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Reflecting satellites, big business
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 |
#3
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Reflecting satellites, big business
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 -- 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? |
#4
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Reflecting satellites, big business
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. |
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Reflecting satellites, big business
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
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Reflecting satellites, big business
AES wrote in news:siegman-9D530E.09164329042010
@bmedcfsc-srv02.tufts.ad.tufts.edu: 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 Home page for "Apache Point Observatory 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
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Reflecting satellites, big business
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
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Reflecting satellites, big business
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
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Reflecting satellites, big business
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
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Reflecting satellites, big business
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 |
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