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From: eugene@eos.UUCP (Eugene Miya)
Newsgroups: sci.misc,comp.graphics
Subject: Re: satellite pictures
Message-ID: <5971@eos.UUCP>
Date: 12 Jan 90 18:11:17 GMT
References: <1523@castle.ed.ac.uk> <556@erm.oz> <7113@lindy.Stanford.EDU>
Reply-To: eugene@eos.UUCP (Eugene Miya)
Followup-To: sci.misc
Organization: NASA Ames Research Center, Calif.
Lines: 60

In article <7113@lindy.Stanford.EDU> rick@hanauma.UUCP (Richard Ottolini) writes:
>In article <556@erm.oz> sns@erm.oz (Stuart Nixon) writes:
>>In article <1523@castle.ed.ac.uk>, nick@lfcs.ed.ac.uk (Nick Rothwell) writes:
>>> Recently I heard of satellite pictures that revealed buried ruins
>>> in the Sahara desert. I have a simple question. Is this true ?
>
>Radar can see several meters below below the surface under optimal conditions
>depending on wavelength, dryness, and soil grain size.
>I've heard some sites have been imaged directly by radar rather than by
>secondary visual effects, and I believe the Sahara was one of these
>successful places.

Well, er,... not quite.

We are certainly seeing substructure in some images, but.... there is
a lot we don't know or understand.

How imaging radar works is dependent on lots of things: wavelength
(which affect whether or not you can see below a surface be it ground
or a forest canopy), a set of concepts like surface roughness and surface
length, the dielectric constant of the soil (the presence of water
which affect this and the wavelength selected), the power of the system, the
structure of the chirp sent out, etc.  But it has limits: radar is like
working in the dark at night, you only see what you shine your light on.
Shadows take on a different meaning.  Images take on a quality of false
3-D.  This causes problems such as range and azimuth ambiguity.
Radar is not a miracle sensing system, but somewhat complex and needs a
lot of reasearch to understand how to use it and how it works.

Wavelength is important because one works in the region where things
like raindrops can make a difference.  Too big and you see them,
too small, you don't.  Same goes for soil particle size, etc.

Radar works by taking a small antenna and moving it linearly along a track
and synthesizing an image by integrating multiple views (looks) along
that track.  Motion is an integral part of constructing the image and no
two scan lines are taken from the same point.

One study on penetration of dense forest canopy was thought be a subsurface
example.  It turns out that was false, there were subtle variations on
vegetation not visible to eyes.  The human eye is at times an unreliable
means of information transmital.  Many people are unable to distinguish
a radar image some say a black and white negative enlargement.  It takes
a bit of training to recognize features thats why images have false depth

I worked on the Seasat-1 SAR, the preliminary studies for what is
now the Magellan spacecraft.  Along side of me were people working on the
SIR (Shuttle Imaging Radar) and various airborne radars.
Simple little description of imaging radar systems.  To learn more,
the material is complex enough you should take a class (or pay me
to explain it to you).

Another gross generalization from

--eugene miya, NASA Ames Research Center, eugene@aurora.arc.nasa.gov
  resident cynic at the Rock of Ages Home for Retired Hackers:
  "You trust the `reply' command with all those different mailers out there?"
  "If my mail does not reach you, please accept my apology."
  {ncar,decwrl,hplabs,uunet}!ames!eugene
  Do you expect anything BUT generalizations on the net?