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From: MEDELMA@CMS.CC.WAYNE.EDU (Michael Edelman)
Newsgroups: sci.military
Subject: OTH radar
Message-ID: <1991Mar4.210921.7716@cbnews.att.com>
Date: 4 Mar 91 21:09:21 GMT
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From:         Michael Edelman <MEDELMA@CMS.CC.WAYNE.EDU>
Despite the claims of the Austrailian team, Over-the-horizon radar is
not the stealth killer they claim it is. I first heard this claim
shortly after the F-117 was unveiled, at which time it was claimed the
the Austrailian radar wopuld defeat stealth by means of its low
frequency.

First, OTH is not new. The Sioviets have had a operational OTH system
(actually, many systems) since at least the early 70s. This system is
widely known to amateur radio hobbyists as the "Russian Woodpecker"
because of its sound. Owing to the need for ionospheric refraction, the
radar is low in frequency; the Woodpecker varies its frequency to take
advantage of the MUF (Maximum Usable Frequency). For the last few years,
I have mainly heard the Woodpecker in the vicinity of 21 MHz. This implies
much lower resolution- 21 MHz corresponds to about 15 meters wavelength.
As the sunspot numbers decline (We're on the downside of the 11-year
cycle) it will be necessary to use lower frequencies, and at some times
the highest useable frequencies will be in the 80-meter range...and below.

Second, because of the great distances, the repetition rate for OTH
radars is much lower than for short distance radars. Again, lower resolution.

Third, and I mentioned this earlier, with low frequencies you don't get as
good a reflection; the anti-stealth theory is to try to get a return by
exciting the entire structure to resonance. This, my radar-designing friend
tells me, is an old idea. It also fails because an aircraft structure is
extremely broadbanded as regards its resonance. Consequently it is a
very low-Q resonator, and provides a very weak return. Add to that the
attenuation effects of the aircraft's conductive skin, and you have an
extremely weak return.

The Soviet OTH radar is designed to serve as part of an early warning
system. It can tell you that there are *approximately* n aircraft (and
possibly cruise missiles) at a distance of roughly x km at a bearing of
y, plus or minus. It's intended to tell you that 200 B-52s have crossed
your border; it would be completely ineffective at locating F-117s or B2s
for intercept.
   --mike edelman   medelma@cms.cc.wayne.edu