Re: Spy Satellites

[brian whatcott <inet@INTELLISYS.NET>]

At 16:48 12/12/99 -0700, Shawn Knudsen wrote:
> I advise a youth group for my church and while talking with
> a couple of teenage boys the subject of spy satelites came
> up.  One of the boys mentioned that he saw in a Scholastic
> News Magazine that the U.S. has spy satelites that can read
> (resolve) the date on a dime from space.  I scoffed at that
> remark but told the boys that I would look into the matter.
> Does anyone on this list have solid information that I could
> take back to these curious boys?


Your first recourse is to physics. The somewhat arbitrary
Rayleigh criterion is lambda/(objective diameter) for limiting
angular resolution. (As you know, this is the measure that
places an adjacent Fraunhofer max in the first null of a given
 line feature so that they are quite merged.)

   If one takes an arbitrary altitude of 1000 km, and the
dime size date as 2 mm (numbers picked out of the air)
the angular resolution needed would exceed
2E-3/1E6 = 2E-9 radians.
For a blue image at 400 nanometers (4E-7) the aperture implied is
4E-7/2E-9 = 2E2 or 200 meters.

So you can be reasonably sure that direct dimeshots from 1000 km
are not in use. But as always, with theoretical constructs in
Physics, you need a cautious, even incredulous approach.

 If radio astronomers wish to resolve details of star systems other
 than the Sun, they first build enormously expensive and unwieldy
dishes of hundreds of meters diameter fixed in natural bowls,
then step on to the same technique used in optical interferometry
and funnel in data from two widely seperated points on Earth -
like across an Earth diameter, then finally trap datasets from
orbital extremes about six months apart
 (with a little fancy footwork, it's true)

 In view of this open shelf technology that's widely known, you
would be surprised (at least I would) if the spy geeks had not
composed successive Earth views from 200 meters or greater seperations.

And so I can conceive that (given numerous constraints of practicality)
such a dimeshot feat could be accomplished.
I suspect that the start point might be a low flying airplane -
even a regular commercial flight with a little 'black' cargo, in
order to ease the physical barriers.

Some useful URLs

 http://www.terraserver.com/
 This is where you find a shot of your own house.

 http://www.sat.dundee.ac.uk/pdusfaq.html
  This is an overview of satellite topics.

 http://helios.cnig.pt/~mario/aperture/table-vhsr.html
This is a Portuguese table of high res satellite (commercial)
resources in the 1 meter up range.



brian whatcott <inet@intellisys.net>
Altus OK