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From: budden@trout.nosc.mil (Rex A. Buddenberg)
Newsgroups: sci.military
Subject: Re: OTH radar
Message-ID: <1991Mar6.041854.24944@cbnews.att.com>
Date: 6 Mar 91 04:18:54 GMT
References: <1991Mar4.210921.7716@cbnews.att.com>
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From: budden@trout.nosc.mil (Rex A. Buddenberg)

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.  
  So while you get some good warning capabilities (you can't fly 'under'
OTH radar), that's about all you can get.

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.
  In practical terms, aircraft are generally easy enough to paint,
but the missile warning radars tend to bin-sort the contacts and only
really pay attention to those with combat aircraft and cruise missile
types of speeds.  Ships, on the other hand, are pretty tough...if
he's going fast enough to show up, he's not likely to be a dope
smuggler unless it's the cigarette boat on the last leg.
  The guys at SRI have their test bed out by Los Banos; during one
of their tests, they claimed to be eyeballing rush hour traffic
in Houston.

To the F-117.  The stealth characteristics tend to minimize the
radar return, but this can be countered to a degree -- almost all
HF radar designs are bi- or multi-static.  And OTH radars have
very sensitive receivers anyway because of their nature -- picking
up a backscatter that refracts off the ionosphere in both directions
is not a trivial exercise.  But the key is the ability to detect
and correlate the doppler shift, which is a characteristic that
is unaffected by stealth technology.

Rex Buddenberg