://en.wikipedia.org/wiki/Specular_reflection

*We could get even higher concentration of the image by using
parabolic mirror reflectors.

* *Bob Clark


Yes, and it was all doable as of more than a decade ago. However, at
the rate we're going, perhaps another century is required.

~ BG
  #10  
Old June 29th 09, 10:10 AM posted to sci.astro,sci.astro.seti,sci.physics
Chris
external usenet poster
 
Posts: 53
Default High data rate space transmissions through visible light communication.

Been there done that.

The data rate depends on noise and the signalling rate. With optical
signaling the signalling rate can be gigahertz but in the presence of noise
the data rate goes down due to the need of retransmission due to missing
bytes.

To reduce noise you increase the signal to noise ratio. You can receive
information with low data rate when the signal is below noise level. You
just use a very narrow bandwidth and a low signalling rate.

To increase the signal to noise ratio you have to concentrate the energy
into a beam intercepting the receiver.

And the receiver needs a similar beam antenna intercepting the transmitter.

all good stuff shannon worked it all out back in 1930 or so.

It is what NASA use every day.

--
Chris.
Remove ns_ to reply
"Sam Wormley" wrote in message
news:SkW1m.165323$DP1.155290@attbi_s22...
Plenty of bandwidth in the radio and microwave spectrum.

A Mathematical Theory of Communication by Claude E. Shannon
http://cm.bell-labs.com/cm/ms/what/s...day/paper.html
http://cm.bell-labs.com/cm/ms/what/s...hannon1948.pdf

Claude E. Shannon
http://scienceworld.wolfram.com/biography/Shannon.html





 




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