Re: CU Crypto Session Sat
Phil Karn comments on spy-satellite resolution:
[Technical argument with which I agree, leading to approximate one-foot resolution limit, deleted.]
Phil's argument was for a 2-meter aperture at typical slant ranges in the visible-light band. I once did the calculation, also for a 2-meter aperture, with other circumstances being as optimal as I could make them; namely, looking straight down from a rather low perigee (I picked 200 Km), working in the near UV (where it still penetrates the atmosphere reasonably well -- I picked 3000 Angstroms as a round number), and with perfect seeing (which depends on luck, weather and exposure times, and perhaps on telescope and/or image-processing technology). For a circular aperture, the nominal resolving power (in radians) -- that is, the Airy disc radius to the first minimum -- is 1.22 * wavelength / aperture diameter, which for this case works out to 0.183 microradian. Multiplying by 200 Km gives 3.66 cm resolution on the ground. If one shapes the aperture to match the pattern under study, one can drive that factor of 1.22 down to as little as 0.5, but such shaping would likely be useful only for specific patterns not likely in the actual observation. I am told that careful image processing can sometimes resolve things a little below the Airy-disc limit, but not far -- the information really goes away fast at higher angular frequencies. So all in all, I am inclined to think that the best ground resolution attainable with a 2-meter aperture from orbit is about an inch. That is in fact just about enough to read a poker hand -- the spots on the cards are an inch or so apart -- but you might have trouble telling the face cards apart, as well as telling hearts from diamonds and clubs from spades. That is, if cards were well spread out you might see that a certain card had five black spots on it, or had a "face". I should probably explain about "Airy disc": The term crops up often in the study of astronomical imaging. The image of a point light source by perfect optics is a bulls-eye, a bright central spot surrounded by alternating light and dark rings, called the "Airy disc" after the physicist who first described it analytically. The 1.22 * wavelength / aperture is the angle from the center of the bright spot to the middle of the first dark ring. Of course, a possible way around this limit is to put up a larger, segmented mirror... -- Jay Freeman PS: References to physics texts on request...
Phil Karn comments on spy-satellite resolution:
[Technical argument with which I agree, leading to approximate one-foot resolution limit, deleted.]
... goes away fast at higher angular frequencies. So all in all, I am inclined to think that the best ground resolution attainable with a 2-meter aperture from orbit is about an inch. That is in fact just about enough to read a ... Of course, a possible way around this limit is to put up a larger, segmented mirror... -- Jay Freeman Could the same effect (as a segmented mirror) be achieved by taking multiple pictures (from the same mirror) and processing them together? E.g. does synthetic aperture radar actually produce higher resolution than achievable from a single "snapshot"? If so, then this might work (at least for slow-moving targets :-)... Enchoiring Mimes Want to Know! -- dat@ebt.com (David Taffs)
A couple of years ago, IEEE Spectrum did an article which took the premise that spy-satellite optics could be made that were as good as the Hubble Space Telescope optics (for various reasons, pointing Hubble at the earth "just wouldn't work" :-) They came up with some number like "1 foot resolution" -- and then did some processing on a photograph to demonstrate what that meant. The picture used was a rear view of a VW Bug, with a copy of Isvestia resting on the upper edge of the trunk. Basically, you could tell there was something sitting there, but you couldn't read the headlines :-) Unfortunately, my library is at the moment unindexed, due to a recent move, or I'd include a reference to the article; perhaps someone else here saw it... it covers the physics involved rather well, and lists a lot of the relevant engineering details. _Mark_ <eichin@paycheck.cygnus.com> ... just me at home ...
Mark W. Eichin wrote:
number like "1 foot resolution" -- and then did some processing on a photograph to demonstrate what that meant.
The picture used was a rear view of a VW Bug, with a copy of Isvestia resting on the upper edge of the trunk. Basically, you could tell there was something sitting there, but you couldn't read the headlines
A previous poster suggested 1 inch with the latest technology on a clear day. In any case this has grave implications for the privacy in outdoor activities like under-the-sky-copulation. A simple protection is available: a heat source to produce chaotic air turbulence. A campfire?
: : Mark W. Eichin wrote: : : > number like "1 foot resolution" -- and then did some processing on a : > photograph to demonstrate what that meant. : > : > The picture used was a rear view of a VW Bug, with a copy of Isvestia : > resting on the upper edge of the trunk. Basically, you could tell : > there was something sitting there, but you couldn't read the headlines : : A previous poster suggested 1 inch with the latest technology on a clear : day. In any case this has grave implications for the privacy in outdoor : activities like under-the-sky-copulation. A simple protection is : available: a heat source to produce chaotic air turbulence. A campfire? : Oh come on! Who cares who you're screwing outside? Do you honestly think that any of us here is doing anything that warrants that kind of scrutiny by the intelligence community? Just because something is possible doesn't make it cheap and easy to use. Besides, on with that sort of angular resolution, the area of the image is also reduced, so they'd have to be pretty interested in you in particular, and have a real good idea of where to look for you when the satellite was overhead. Besides, do you really think that your activities are so important that the NRO would be willing to devote the time and money necessary to find you and photograph you (or any other member of this list) with their precious resources? It's not like the sky is blanketed with these satellites to the point where anyone in the US would have to worry about what they were doing outdoors..... Of course, perhaps the comments were meant facetiously, and I've meerly overreacted here -- wouldn't be the first time. Brendan
A couple of years ago, IEEE Spectrum did an article which took the premise that spy-satellite optics could be made that were as good as the Hubble Space Telescope optics (for various reasons, pointing
July 1986. Most of the issue was devoted to articles on "national technical means" for verifying arms control agreements. This has always been jargon for spy satellites and other not-so-public capabilities. Phil
Could the same effect (as a segmented mirror) be achieved by taking multiple pictures (from the same mirror) and processing them together? E.g. does synthetic aperture radar actually produce higher resolution than achievable from a single "snapshot"? If so, then this might work (at least for slow-moving targets :-)...
