Engineers Devise Invisibility Shield

GerritHoll points out an article in Nature according to which "researchers at the University of Pennsylvania 'say that a "plasmonic cover" could render objects "nearly invisible to an observer.' Earlier attempts at invisibility worked by colouring a screen to match its background, like a chameleon. The described technique is new, because it works by the concept of reducing light scattering. It is not a 'magic cloak,' however, because it will not work for the full range of visible light and needs to be adjusted precisely for the shape of the object. However, the concept could find an application in stealth technology."

Comments

Indeed, it's pretty far from advertised...

[Vthornheart]

Those who read the article until the end will note that they save the kicker for the very end:

This technology would only work for microscopic objects (as they must be the same size as the wavelength of light hitting it), and only a single wavelength. So in other words, for you to get a nice, new cloak of invisibility you'll need to be microscopic in size and constantly in environments with only one wavelength of visible light hitting you. =)

Well, back to the drawing board.

Re:Indeed, it's pretty far from advertised...

[digitalchinky]

To say that a stealth aircraft is invisible to RADAR is a tad misleading. There are numerous ways to exploit the nature of the technology - current stealth for the most part relies on surfaces that reflect much of the energy anywhere but directly back to the emitter.

A simple example. Radar 'transmitter' and 'receiving' system located at an offset from each other. A distance measuring a few hundred meters to a number of kilometers. Scatter from stealth aircraft is easily picked up. (I speak from experience here) RADAR absorbing material is not very sponge like, just drops the return by a couple of DB for typical RADAR/EW emitters (400MHz-6GHz ish), nothing huge. Stealth is not really as complicated as 'they' say.

Stealth can also be picked up by most primary RADAR emitters (Air traffic control for example), it's more likely to be 'filtered' off the PPI, but can still be seen if the operator desires. Think small flock of birds. The kind of crud that is marginal and usually ignored.

Until non-microwave-reflective material can be used to build the entire aircraft, it will only ever work against low-tech level targets. By non-reflective I mean 'not reflect anything between 0khz-100GHz'

Re:Indeed, it's pretty far from advertised...

[X0563511]

http://www.its.bldrdoc.gov/pub/oprad/main.pdf

2.2 mb PDF


Look on page 11 (numbered page, not the PDF's page) at the bottom, at TPS-43E.

The placement of those diagrams is annoying, that sentence isn't finished untill page 16. Here's what it says:

The TPS-43E is an Air Force 3-D, stacked-beam, surveillance radar and is air and ground transportable. The range of the TPS-43E is 481 kilometers (260 nmi), transmitting 4 MW using a linear beam twystron. The transmitter control can select any one of 16 frequencies in the range from 2900 to 3100 MHz. It can operate on six different PRR's; the average PRR is 250 Hz using a 6.5 uS pulse.

I boldfaced the really important parts. That sounds like a LOT of energy. It all depends on the class of radar. When i said beam above, I meant that they narrow their sweep path and shorten the period.

Re:Indeed, it's pretty far from advertised...

[josecanuc]

If the pulse is 6.5uS long and occurs at a rate of 250 Hz, then the total duration that the transmitter is on is 0.001625 seconds for every 1 second of real time. That's a duty cycle of less than 1%.

That would mean the average power (average power would dictate heating effects) is 6500W (4MW * 0.1625%), which is roughly equivalent to 4 decent microwave ovens.

Now take that amount of power and point it at an aircraft 200km away (well within the range of 481km). Without doing the calculation to find out the exact value of the intensity at 200km, I will just say that the intensity of the radar beam at 200km will be 0.000025 times smaller than at 1km. And at 1km it would be 0.000001 times smaller than at 1m, which is comparable to the range of a household microwave. So you want to stack 4 or 5 microwave ovens together, collate their radiators so that all of the energy is radiating in one general beam, and try to heat up an aircraft far away...

In short, radars do not cause significant heating on aircraft, even if the aircraft absorbs every photon that hits it. Radars do not run at 100% duty cycle, or even at 5% duty cycle. When you're generating 4 MW at those frequencies you make a lot of heat in the resonator/amplifier (klystron, twystron, etc.), so you can't just keep it on all the time or it would melt.

