robert@uop.edu (Doc Ness) (12/29/87)
Well, how about that AP headline? "Satellite saw Waite, newspaper says" and "National Security Agency Officials" talked of the "Lacrosse" satellite parked over Lebanon. Comments? (I bet NSA is pissed off either way!)
cgs@umd5.umd.edu (Chris Sylvain) (01/04/88)
In article <873@uop.edu> robert@uop.edu (Doc Ness) writes:
? Well, how about that AP headline?
? "Satellite saw Waite, newspaper says"
? and "National Security Agency Officials" talked of the "Lacrosse"
? satellite parked over Lebanon.
?
? Comments?
?
? (I bet NSA is pissed off either way!)
Uh-huh.... apply NaCl liberally..
Really now, does Waite have a highly distinctive bald-spot on the top of
his head? or maybe he was looking down into a bird-bath ?
--
--==---==---==--
.. 'Twas brillig, and the slithy toves ..
ARPA: cgs@umd5.UMD.EDU BITNET: cgs%umd5@umd2
UUCP: ..!uunet!umd5.umd.edu!cgsshibumi@well.UUCP (Kenton A. Hoover) (01/05/88)
>In article <873@uop.edu> robert@uop.edu (Doc Ness) writes: >? Well, how about that AP headline? >? "Satellite saw Waite, newspaper says" >? >? (I bet NSA is pissed off either way!) > >Uh-huh.... apply NaCl liberally.. No, it was the "Terry Waite" on his hostage jersey (number 17?). Or perhaps the satellite heard some car honk when they saw another car's "Honk if you have Terry Waite in your trunk" bumpersticker. Or perhaps a "Terry Waite on board" sign in the window. Or perhaps some person at AP need to change whatever it is they're smoking. -- ! Kenton A. Hoover {hoptoad,hplabs,lll-lcc,ptsfa}!well!shibumi ! ! SNAIL: 1748 Clement Street ! ! Prescriptive Technology San Francisco, CA 94121 ! ! "Evil will always triumph over good because good is dumb!" !
wb8foz@netsys.UUCP (David Lesher) (01/07/88)
>>? Well, how about that AP headline? >>? "Satellite saw Waite, newspaper says" >> >>Uh-huh.... apply NaCl liberally.. > On the other hand, several public sources have reported that resolutions good enough to read the brand of coffin nails being smoked by the guard on duty in Moscow are in use. If you combine that with the art of *interpretation*, it is not unreasonable that the distinctive, big bulky fellow walking outside the isolated house in a area of interest, surrounded by guards with AK-47s is Terry Waite. I suggest the book "Deep Black" on the subject of remote imagery. -- Have you ever WATCHED cable TV, Judge Kennedy? decuac!netsys!wb8foz
jfc@athena.mit.edu (John F Carr) (01/10/88)
In article <1952@netsys.UUCP> wb8foz@netsys.UUCP (David Lesher) writes: >>>? "Satellite saw Waite, newspaper says" >>>Uh-huh.... apply NaCl liberally.. >On the other hand, several public sources have reported that >resolutions good enough to read the brand of coffin nails >being smoked by the guard on duty in Moscow are in use. >If you combine that with the art of *interpretation*, >it is not unreasonable that the >distinctive, big bulky fellow walking outside the isolated house >in a area of interest, surrounded by guards with AK-47s is >Terry Waite. I suggest the book "Deep Black" >on the subject of remote imagery. I haven't read the book, but there is a simple argument against such resolution. Cameras in orbit are limited either by diffraction or atmospheric conditions. A telescope at sea level, in perfect weather, is typically limited to no better than 1" (that is a unit of angle: 1 arc-second, not 1 inch). At 200 km this is a linear distance of 1 meter. I would believe that looking down from orbit would give a different result (though I don't know). An *absolute* limit on the resolution of an image is diffraction in the camera. For visible light (wavelength 5 X 10^-7 m), and a 1 meter lens or mirror on the satellite, the limit is about 10 cm. To distinguish an object of size 1 cm, a 10 meter lens/mirror would be needed. The Space Telescope, which has not been launched and is to my knowledge the largest mirror in or intended to be in space, is about 2 meters. The biggest telescope in the world is 6 meters. There are techniques for processing images distorted by the atmosphere or an imperfect lens, but they can not get an image to be better than the diffraction limit. I would be impressed if any intelligence agency could regularly scan a city at 4 inch resolution while also doing its other (presumably more important) business. (assuming B+W image, 256 intesity levels, 10 cm pixels, 2 km square area : 400 MB per image.) Unless the miltary or intelligence agencies are far ahead of anything I have read about in computer pattern recognition (or somehow learned exactly where to look), I would not believe the report "Satellite saw Waite". (Unless someone was *extremely* lucky). --John Carr
