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From: anderson@erim.org (Rod Anderson)
Newsgroups: sci.military
Subject: Re: Firing on FLIR Contacts
Message-ID: <1991Jun4.040323.9358@amd.com>
Date: 3 Jun 91 23:35:33 GMT
Sender: military@amd.com
Organization: Advanced Micro Devices, Inc.
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Approved: military@amd.com


From: anderson@erim.org (Rod Anderson)


lafford@serum.kodak.com (Rick Lafford) writes:
>Anyone have data indicating how much FLIR sensors are degraded
>by various atmospheric conditions?

	I don't have any data on actual Desert Storm atmospheric conditions, 
but perhaps the following general comments apply.

	Most military FLIRs operate in the long wave infrared, from 8 to 12
micrometers.  Water vapor is the dominant absorber in this band, although 
carbon dioxide and other gases contribute some.  To first order, FLIR 
performance is inversely related to absolute humidity, the total amount
of the dominant absorber along the line of sight.  Given that this 
amount was probably very small under DS conditions, good propagation would
result.
	Other effects could be important, however.  Scattering by 
atmospheric aerosols (dust, sand, salt, etc.)  can be a significant source
of extinction.  The degree of extinction is a function of the optical
properties of the aerosol, its size distribution, shape, and extincted 
wavelength of interest.  For small particles (less than .1 wavelength) the
scattering is proportional to the inverse fourth power of the wavelength.
(The familiar Rayleigh scattering case.)  Wind-blown sand (of say, 1-4um)
which would obscure visual detection at 0.5 um would be much less of a 
problem in the 8-12um band.  

	Another effect of atmospherics on performance is the target source
signature.  For example, high cloud cover would affect the solar loading
on a vehicle, and thus its thermal signature.  Note that this is 
independent of atmospheric absorption along the line of sight.  Clouds
along the line of sight are another matter.  Unless they are VERY thin,
don't count on seeing through them, in spite of the 1/4th dependence.

	A comparison of weather data from DS with Central Europe would be
very interesting. Lack of all-weather has always been a serious criticism of
FLIRs.  I wonder how Desert Storm will change the conventional wisdom.

-Rod Anderson