Re:Indeed, it's pretty far from advertised...

[harrkev]

I was recently trying to design a space ship for an RTS mod that was designed similarly to the stealth fighter. I realized that if you didn't know the direction of the incoming radar beam (which you wouldn't in 3D combat) it was going to be nearly impossible to design.

Which was exactly why it took a computer to be able to design stealth aircraft. The F-117 is an ugly faceting monstrosity, but it is faceted because at the time that it was designed, the computers were not powerful enough to do continuous surfaces. But advancing computing power has enabled the design of the F-22 and F-35.

But one key of stealth design is to avoid any 90-degree angles -- between ANY two surfaces. If you accomplish this, then you have done much to reduce the radar signature.

Re:Indeed, it's pretty far from advertised...

[josecanuc]

Yes, you're right. I don't know the exact calculation, but if we do something like assume that the antenna (dish) gain is something like 80db (equivalent to 25X effective increase power, caused by directional beam, a somewhat unrealistically high gain figure...), you just insert a (*25) factor into the chain and end up not much better off.

I found a free space loss calculator and put in 3 GHz and 100km and it came up with about -142dB, so let's play with that. You've got an +80db gain antenna and -142dB loss due to distance, which totals up to a system loss of -62dB, or -20.67 times loss.

Assuming the aircraft at 100km absorbs the whole beamwidth's worth of energy, that amounts to (6500W / 20.67)=315W. So the total heating of an aircraft illuminated by this particular radar, assuming total absorption of the beam by the aircraft, an unreasonably high 80dB gain of the dish antenna, and the airplane being the target of the beam for an extended period of time would be like putting 3 100W lightbulbs near it.

Re:Invisibility cloaking

[JVert]

The photograph was taken through a viewfinder that uses a combination of moving images taken behind the wearer to give a transparent effect.

ergo, it doesn't do a damn thing this is just photoshop of an "artist conception".

been investigated a bit before

[mnemonic_]

U.S. Air Force scientists looked into generating a field of plasma around an aircraft to reduce aerodynamic drag. One unexpected effect was a reduction of RCS (radar cross section, a rough measure of radar visibility), though to my knowledge the research has not been pursued (it probably continues in classified state, just like the plasma toroid ABM system 7 years ago...). Of course, this is EM radiation in the radio portin of the specturm, not optical.

Russian electrodynamicists are also infamously known for proposing "plasma stealth" devices, which have yet to be demonstrated veritably well. Every few months something pops up about how they've solved high power requirements, reduced weight of the devices, eliminated interferce with the aircraft's EM devices (radar and comm/nav, which critical to everything) and problem Y. And then, you see nothing of it in any journal or trade publication. Just claims, and it seems, nothing more.

Notably, plasma radar stealth has an opposite effect of the optical stealth. The aircraft would glow like a lightbulb, and leave a trail of glowing plasma in its wake. Also notably, aircraft at high hypersonic speeds induce a local plasma air environment, due to the tremendous energy of the aerodynamics.

Re:Sounds like someone's been tokin' the hookah

[MooseByte]

"Could a small-sized object be hidden from radar by this "invisibility" shield?"

Millimeters to centimeters typical for radar. If you're looking to hide a large object, as in plane/ship length, you need to get into HF radio wavelengths (10-160m).

So you could hide it from... ham radio operators. On a single section of one band. Yeah, the Romulans ain't sweatin' this one. :-)

Research abstract

[FleaPlus]

Here's an obligatory link to the pre-print research paper and the abstract:

http://arxiv.org/abs/cond-mat/0502336

Achieving transparency with plasmonic coatings

Andrea Alu, Nader Engheta

The possibility of using plasmonic covers to drastically reduce the total scattering cross section of spherical and cylindrical objects is discussed. While it is intuitively expected that increasing the physical size of an object may lead to an increase in its overall scattering cross section, here we see how a proper design of these lossless metamaterial covers near their plasma resonance may induce a dramatic drop in the scattering cross section, making the object nearly invisible to an observer, a phenomenon with obvious applications for low observability and non invasive probe design. Physical insights into this phenomenon and some numerical results are provided.