jnp@calmasd.GE.COM (John Pantone) (01/12/88)
> >>>? "Satellite saw Waite, newspaper says" > >>>Uh-huh.... apply NaCl liberally.. > >On the other hand, several public sources have reported that > >resolutions good enough to read the brand of coffin nails > >being smoked by the guard on duty in Moscow are in use. > I haven't read the book, but there is a simple argument against > such resolution. Cameras in orbit are limited either by diffraction > or atmospheric conditions. A telescope at sea level, in perfect > weather, is typically limited to no better than 1" (that is a unit > of angle: 1 arc-second, not 1 inch). At 200 km this is a linear > distance of 1 meter. Except sattelites don't use cameras - in that sense of the word. They use CCD's and all sorts of bizarre imaging equipment - but they DON'T take pictures - I don't think that the same limitations apply (although they no doubt have other limitations). I've seen a message on this subject from someone at NASA - can anyone talk? -- These opinions are solely mine and in no way reflect those of my employer. John M. Pantone @ GE/Calma R&D, 9805 Scranton Rd., San Diego, CA 92121 ...{ucbvax|decvax}!sdcsvax!calmasd!jnp jnp@calmasd.GE.COM GEnie: J.PANTONE
karn@faline.bellcore.com (Phil R. Karn) (01/12/88)
> On the other hand, several public sources have reported that > resolutions good enough to read the brand of coffin nails > being smoked by the guard on duty in Moscow are in use. Nonsense, assuming you're talking about orbiting reconaissance satellites (as opposed to low-flying aircraft). Do a little math and determine the visible-light resolution limit of a 2-meter diameter mirror (roughly the largest that could be comfortably carried inside the payload fairing of a Titan from Vandenburg) at a distance of 500 miles (the perigee of a KH-11 orbit plus allowance for slant range). You'll end up with something on the order of a foot, which just so happens to be the resolution of the KH-11 aircraft carrier construction pictures that showed up in Av Week last year. Making a project secret doesn't exempt it from the laws of physics. (Too bad the SDI folks don't realize this. :-)) Phil
baum@apple.UUCP (Allen J. Baum) (01/12/88)
-------- [] In article <2209@bloom-beacon.MIT.EDU> jfc@athena.mit.edu (John F Carr) writes: > >There are techniques for processing images distorted by the >atmosphere or an imperfect lens, but they can not get an image to be >better than the diffraction limit. In fact, it is possible to get better than the diffraction limit, as synthetic aperature radars have shown. Take several images from different points,(from a satelite, easily done by just taking a few sequential pictures, since the stelite is moving) and image-process your little brains out. -- {decwrl,hplabs,ihnp4}!nsc!apple!baum (408)973-3385
daveb@eneevax.UUCP (David Bengtson) (01/13/88)
