Xref: utzoo sci.space:4177 sci.crypt:735
Path: utzoo!linus!husc6!hao!gatech!rutgers!bellcore!faline!karn
From: karn@faline.bellcore.com (Phil R. Karn)
Newsgroups: sci.space,sci.crypt
Subject: Re: satellites
Summary: synthetic aperture radars
Message-ID: <1705@faline.bellcore.com>
Date: 12 Jan 88 19:20:10 GMT
References: <873@uop.edu> <2166@umd5.umd.edu> <4910@well.UUCP> <1952@netsys.UUCP> <7169@apple.UUCP>
Organization: Bell Communications Research, Inc
Lines: 19

> In fact, it is possible to get better than the diffraction limit, as synthetic
> aperature radars have shown. Take several images from different points,(from
> a satelite, easily done by just taking a few sequential pictures, since the
> stelite is moving) and image-process your little brains out.

Aperture synthesis doesn't "get better than the diffraction limit", it
decreases the diffraction limit by building a system with a very large
effective aperture. It's like having a telescope with a very large
virtual mirror where only small portions of the signal gathering area is
actually "filled in".

You can get extremely high resolution images if you *simultaneously*
photograph the same target from two or more widely separated satellites,
and then *coherently* add the two images with an accuracy on the order
of a small fraction of a wavelength. This is entirely practical at radio
wavelengths (VLBI and SAR being two examples), but at optical
wavelengths? Good luck!

Phil