dat@ebt.com (David Taffs)
Yes, but the positional accuracy required (on the order of the wavelength) would be prohibitive to achieve. (Such things may be possible for the NRO's DSP (more acronym overloading: DSP stands for Defense Support Program) satellites to implement. I haven't heard any speculations that this is actually being done.) Synthetic Aperture Radar is feasible becuase the wavelengths are so much larger. The new Keck Telescope will eventually use a second telescope, now under construction, located some distance away, for very long baseline interferometry...I have no idea if it can be made to work as an actual synthetic aperture. Jay Freeman man know. --Tim May -- .......................................................................... Timothy C. May | Crypto Anarchy: encryption, digital money, tcmay@netcom.com | anonymous networks, digital pseudonyms, zero 408-688-5409 | knowledge, reputations, information markets, W.A.S.T.E.: Aptos, CA | black markets, collapse of governments. Higher Power: 2^859433 | Public Key: PGP and MailSafe available. "National borders are just speed bumps on the information superhighway."
From: tcmay@netcom.com (Timothy C. May)
Could the same effect (as a segmented mirror) be achieved by taking multiple pictures (from the same mirror) and processing them together? E.g. does synthetic aperture radar actually produce higher resolution than achievable from a single "snapshot"? If so, then this might work (at least for slow-moving targets :-)...
dat@ebt.com (David Taffs)
Yes, but the positional accuracy required (on the order of the wavelength) would be prohibitive to achieve. (Such things may be possible for the NRO's DSP (more acronym overloading: DSP stands for Defense Support Program) satellites to implement. I haven't heard any speculations that this is actually being done.) Synthetic Aperture Radar is feasible becuase the wavelengths are so much larger. The new Keck Telescope will eventually use a second telescope, now under construction, located some distance away, for very long baseline interferometry...I have no idea if it can be made to work as an actual synthetic aperture. Jay Freeman man know. I wasn't thinking so much of interferometry techniques (although my reference to synthetic radar certainly implies them), but rather something on the order of a filter which might work (independent of the wavelength of light) as follows: Take, for example, the square box pixellation (is this the right word here?) used to blot out people's faces on TV sometimes. Put a long (preferably continuous) series of images into the computer, and build a model of the movement of the person's head (the camera isn't perfectly still; assume that the person, however, does stay still). Use the data about how adjacent pixels change over time to improve the model of what the person's face really looks like. This is independent of the wavelength of light -- it does of course depend on the resolution of the square pixels used to blot the peron's face, but not particularly on the wavelength or resolution of the camera (assuming it is much better than the square blotches). I first noticed this effect watching Court TV's coverage of the William Kennedy Smith rape trial (I was home sick at the time), while the victim testified. I felt that as the person (and camera) moved around, I could gradually form a better opinion of what the person looked like than just provided by the square blotches, by noting when and how the (macro-)pixels changed. Of course, just filtering a single frame would be better than looking at the sharp-edged squares. I'm talking about averaging all these filtered images over time, compensating for movement of the camera and subject. It would seem to me that over long enough time, perhaps using more sophisticated mathematics than just averaging (although just plain averaging seems like the right operation here), if there was actually enough movement to provide enough resolution, you could eventually get to a real photographic-quality image of the person. This process might be similar to CAT scans, where a lot of low-resolution "pictures" are combined to create a high-resolution image, except the distribution would be temporal rather than spatial. ObCryptoJustification: I think is relevant to c'punks, because it involves decryption of an encrypted signal (recovering the face of a person when it was intentionally distorted). Does this mean that if people like Court TV really want to blur people's faces, they need to add crypto-secure noise instead of just averaging the micro-pixels into macro-pixels? I think so! ObRandomOtherThreadWithMarginalCryptoJustificationButInReplyToOtherCpunksMsgs: and also ObAdditionalMetaDiscussionAboutWhatIsAppropriateForThisList: I also thought the license plate joke was definately relevant to c'punks, because it was actually a code, where the cleartext domain was conceptual rather than textual, just like this mail talks about a domain in 2-space (or 3-space) images, rather than text. Also, the fact that the "plaintext" was actually a pun involving multiple coding schemes made it relevant to this list also IMHO. Also, I think short humor is appropriate for any list, at least if it is both funny and computer-related, but I admit that may be stretching it for some here. I assume that coding (as distinguished from ciphering) is indeed relevant to this list... -- dat@ebt.com (David Taffs)
participants (7)
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unknown@example.com -
Brendan McKenna -
freeman@netcom.com -
Mark W. Eichin -
Mats Bergstrom -
Phil Karn -
tcmay@netcom.com