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From: chidsey@smoke.brl.mil (Irving Chidsey)
Newsgroups: sci.military
Subject: Re: OTH radar
Message-ID: <1991Mar14.014148.16999@cbnews.att.com>
Date: 14 Mar 91 01:41:48 GMT
References: <1991Mar4.210921.7716@cbnews.att.com> <1991Mar6.041854.24944@cbnews.att.com>
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Organization: U.S. Army Ballistic Research Laboratory, APG, MD.
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From: Irving Chidsey <chidsey@smoke.brl.mil>

In article <1991Mar6.041854.24944@cbnews.att.com> budden@trout.nosc.mil (Rex A. Buddenberg) writes:
<
<A few more points about OTH radar.
<
<The wavelength numbers only tell you resolution; they don't
<translate into ranging errors.  Since all HF radars use ionospheric
<refraction, range measurement is critically dependent on the
<virtual height of the reflecting layer of the ionosphere (the ionosphere
<actually refracts, rather than reflects, so in addition to 'height',
<you need to know something about density too).  This means that
<real time sounders are needed to calibrate things, or a benchmark
<such as known traffic around a known airport.  
<
<Second.  The signal processing is different than in conventional radars. 
<In an OTH arrangement, everything reflects -- trees, ocean, mountains,
<you name it.  The key to making OTH radar work is the doppler signal
<processing -- the doppler shift is what allows you to sort out the
<moving targets and drop the clutter out of the picture.  So the
<faster a target is moving toward or away from the radar antennae,
<the more likely he is to paint.  
<  Conversely, the slower the target you want to find, the more
<fancy signal processing you'll need.

	Whole bunch deleted.
	The ionosphere is, unfortunately, not a benign, static refractor
like a lens or a prism.  It is not even in equilibrium.  To be exact,
or at least, more exact, it is in a sort of dynamic equilibrium.  The
free electron density is maintained by a very dynamic balance between
source terms, the neutral gases are ionized by Ultra Violet light from
the sun; and sink terms, the electrons eventualy recombine with positive
ions or attach to neutral molecules and become negative ions.  The negative
ions don't last long during the day, they are vulnerable to visible and
near Infra Red light.  The negative ions last fairly long at night, and
are stripped of their electrons by the first light of dawn.  The 'reflection
level' may drop from a night-time value of 250 km, to a daytime value of
100 km in a few minutes.  

	When the 'reflection' layer drops much below 100 km, absorption 
becomes a problem.  The height at which 'reflection', really refraction, 
occurrs is frequency dependent; radio amateurs and commercial radio 
communication companies are assigned multiple frequencies so that they 
can usualy find one or more that fits between the highest frequency that 
will propagate over the horizon and the lowest frequency that will not be 
absorbed.  Sometimes the usable band disappears.

	The solar wind is both ionized and gusty, and when it hits the
edge of the earths magneto-sphere, the interaction is quite dynamic.
The magneto-pause moves in and out as the solar wind pressure pushes on 
it.  The magneto-pause, in turn, pushes on the earths magnetic field.
The changes in the magnetic field are considerable, even in the lower
ionosphere, which moves around a surprising ammount  in reaction to
the solar wind.  The ionosphere is also lumpy, at whatever scale you 
choose to measure it, and there are very strong winds blowing at iono-
spheric heights.  All of this huffing and puffing and heavy breathing
affects the radar path length and makes separating moving targets from
a stationary clutter difficult.  The clutter refuses to sit still.


								Irv

-- 
I do not have signature authority.  I am not authorized to sign anything.
I am not authorized to commit the BRL, the DOA, the DOD, or the US Government
to anything, not even by implication.  The do not tell me what their policy is.
			Irving L. Chidsey  <chidsey@brl.mil>