Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!att!cbnews!military
From: nadel@aerospace.aero.org (Miriam H. Nadel)
Newsgroups: sci.military
Subject: Re: Infra-red detection
Message-ID: <12569@cbnews.ATT.COM>
Date: 23 Dec 89 06:05:17 GMT
References: <12535@cbnews.ATT.COM>
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From: nadel@aerospace.aero.org (Miriam H. Nadel)

In article <12535@cbnews.ATT.COM> GA.CJJ@Forsythe.Stanford.EDU (Clifford Johnson) writes:
>
>While not an expert, I believe the distance of the sensor from the
>source is also important, especially because the atmosphere absorbs
>radiation except at certain wavelengths.  Thus, viewed
>from space, an oil fire has the same infra-red radiation pattern
>as a missile exhaust plume.  Given this look-alike phenomenum,
>the term "signature" is very misleading.
>
The atmosphere is more important as a source of refraction.  The observed
position of an IR source depends, in part, on how much atmosphere you're
looking through.

>Are satellite sensors sufficiently sensitive to detect residual
>differences in spectroscopic distributions as to reliably
>differentiate between various hot sources?  I don't believe so.
>Can anyone enlighten me?

Generally satellite sensors are relatively broad band.  Differentiation between
various IR sources has more to do with their pattern of motion than with
their spectroscopic distribution.  To use your example, an oil fire will 
be in a fixed location with respect to the earth, while a missile exhaust
plume will have a characteristic motion, depending on the type of missile.
One can detect relatively small changes in position, depending on the
exact configuration of the sensor focal plane.  It isn't unusual to have
several hundred separate sensor cells.

By the way, the biggest source of noise is probably IR radiation from stars.
A common approach is to use a catalog of known star positions to remove
them from the data stream.  This has more to do with reducing processing
load than with the possibility of false detections, however.

Miriam Nadel
-- 
One of the 60% of Americans who do not spend a lot of time on their personal
appearance.

nadel@aerospace.aero.org