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...