Xref: utzoo sci.space:4143 sci.crypt:725
Path: utzoo!linus!necntc!ames!umd5!eneevax!daveb
From: daveb@eneevax.UUCP (David Bengtson)
Newsgroups: sci.space,sci.crypt
Subject: Re: satellites
Keywords: resolution
Message-ID: <1175@eneevax.UUCP>
Date: 12 Jan 88 17:11:48 GMT
References: <873@uop.edu> <2166@umd5.umd.edu> <4910@well.UUCP> <1952@netsys.UUCP> <2209@bloom-beacon.MIT.EDU> <2604@calmasd.GE.COM>
Reply-To: daveb@eneevax.umd.edu.UUCP (David Bengtson)
Organization: Elec. Eng. Dept., U of Maryland, College Park, MD 20742
Lines: 46

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  
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