In article <2604@calmasd.GE.COM> jnp@calmasd.GE.COM (John Pantone) writes: >> >>>? "Satellite saw Waite, newspaper says" >> >>>Uh-huh.... apply NaCl liberally.. >> >On the other hand, several public sources have reported that >> >resolutions good enough to read the brand of coffin nails >> >being smoked by the guard on duty in Moscow are in use. > >> I haven't read the book, but there is a simple argument against >> such resolution. Cameras in orbit are limited either by diffraction >> or atmospheric conditions. A telescope at sea level, in perfect >> weather, is typically limited to no better than 1" (that is a unit >> of angle: 1 arc-second, not 1 inch). At 200 km this is a linear >> distance of 1 meter. > >Except satelites don't use cameras - in that sense of the word. They use >CCD's and all sorts of bizarre imaging equipment - but they DON'T take >pictures - I don't think that the same limitations apply (although they no >doubt have other limitations). That's true, they don't use cameras, in the sense of a piece of film isn't currently used, However, the concept of diffractive losses apply to any situation where an image is formed, and light, or in the more general sense, Electromagnetic waves, are passed through an aperature. From Jenkins & White, Fundamentals of Optics p331 Minimum angle of resolution in seconds = 1.220 * ( lambda / D ) Where D is the diameter of the aperture and lambda is the wavelength of the light. It is physically impossible, using 1 image, to get below this limit. For the visible range ( lambda ~ 555 nanometers ) and D = 2 meters, theta = 3.39e-7 radians For an orbit of 200 miles, features resolve to ~ 5 inches. Not bad, but that assumes a perfect atmosphere. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ I cannot comment on the possibility of image processing, since I know squat about that topic David Bengtson If you think that I speak for Laboratory for Plasma Fusion the University, I've got a Bridge University of Maryland to sell you :-) College Park Md 20742 {your keyboard} !uunet!mimsy!eneevax!daveb eneevax.umd.edu
msb@sq.uucp (Mark Brader) (01/13/88)
> A telescope at sea level, in perfect > weather, is typically limited to no better than 1" (that is a unit > of angle: 1 arc-second, not 1 inch). At 200 km this is a linear > distance of 1 meter. I would believe that looking down > from orbit would give a different result (though I don't know). Probably it would be considerably better, since the atmosphere is at the right end of the light path. However, diffraction is still a limit, as you say. They might not be using visible light; how many times the frequency of violet light must you go to before viewing becomes impossible? > I would be impressed if any intelligence agency could regularly scan > a city at 4 inch resolution while also doing its other (presumably > more important) business. This could have BEEN considered important business, or they could have gotten lucky, or, as you say, they could have known where to look. I'm not saying that people CAN be spotted by satellite, only that it's not as easy to dismiss as one might think. Mark Brader "You wake me up early in the morning to tell me I am Toronto right? Please wait until I am wrong." utzoo!sq!msb, msb@sq.com -- John von Neumann, on being phoned at 10 am
karn@faline.bellcore.com (Phil R. Karn) (01/13/88)
> In fact, it is possible to get better than the diffraction limit, as synthetic > aperature radars have shown. Take several images from different points,(from > a satelite, easily done by just taking a few sequential pictures, since the > stelite is moving) and image-process your little brains out. Aperture synthesis doesn't "get better than the diffraction limit", it decreases the diffraction limit by building a system with a very large effective aperture. It's like having a telescope with a very large virtual mirror where only small portions of the signal gathering area is actually "filled in". You can get extremely high resolution images if you *simultaneously* photograph the same target from two or more widely separated satellites, and then *coherently* add the two images with an accuracy on the order of a small fraction of a wavelength. This is entirely practical at radio wavelengths (VLBI and SAR being two examples), but at optical wavelengths? Good luck! Phil
lwall@devvax.JPL.NASA.GOV (Larry Wall) (01/13/88)
In article <1175@eneevax.UUCP> daveb@eneevax.umd.edu.UUCP (David Bengtson) writes:
: For the visible range ( lambda ~ 555 nanometers ) and D = 2 meters,
: theta = 3.39e-7 radians
: For an orbit of 200 miles, features resolve to ~ 5 inches. Not bad, but
: that assumes a perfect atmosphere.
