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)

> 
> 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
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W.A.S.T.E.: Aptos, CA  | black markets, collapse of governments.
Higher Power: 2^859433 | Public Key: PGP and MailSafe available.
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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)

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