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Comment by lordgilman | Hacker News Reader

This has come up in the past on HN. As I understand it the newspaper story is bull. As for advancements in technology the answer is likely no - producing that technology requires an entire toolchain/industry that the NSA is unlikely to replicate with its size. The only shot the NSA has at pulling ahead of us is with entirely mathematical things like crypto (which they did at least in the 70s with differential cryptanalysis). With math you can simply hire a bunch of smart people and throw them in a room together which is much less capital intensive than the massive, fundamental research needed to advance technology ahead of the industry.

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>This has come up in the past on HN. As I understand it the newspaper story is bull.

Pretty much. The same effect that causes stars to twinkle limits the resolution of space-based spy satellites imagery of the ground.

http://en.wikipedia.org/wiki/Astronomical_seeing

That's for distant stars though, isn't it? I mean, just with Google Maps you can see the mirrors on a car. Newsprint isn't that much of a step up.

Google Maps uses aerial photography from planes for the high-resolution layers of their maps, not satellite imagery.

Huh, interesting! I never knew that. So then I'll need to ask my father about what he was told again. Maybe something got misinterpreted along the way.

It probably was that you can see newsprint, not read it.

All of the detailed views in Google Maps are actually aerial photography rather than satellite images. Much easier to get that kind of detail from 1500ft.

While it's true that the stars are very distant, the only atmosphere that the light passes through is same atmosphere the satellites have to peer through (resulting in the same amount of distortion)

Hm, but with angles, distance matters to right? Bending light 10 degrees will mean a much bigger difference light-years away than a couple hundred miles away. That said, I could easily be missing something...

The light isn't being bent light-years away. It is being bent at the beginning of the atmosphere, resulting in the same degree of bending as the satellite has to deal with.

Not just "seeing" (atmospheric turbulence), but also diffraction.

> As I understand it the newspaper story is bull

Could you clarify what you mean by this?

And yeah, I should have been more clear in my original question; I was lumping mathematical advances under "technology".

The resolution of a lens at a given wavelength is determined by its diameter (Rayleigh function). We know how big the launch vehicles are, so we can estimate the largest size a spy satellite's mirror could be, and we can use that to compute the maximum resolution a satellite could have; it turns out to be something around 5-10 cm. In order to resolve a newspaper from near-earth orbit, you'd need a lens bigger than the ISS, and if such an object existed it would be one of the brightest objects in the sky.

Would it be possible to improve the resolution of optical imagery using a synthetic aperture?

Beyond my knowledge, sorry.

Yes, much finer optical resolution is possible if you use multiple instruments separated by a controlled interval.

In radioastronomy, there are arrays of dishes that use this technique. There are good pictures in the Wikipedia article. [0]

One military implementation was a connected three-satellite constellation, built by the US Navy for scanning the surface of the world's oceans. [1]

But those tools work in (relatively) long wavelengths.

I have never heard of free-flight multi-aperature interferometry that works in optical or near-optical wavelengths.

[0] "Very-long-baseline interferometry" https://en.wikipedia.org/wiki/Very-long-baseline_interferome...

[0] "How VLBI Works" https://en.wikipedia.org/wiki/Very-long-baseline_interferome...

[1] "White Cloud | PARCAE | NOSS" https://en.wikipedia.org/wiki/Naval_Ocean_Surveillance_Syste...

Yes, aperture synthesis is possible with optical wavelengths. I don't know how practical it would be to actually use with spy satellites, or how much of an improvement they could see with it.

Keep in mind that there are a lot of people that track satellites, even spy satellites, as a hobby. I haven't heard of anyone discovering two or more satellites orbiting in the sort of tight formation you would expect would be required for this.

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