You guys are all assuming a big round mirror. Now, it's true that for looking
at faint stars you need a lot of mirror acreage, but there's plenty of light
bouncing off of Lebanon. You don't need a huge round mirror to get the
aperature you want--just build a frame that will stay rigid in microgravity
and hang several smaller mirrors on it with a common focus. What's the
resolution for a mirror with an effective aperature of, say 20 meters? How
many 1 meter mirrors would it take to get the interferometry to come out right?
Larry Wall
lwall@jpl-devvax.jpl.nasa.govlgy@pupthy2.PRINCETON.EDU (Larry Yaffe) (01/13/88)
In article <2604@calmasd.GE.COM> jnp@calmasd.GE.COM (John Pantone) writes: >> [[ Quoted comments about limits on image resolution omitted. ]] > >Except sattelites don't use cameras - in that sense of the word. They use >CCD's and all sorts of bizarre imaging equipment - but they DON'T take >pictures - I don't think that the same limitations apply (although they no >doubt have other limitations). > >John M. Pantone @ GE/Calma R&D, 9805 Scranton Rd., San Diego, CA 92121 >...{ucbvax|decvax}!sdcsvax!calmasd!jnp jnp@calmasd.GE.COM GEnie: J.PANTONE The fact that CCD sensors are used in place of film is completely irrelevant to the issue of diffraction-based resolution limits. Diffraction by the primary mirror of the imaging system limits the best acheivable focusing of the incoming light irrespective of what sort of sensor is used to record the picture. On a marginally related note - an earlier posting (sorry, no reference), implied that spy satellite mirrors are no larger than the primary mirror of the Hubble Space Telescope. I doubt that this is true (but not by enough to change the improbability of recognition of Waite from orbit!). The size of the space telescope mirror was far more strongly influenced by economic realities than by technical limits. Supposedly, when the bids for the fabrication of the space telescope mirror were solicited, the lowest bids came not from traditional optical companies like Corning, but from defense companies like Lockheed. Seems that they already had lots of experience... ------------------------------------------------------------------------ Laurence G. Yaffe Department of Physics lgy@pupthy.princeton.edu Princeton University ...!princeton!pupthy!lgy PO Box 708, Princeton NJ 08544 609-452-4371 or -4400
lee@srs.UUCP (Lee Hasiuk) (01/13/88)
> That's true, they don't use cameras, in the sense of a piece > of film isn't currently used, However, the concept of diffractive losses > apply to any situation where an image is formed, and light, or in the > more general sense, Electromagnetic waves, are passed through an aperature. Actually, this may not be completely true. According to the 'The Puzzle Palace', some reconnaissance satellites have the capability of ejecting film cannisters which can be picked up by airplanes as they fall to Earth. This technology was developed in 1960. > > From Jenkins & White, Fundamentals of Optics p331 > > Minimum angle of resolution in seconds = 1.220 * ( lambda / D ) > > Where D is the diameter of the aperture and lambda is the wavelength > of the light. It is physically impossible, using 1 image, to get below > this limit. > For the visible range ( lambda ~ 555 nanometers ) and D = 2 meters, > theta = 3.39e-7 radians > For an orbit of 200 miles, features resolve to ~ 5 inches. Not bad, but > that assumes a perfect atmosphere. > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > I cannot comment on the possibility of image processing, since > I know squat about that topic > In a complex analysis class, we were told that the diffractive 'limits' of lenses and mirrors could be bypassed to a certain degree through the use of analytic continuation. Anyone care to comment?
