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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 |
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