[amsat-bb] Re: Malaysian airliner puzzle
antonio at qti.qualcomm.com
Mon Mar 17 18:08:44 PDT 2014
At 12:40 PM 3/17/2014, Bruce wrote:
>i am sure that the u.s. as well as other countries have spy
>satellites that have been taking pictures 24/7 with resolution that
>can see a person on the ground. that being said, i would further
>venture to say that none of them will want to show pictures of the
>jet as it would give away their secret ability.
Hey, good to see a post that's on topic. Satellites!
Actually, the capability is not very secret. Its pretty much out in
the open. Although every nation keeps the details secret, but you
don't need details to understand what kind of a picture is
possible. Physics will tell you that.
You can calculate the resolution of a spy satellite. Its just
physics. You know the diameter of the aperture (lens or
mirror). You know that because dimensions for many classified
objects have been published. Also you know the diameter of the space
shuttle's payload bay. You know the wavelength of light. Simple
formula gives you the resolving power as an angle. To turn that into
the size of an object you need to know a distance. You can look up
the orbits of many classified satellites, so know how far away they
are. But because you get more resolution by being closer, you know
the best pictures will come from low orbit, and here the limit is the
Earth's atmosphere. No need to know classified orbit data. Just
presume there are some spy satellites in low but stable orbits,
perhaps 300 to 500 km.
So, yes, they should be able to see something smaller than a person,
but here's the kicker... They can't do that everywhere at
once. Because they are in orbit close to the earth, they can only
see stuff that's under them. This is a tiny fraction of the
earth. Also, within that area, you have to tell them where to
point. A simple number-of-pixels thing. Looking at the whole
footprint at max resolution would be an enormous # of pixels. We
would have to guess the limit, but its not hard to guess that the
maximum image is way smaller than the footprint. This is great for
taking pictures of your opponent's military installations, and maybe
even seeing what assets there come and go over time, but it is
unlikely to be of help in tracking an airplane that unexpectedly
flies some unknown path to some unknown location.
Also, the plane flew at night, in the dark.
I'm sure the big boys are thinking about new capabilities as we
speak, and writing them into next year's budget!
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