karn@faline.bellcore.com (Phil R. Karn) (01/14/88)
> ...You don't need a huge round mirror to get the > aperature you want--just build a frame that will stay rigid in microgravity > and hang several smaller mirrors on it with a common focus. What's the > resolution for a mirror with an effective aperature of, say 20 meters? How > many 1 meter mirrors would it take to get the interferometry to come out right? Yes, but those mirrors will have to be positioned to accuracies of a fraction of a wavelength. That's easily done at radio frequencies, but optical frequencies are a different story. I'm not saying it can never be done, only that it's well beyond our capabilities at present to do it on spacecraft. Not to say that there probably isn't a few billion buried somewhere in NRO's black budget for R&D into this sort of thing. (Can you say 'Welfare for Engineers?' Good! I knew you could!") :-) Don't be *that* sure you don't need light-gathering capability, even when imaging the sunlit earth. Remember you are imaging a narrow field with a very long "lens", so the f-rating will be very large. You're also moving along at a good clip (~7 km/sec) so there's something to be said for being able to use short exposure times. Any photographer can tell you that ASA 400 film isn't all that fast when you're using handheld telephoto lenses, even on a sunny day. Phil
fiddler%concertina@Sun.COM (Steve Hix) (01/14/88)
In article <531@srs.UUCP>, lee@srs.UUCP (Lee Hasiuk) writes: > > Actually, this may not be completely true. According to the 'The Puzzle > Palace', some reconnaissance satellites have the capability of ejecting > film cannisters which can be picked up by airplanes as they fall to Earth. > This technology was developed in 1960. The old Discover series of satellites, generally launched from Vandenberg AFB, tossed down film cannisters after they had exposed all the film (most of the time). The exposed film, drifting down with a parachute off the coast of someplace like Hawaii, would be snagged in mid-air by either a HC-130 or a large helicopter towing a trapeze-like device. There was usually time to make several passes before the thing hit the water. (When that happened, whoever pulled the short straw got to go swimming.) The disadvantage of this system (while nobody could intercept and decode any transmissions) was that you had to wait some variable amount of time before the polar-orbiting satellite was in a position to launch its package where you wanted to be. It's probably cheaper to dump a radio signal down to one or more ground stations (once you have them built, anyway) than sending a plane 600 miles out to sea to catch a film drop. (Somewhat quicker, too.) seh
jgy@hropus.UUCP (John Young) (01/16/88)
Are you safe from satellite scanning if you carry an umbrella, or could they scan from a *wide* range of angles (>100 degrees)
jnp@calmasd.GE.COM (John Pantone) (01/16/88)
In article <1988Jan12.180815.685@sq.uucp>, msb@sq.uucp (Mark Brader) writes: > I'm not saying that people CAN be spotted by satellite, only that it's > not as easy to dismiss as one might think. Thank you Mark. O.K. - I've finally reached saturation. As a certifiable old-fart who was around during the Cuban missle crisis (1962) I can guarantee that an object as small as an automobile and/or truck was plainly visible by sattelite in '62. I'm quite confident that you could have made out the make of the car or truck - even in the crummy newspaper photos they were showing at the time. Even if technology hasn't progressed since then - seeing a person is surely possible - since those photos were released publicly I can imagine that they had better ones which they kept secret. I can't even begin to believe that a sattelite would have trouble resolving a person-sized thing. (Image enhancement, long-baseline effects, microwave imaging, etc.) -- These opinions are solely mine and in no way reflect those of my employer. John M. Pantone @ GE/Calma R&D, 9805 Scranton Rd., San Diego, CA 92121 ...{ucbvax|decvax}!sdcsvax!calmasd!jnp jnp@calmasd.GE.COM GEnie: J.PANTONE
