MEDELMA@CMS.CC.WAYNE.EDU (Michael Edelman) (03/05/91)
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
budden@trout.nosc.mil (Rex A. Buddenberg) (03/06/91)
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
chidsey@smoke.brl.mil (Irving Chidsey) (03/14/91)
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>