--- On Sun, 6/28/09, Robert Clark <bobbygc2001@yahoo.com> wrote: > From: Robert Clark <bobbygc2001@yahoo.com> > Subject: What factors determine the brightness of a satellite flare? > To: seesat-l@satobs.org > Date: Sunday, June 28, 2009, 5:17 AM > > I copied below a post to sci.astro about the possibility > of sending naked-eye-visible light communications from a > Google Lunar X Prize entrant. It's based on the brightness > seen with some satellite flares. > What I need to know is what are the factors that > contribute to the brightest satellite flares since then we > could incorporate these facts into the design of the > reflective surfaces at the Moon to produce the brightest > images. > > I forgot to attach that sci.astro post. Here it is: =============================================================== Newsgroups: sci.astro, rec.radio.amateur.space, rec.radio.amateur.antenna, sci.astro.seti, sci.physics From: Robert Clark <rgregorycl...@yahoo.com> Date: Sat, 27 Jun 2009 23:54:14 -0700 (PDT) Subject: High data rate space transmissions through visible light communication. 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 here: 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 ===================================================================== ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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