clt@pur-phy (Carrick Talmadge) (01/16/88)
In article <1988Jan12.180815.685@sq.uucp> msb@sq.UUCP (Mark Brader) writes: >> A telescope at sea level, in perfect >> weather, is typically limited to no better than 1" (that is a unit >> of angle: 1 arc-second, not 1 inch). > >Probably it would be considerably better, since the atmosphere is at >the right end of the light path. Mark is right here. This is a point which I screwed up on last fall when this question was last discussed: You'll do a lot better looking in from the outside than 1 arc-second. Numbers that were floating around at that point were maximum resolutions of about 3-4 cm. >However, diffraction is still a limit, >as you say. They might not be using visible light; how many times the >frequency of violet light must you go to before viewing becomes impossible? Depends. Violet light is usually classified to be around 4200 angstroms. The transmission coefficient of the atmosphere plummets somewhere between 2900-3000 angstroms (at 2900 angstroms, it is less than 1e-6) [Ref: International Critical Tables, Vol. V, page 268]. Over Antartica, with the hole in the ozone layer, you'll probably do better, though. :-) Anyway, you'll gain *maybe* a factor of two by going as far as is practical into the ultraviolet. Carrick Talmadge clt@newton.physics.purdue.edu -------- When you have eliminated the impossible, whatever remains, no matter how improbable, must be the truth. -- Sherlock Holmes Yes, but what happens if nothing's left? -- Watson's reply
wyatt@cfa.harvard.EDU (Bill Wyatt) (01/17/88)
> > O.K. - I've finally reached saturation. As a certifiable old-fart who was > around during the Cuban missle crisis (1962) I can guarantee that an object > as small as an automobile and/or truck was plainly visible by sattelite in > '62. [...] Except they used U2's, not satellites. -- Bill UUCP: {husc6,ihnp4,cmcl2,mit-eddie}!harvard!cfa!wyatt Wyatt ARPA: wyatt@cfa.harvard.edu (or) wyatt%cfa@harvard.harvard.edu BITNET: wyatt@cfa2 SPAN: cfairt::wyatt
bill@utastro.UUCP (William H. Jefferys) (01/17/88)
In article <2619@calmasd.GE.COM> jnp@calmasd.GE.COM (John Pantone) writes:
~
~O.K. - I've finally reached saturation. As a certifiable old-fart who was
~around during the Cuban missle crisis (1962) I can guarantee that an object
~as small as an automobile and/or truck was plainly visible by sattelite in
~'62. I'm quite confident that you could have made out the make of the car
~or truck - even in the crummy newspaper photos they were showing at the time.
Actually, the photographs made public by the Kennedy administration
during the Cuban missle crisis were taken by high-flying U-2 aircraft,
not by satellites. They carried cameras of much smaller aperture
(and correspondingly lower resolution) than the present generation of
spy satellites does.
Bill Jefferys
--
Glend. I can call spirits from the vasty deep.
Hot. Why, so can I, or so can any man; But will they come when you
do call for them? -- Henry IV Pt. I, III, i, 53parker@epiwrl.EPI.COM (Alan Parker) (01/17/88)
In article <2619@calmasd.GE.COM> jnp@calmasd.GE.COM (John Pantone) writes: >As a certifiable old-fart who was >around during the Cuban missle crisis (1962) I can guarantee that an object >as small as an automobile and/or truck was plainly visible by sattelite in >'62. I think you are confusing sattelite imaging with U-2 pictures here.
