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U.S. Allows Sale of Half-Meter Satellite Photos
kreinsch writes "According to an article in today's Washington Post, the U.S. Government quietly granted a license to Space Imaging, Inc. two weeks ago to allow the sale of satellite photos with half-meter resolution, as compared to the current one-meter resolution available." As the article points out, this effectively ends the monopoly the spy agencies had on this high-end imagery.
The resolving power of any telescope can be calculated by the formula Theta = 115.8/D, .05 seconds of arc.
.5 seconds of arc in most cases, although i'm not sure this is as critical for taking pictures of terrestrial objects as it is for astronomical ones. I'm sure someone will correct me if I'm wrong on any of this.
.^
where Theta is angualr resolution expressed in seconds of arc, and D is the diameter of the objective of the telescope/camera, which is what a spy satellite is. Now, the Hubble telescope has an objective of 2.4 meters, which is probably pretty close to the maximum diameter that will fit inside current launch vehicles, so the NRO satellites can't be much bigger than this. so that works out to around
to figure out actual size from angular size and distance use the formula
angular size(in degrees) = 57.3*actual size / distance
which works out to right around 10 centimeters, if i've done the math right. so unless they have multi-segment meirros for their satellites or some other unknown capability that is about hte theoeretical limit of their resolution. Interestingly though, atmospheric turbulence (what astronomers refer to as 'seeing') limits actual performance to
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The limit of resolution on a camera is the diffraction limit. That's the point where the wavelength of the light is larger than the angular distance of the object you're viewing. The formula is (angular resolution) = 1.22 * (wavelength)/(telescope diameter) in radians. To convert radians to length, multiply by the distance from your target (which is a good approximation at large distances.)
:)
Most low orbit satellites are about 700 km up. Visible light is around 300-600 nanometers...call it 400 nm.
So the theoretical minimum telescope needed to have a one-centimeter resolution on the ground would be diameter = 1.22*400e-9*700e3/1e-2, or about 34 meters across. For reference, the Hubble's mirror is 2.4 meters diameter.
So it's possible. Just not bloody likely given current limits on what we can build in space.
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A company should not need a license to sell photographs that it took with a satellite that it designed, built, and owns. The US government does not own the entire planet.
I don't think it's *that* detailed, guys.
Here's one of the early "meter" images.
Sure, you can see the road, and big buildings, but you can't really identify a person...
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Not true. To get a 5-mm-resolution 500x500 image from a height of 300 km, you'd have to have a field of view of 0.03 arc seconds. For comparison, atmospheric turbulence normally makes it pointless to try to make telescopic images with fields of view of less than about 1000 arc seconds. That's why the Hubble Space Telescope got built -- to do astronomy without that nasty atmosphere in the way.
Nevertheless, whith that resolution you can get enough information about goods productions, plantations, petrol explorations, building surfaces, electric/energy installations, radio installations, satellital antennas (which are forbidden in some countries), and so on.
If someone's afraid of the secret police knocking on his door about his satellite antenna, I think he has more to worry about from neighborhood informants than from space-based imaging. He could always put his satellite antenna under a blanket or something.
I am not afraid of my privacity, I don't have anything important to hide, but privacity, in the sense that is technological expensive to peep you, is a fundamental value in most of "western minds"*.
You need to be realistic about the privacy you expect. When you do something outdoors, you don't normally have an expectation of privacy.
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Does this bug other people as much as it does me?
Half-meter resolution doesn't mean that it "clearly shows objects as small as 19 inches!" This is quite misleading.
It simply means that an object of 19 inches can register in the image- and "register" simply means that a dot on the image might be brighter or darker depending on the overall colour of the object.
"To clearly show an object" implies that you'll be able to identify it. Some might even think that you'd be able to see features and details of the object. Nothing could be further from the truth.
This is better:
At half-meter resolution... forestry officials can count trees, and urban planners can view streetscapes, even discerning manhole covers.
But I wish they had put that at the top of the article, not at the end of the last paragraph!
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Of course, the optical resolution doesn't really matter, they can always zoom in the picture to get more detail. You know, like they do on The X-Files.
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