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#11
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Detecting the high def TV for the Google Lunar X Prize.
On Jun 19, 11:20*am, Robert Clark wrote:
On Jun 16, 6:57 pm, Robert Clark wrote: 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. Not easy, not for the precision required. You need not only precise time (straightforward), but also precise location (not so straightforward) You're interested in roughly 20MHz, as I recall. Wavelength of 15 meters. In time, about 45 nanoseconds. Let's start with a real relaxed requirement, comparable to the mirror flatness for a telescope of lambda/14. That means a time knowledge of about 3 ns and a position knowledge of 1 meter, in absolute terms. Typical GPS receivers that have a 1pps output are good to about 20-30 nanoseconds. Using that to discipline a quartz oscillator, you can do a bit better, but it's non trivial to get to the 1-2 ns range. Remember, you're also planning on integrating over time, so you have to hold that tolerance for a long time. It would be difficult to determine your position to an absolute accuracy of 1 meter, much less the phase center of the antenna (which will change as a function of the angle of incidence, quite substantially, unless you're putting those dipoles up 100s of feet in the air. 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: snip You need to go beyond looking at press releases from radio astronomers. *However, instead of using the satellite TV dishes, we could use individual dipole antennas attached to each house. You would need to communicate high data rates for the signals detected so you would need broadband internet access for this. *These dipole antennas as per the Radio JOVE project are just simple vertical wires so could be attached to the house when the installer is connecting the wiring for the broadband. Possibly you could use the same external wiring as for the broadband but that might cause interference with the internet signals. Radio Jove uses a pair of horizontal dipoles connected together to create a single narrower lobe pointing up. *As shown on the Radio JOVE page the receivers for these dipole antennas are quite simple so would contribute minimally to the cost of installation. Who's paying, and how minimal? I don't think so. You do need accurate positional determination and timing synchrony for each receiving system to do the very long baseline interferometry, but at these decametric wavelengths this would be easy to do with GPS receivers carried by the installers. No they can't. You need position accuracy of sub-1 meter accuracy, and that isn't achievable by simple handheld devices, like your Garmin E-trex, etc. A surveyor using a survey GPS system can get there, although absolute position (relative to, say, the center of the earth, or some standard datum) to 1 meter would be very challenging. There's also the not so little problem of tidal bulge. Your position changes in absolute (relative to a stellar reference) terms several tens of cm. On top of that, tectonic plate movement is on the order of several cm/year, which is in the same general ballpark as your accuracy requirement. To do the kind of large area combining you're contemplating requires geodetic quality surveying or some form of in-situ calibration using known sources (which the folks doing LOFAR and SKA have thought about). When DSN does accurate interferometric measurements of deep space probes (a process called Delta DOR) they use a "common view" quasar as a timing reference, because the Hydrogen maser normally used for VLBI kinds of things isn't good enough. Over time you could keep the systems in synchrony by timing stamps accessed over the internet. NTP over the internet is only good to tens of milliseconds. You need nanosecond precision. *New broadband subscribers would automatically get the dipole antennas. At the rate of increase of broadband subscribers, it would only take 3 months to reach 10 million separate dipoles. If each installer when setting up a new system, also retrofitted an another existing broadband system, then you could reach the full coverage of all the broadband subscribers dipoles in 6 years. Let's see, leaving aside the surveying and time synchronization problems, in economic terms this is a non-starter. Say it costs $100 for each "station"... that's a billion dollars for your 10 million stations. And $100 is a very, very low cost estimate, because installer time isn't free (probably about $25/hr with all benefits, insurance, equipment, added in). BTW, if you really want to do something like this, think in terms of an addon to a cell site. They already have to have nanosecond precision timing and surveys in order to do E-911 position trilateration. *The number of world-wide broadband subscribers will be 500 million by 2010. At current growth rates it would be 900 million within the 6 years it took to equip each broadband subscriber system with one of the antenna dipoles. This is nearly two orders of magnitude better sensitivity than a 10 million dipole system. You could detect out to 100 light-years, opening up many more stars to the possibility of detection. I think the 100 billion dollars could be better spent in other ways, if looking for planets is your goal. Check out Terrestrial Planet Finder (TPF) for one approach. * * * Bob Clark |
#12
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Detecting the HDTV for the Google Lunar X Prize, applications to theSETI search.