truett@cup.portal.com (01/17/88)
In order to beat the apparent diffraction limit, you don't even need a rigid frame for the multiple mirror setup. The method is called active optics and it works like this: First you drop a small corner reflector somewhere in the Lebanese highlands. Then you put an array of mirrors and CCD detectors into orbit together with a small laser. Now, use the reflected signal of the laser from the corner reflector to get the optical sensors all phase to withing a fraction of a wavelength of light (or infrared, that's even easier). The resulting system has an effective appature much bigger than a sigle mirror. I suspect that arrays with an apparent aperture of 20-30 meters can be orbited easily. Another note. Each of the optical sensors is using folded optics with a very long focal length (probably up to ten meters) and for pictures in daylight, a very high f/stop would be used resulting in extreme depth of field. There is also nothing that says the observation had to be in visible light. I believe that synthetic aperture radar achieves a resolution of half the antenna diameter. It should not be too difficult to put a microwave radar on a satellite that can achieve a three-dimensional image to a resolution of a few inches. Such a system can even provide sequential images giving you a movie of the action below. Now, use the reflected laser light to puat an array of synthetic aperture radars in phase (thus overcoming the higher diffraction limit of microwaves) and the results can be very interesting. Truett Lee Smith, Sunnyvale, CA UUCP: truett@cup.portal.com
jnp@calmasd.GE.COM (John Pantone) (01/19/88)
> > I'm not saying that people CAN be spotted by satellite, only that it's > > not as easy to dismiss as one might think. > > O.K. - I've finally reached saturation. As a certifiable old-fart who was > around during the Cuban missle crisis (1962) I can guarantee that an object > as small as an automobile and/or truck was plainly visible by sattelite in > '62. I'm quite confident that you could have made out the make of the car > or truck - even in the crummy newspaper photos they were showing at the time. I must also be a certifiable old idiot - apparently those photos were from a U-2 not a satellite. Sorry to increase the background noise. -- These opinions are solely mine and in no way reflect those of my employer. John M. Pantone @ GE/Calma R&D, 9805 Scranton Rd., San Diego, CA 92121 ...{ucbvax|decvax}!sdcsvax!calmasd!jnp jnp@calmasd.GE.COM GEnie: J.PANTONE
die@frog.UUCP (Dave Emery) (01/19/88)
Not that it has much to do with resolution from orbit, but on the original topic that started the discussion - NSA picking up pictures of Waite from a satellite "over Lebanon" - This could very well have been accomplished by satellite interception of the video signal from one of the low altitude drones that the Israelis regularly fly over the area. These aircraft probably use microwave wide-bandwidth encrypted digital video links to send back real time imagery - no doubt one of NSA's Aquacade class synchronous spy satellites would do quite nicely at picking up the signal. And for at least the smaller drones, omnidirectional transmit antennas which radiate significant signal at the sky are more or less neccessary because mechanically or electronically steered high gain antennas are impractical from a weight, cost, size, and angular coverage standpoint. (since the drones circle and manuever to get the right camera angle and linger over the target the rf link has to work at almost any azimuth and over quite a range of elevations and vehicle attitudes).
jaa@basser.oz (James Ashton) (01/19/88)
In article <1705@faline.bellcore.com> karn@faline.bellcore.com (Phil R. Karn) writes: >You can get extremely high resolution images if you *simultaneously* >photograph the same target from two or more widely separated satellites, >and then *coherently* add the two images with an accuracy on the order >of a small fraction of a wavelength. This is entirely practical at radio >wavelengths (VLBI and SAR being two examples), but at optical >wavelengths? Good luck! It is true that interferometry is more difficult at optical wavelengths but it is certainly not impossible. The Physics department at this university has sucessfully tested a prototype optical telescope which uses two widely spaced plane mirrors feeding into an intricate optical system. The system was able to compensate for atmospheric jitter and recombine the light to obtain very high resolution. Work is currently in progress on a full scale instrument which will have a maximum mirror separation of 240 metres and the best optical resolution of any telescope. Of course stellar images tend to be simpler than spy sat images but I guess that a spy sat, could use a similar technique to obtain very fine resolution. James Ashton.
cgs@umd5.umd.edu (Chris Sylvain) (01/23/88)
In article <2013@frog.UUCP> die@hydra.UUCP (David I. Emery) writes:
+
+ Not that it has much to do with resolution from orbit, but on
+ the original topic that started the discussion - NSA picking up pictures of
+ Waite from a satellite "over Lebanon" - This could very well have been
+ accomplished by satellite interception of the video signal from one of the
+ low altitude drones that the Israelis regularly fly over the area.
+
I'll take back the NaCl now.. Ahh, an elegant solution, and could well be
'dangerously' close to what really happened! That is, this solution sounds
much closer to reality to me than imaging directly from orbit.
--
--==---==---==--
.. All mimsy were the borogoves, ..
ARPA: cgs@umd5.UMD.EDU BITNET: cgs%umd5@umd2
UUCP: ..!uunet!umd5.umd.edu!cgslawrence@bbn.COM (Gabriel Lawrence) (01/26/88)
Knowing precious little about optical diffraction limits or advanced uses of interferometry, you can take what I have to say as having little technical merit. I do know that according to a Natl. Public Radio report I heard last month, Congress has just 'officially' lifted the ban on the 10 meter satellite optical imaging restriction due to American industrial lobbyists complaining about unfair international competition. It has been widely known that satellites giving much greater resolution have been available outside of the U.S. for quite awhile. I believe the new commercial standard is somewhere in the 3 meter range. To the best of my recollection, the commentator mentioned that while the 10 meter range was sufficient to view the individual cars located in the parking lot of the Pentagon, the new range allowed you to see the contents of the cars through the windows. This would seem to me to easily make the case for a _commercial_ satellite being able to spot an individual held hostage if the location of the prison/barracks and his physical characteristics was a known quantity. The same commentator also acknowledged that military spy satellites were, in all likelihood, quite a bit more powerful/accurate/higher in resolution than the best commercial satellites so I'll let you informed usenetter's draw your own conclusions...
bob@its63b.ed.ac.uk (ERCF08 Bob Gray) (01/28/88)
In article <6309@ccv.bbn.COM> lawrence@ccv.bbn.com (Gabriel Lawrence) writes: >of the U.S. for quite awhile. I believe the new commercial standard is >somewhere in the 3 meter range. To the best of my recollection, the >commentator mentioned that while the 10 meter range was sufficient to >view the individual cars located in the parking lot of the Pentagon, the new >range allowed you to see the contents of the cars through the windows. I know that American cars are supposed to be bigger than European cars, But 3 metres is [almost] 10 feet. What size are the the car windows you can resolve things to this scale through? :-> 3 centimetre resolution sounds more reasonable, but this is close to the minimum being mentioned by previous posters. Bob.
bobcoe@cca.CCA.COM (Robert K. Coe) (01/28/88)
In article <2534@cup.portal.com> truett@cup.portal.com writes:
*First you drop a small corner reflector somewhere in the Lebanese highlands.
*Then you put an array of mirrors and CCD detectors into orbit together with a
*small laser. Now, use the reflected signal of the laser from the corner
*reflector to get the optical sensors all phase to withing a fraction of a
*wavelength of light (or infrared, that's even easier). The resulting system
*has an effective appature much bigger than a sigle mirror. I suspect that
*arrays with an apparent aperture of 20-30 meters can be orbited easily.
I recall that at the time the 200-in. Hale (Palomar Mtn.) telescope was built,
it was reported that if it were not for the curvature of the earth and 3000
miles of atmosphere, the telescope has the resolving power to read the date on
a dime in New York.russ@crlt.UUCP (Russ Cage) (02/03/88)
In article <23760@cca.CCA.COM>, bobcoe@cca.UUCP writes: >I recall that at the time the 200-in. Hale (Palomar Mtn.) telescope was built, >it was reported that if it were not for the curvature of the earth and 3000 >miles of atmosphere, the telescope has the resolving power to read the date on >a dime in New York. Nope, can't be. Angular resolution is approximately lambda/D, which at 5 meters and 6000 Angstroms, is about 1.2e-7 radians for the Hale telescope. At 5 million meters distance, this corresponds to an object of .6 meters in size. You couldn't see the dime, much less read the details off it. (But it is a nice story even if not true.) -- The above are my own opinions and figures and I speak for RSI too, so there. (313) 662-4147 Will you come quietly, or must I use earplugs? Russ Cage, Robust Software Inc. crlt may vanish anytime; use this address: rsi@m-net