On Jun 20, 10:45 am, Jim Lux wrote:
On Jun 19, 11:20 am, Robert Clark wrote: On Jun 16, 6:57 pm, Robert Clark wrote: 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. Not easy, not for the precision required. You need not only precise time (straightforward), but also precise location (not so straightforward) You're interested in roughly 20MHz, as I recall. Wavelength of 15 meters. In time, about 45 nanoseconds. Let's start with a real relaxed requirement, comparable to the mirror flatness for a telescope of lambda/14. That means a time knowledge of about 3 ns and a position knowledge of 1 meter, in absolute terms. Typical GPS receivers that have a 1pps output are good to about 20-30 nanoseconds. Using that to discipline a quartz oscillator, you can do a bit better, but it's non trivial to get to the 1-2 ns range. Remember, you're also planning on integrating over time, so you have to hold that tolerance for a long time. It would be difficult to determine your position to an absolute accuracy of 1 meter, much less the phase center of the antenna (which will change as a function of the angle of incidence, quite substantially, unless you're putting those dipoles up 100s of feet in the air. 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: snip You need to go beyond looking at press releases from radio astronomers. However, instead of using the satellite TV dishes, we could use individual dipole antennas attached to each house. You would need to communicate high data rates for the signals detected so you would need broadband internet access for this. These dipole antennas as per the Radio JOVE project are just simple vertical wires so could be attached to the house when the installer is connecting the wiring for the broadband. Possibly you could use the same external wiring as for the broadband but that might cause interference with the internet signals. Radio Jove uses a pair of horizontal dipoles connected together to create a single narrower lobe pointing up. As shown on the Radio JOVE page the receivers for these dipole antennas are quite simple so would contribute minimally to the cost of installation. Who's paying, and how minimal? I don't think so. You do need accurate positional determination and timing synchrony for each receiving system to do the very long baseline interferometry, but at these decametric wavelengths this would be easy to do with GPS receivers carried by the installers. No they can't. You need position accuracy of sub-1 meter accuracy, and that isn't achievable by simple handheld devices, like your Garmin E-trex, etc. A surveyor using a survey GPS system can get there, although absolute position (relative to, say, the center of the earth, or some standard datum) to 1 meter would be very challenging. There's also the not so little problem of tidal bulge. Your position changes in absolute (relative to a stellar reference) terms several tens of cm. On top of that, tectonic plate movement is on the order of several cm/year, which is in the same general ballpark as your accuracy requirement. ... The NASA Global Differential GPS System. "The NASA Global Differential GPS (GDGPS) System is a complete, highly accurate, and extremely robust real-time GPS monitoring and augmentation system. "Employing a large ground network of real-time reference receivers, innovative network architecture, and award-winning real-time data processing software, the GDGPS System provides decimeter (10 cm) positioning accuracy and sub-nanosecond time transfer accuracy anywhere in the world, on the ground, in the air, and in space, independent of local infrastructure." http://www.gdgps.net/ This would be enough for the positional accuracy at this wavelength. This type of highly accurate receiver would probably have to be used only by the installers as they are likely to be expensive. Perhaps the positional accuracy could be maintained over time by referring to a satellite signal. The "time transfer" accuracy mentioned apparently does mean the many different sites can be put in time synchrony to within sub-nanosecond precision by reference to the atomic clocks on several GPS satellites at the same time: Global Positioning System. 2.) Basic concept of GPS * 2.1 Position calculation introduction * 2.2 Correcting a GPS receiver's clock http://en.wikipedia.org/wiki/Global_...concept_of_GPS Innovation: GPS Time Transfer. Using Precise Point Positioning for Clock Comparisons. Nov 1, 2006 By: François Lahaye, Diego Orgiazzi, Patrizia Tavella, Giancarlo Cerretto. GPS World http://www.gpsworld.com/gpsworld/Inn.../detail/383189 However, JPL radio astronomer Dr. Dayton Jones responded to my question about the required timing accuracy at such long wavelengths, suggesting it might only have to be only at the ten's of nanoseconds to even microseconds range, depending on the bandwidth being detected: Newsgroups: rec.radio.amateur.space, rec.radio.amateur.antenna, sci.astro, sci.astro.seti, sci.space.policy From: (Robert Clark) Date: 18 Jun 2001 10:26:50 -0700 Subject: Will amateur radio astronomers be the first to directly detect extrasolar planets? http://groups.google.com/group/sci.a...56d6bc52a09590 Note that with the Radio JOVE system the bandwidth being detected is usually quite small at the tens to hundreds of khz range, as the emissions consist of short pulses. This would only require timing accuracy at the microsecond range. For the cost, note that for cable, DSL, satellite, internet and/or TV service typically the receivers, modems, routers, etc are only "rented" where you pay a nominal fee every month. If the cost for the dipole and receivers were in the range of $100 dollars per installation then this could be amortized over the life of that broadband internet system, at say $1 dollar a month or even 50 cents a month. Bob Clark |
#13
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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 |
#14
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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 |
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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 |
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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 |
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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 |
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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 |
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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 |
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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 |
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