Re:Invisibility cloaking

[mkro]

A better link would be http://projects.star.t.u-tokyo.ac.jp/projects/MEDI A/xv/oc.html Includes some show-off videos.

500 Nanometer Romulan Warbirds, perhaps...

[cfalcon]

"And crucially, the effect only works when the wavelength of the light being scattered is roughly the same size as the object."

Visible light is around 400nm (violet) to 800nm (red). So, this is only effective for sufficiently tiny battleships.

Re:What about other wavelengths?

[digitalchinky]

RADAR transmissions are most common at the 9GHz band - india band navigation (boating/shipping) - the bane of many EW's existance.

That said, the transmission can be on any part of the entire spectrum. You can find RADAR on most any frequency. Including light. It's all about duty cycle, dopplar, PRF etc... EW radars modulate the pulse so you get high target resolution and your transmitter pumps out much more effective power. millimetric band radar is mostly only useful in fire control systems - short range.

Frequency depends on what the RADAR needs to do.

Re:Invisibility cloaking

[Feztaa]

Watch this demo and maybe you'll understand. ;)

Re:"precise wavelength of most radar waves"

[digitalchinky]

It's not easily possible to hide from any good EW system. They use multiple frequencies, pulse modulation, frequency hopping, staggered pulses, and a hundred other techniques that provide some really fine grained resolution - right out to the MTUR.

You also find RADAR on HF, it's annoying if your day job is to actually listen to the static, sounds a bit like a high pitched fart, transmissions are normally short duration though - less than 30 seconds then the frequency is changed - don't hear it again for a couple of minutes/hours.

Re:Everybody knows

[Anonymous Coward]

Plasmons are not science fiction or a hoax. They are electron waves in the surface of conducting materials. They also allow light to pass through holes very much smaller than the wavelength by converting the light to plasmons and back again on the other side. This was previously thought to be impossible and it has applications in optical microscopy.

BTW plasmons are not my area of expertise but I am pretty sure that the above is correct in principle.

Re:Everybody knows

[conteXXt]

Didn't Wendo O Williams sing for them in the 70s-80s too?

Chameleons

[ndogg]

...match its background, like a chameleon.

Grrr...

Chameleons don't change their colors for this reason. It's a myth. Stop spreading it.

http://www.wsu.edu/DrUniverse/chamel.html

Re:Everybody knows

[ShadeOfBlue]

Yeah, that's all basically correct. I did research on annealing these metal films to try to change their optical properties (we ran into some problems with grain structures in the metal growing during the annealing process).

Most scientifically literate people probably haven't heard of plasmons because they only form when the surface of a metal is milled with a regular array of nanostructures. In this case you have an array of holes on the scale of tens to hundreds of nanometers in diameter. When there's some such repeating nanoscale structure it changes the electron energetics so that the energy to frequency ratio is similar to that of the electromagnetic spectrum, at which point light can couple with the surface electrons and form these longitudinal surface waves (I'm not a physicist yet, so some of this may be a bit shakey).

As the parent said it's these waves that can then travel through the holes milled in the surface out onto the other side, where for some reason or another, they'll reemit the energy stored in them as light. It's pretty cool because they've done tests and the light doesn't just come out of the holes. It's as if the light passes straight through the metal film. Furthermore, they know the light's not simply passing through the film, because they've also measured it and found a very slight delay due to the formation, propagation, and reemission of the plasmons.

The story I heard about the discovery of this phenomenon is kind of amusing. Apparently an English speaking chemist wanted an array of micro wells for some polymer reaction, asked a Chinese chemist if he could do make one. The Chinese chemist thought he was crazy and said it would take six months. Due to the language barrier, the "you're crazy" bit didn't make it through, and six months later the English speaker picked it up looked through it, and said, hey, there's nothing here.

One use they're currently looking into is very specific optical filters which can be built for any wavelength. The grad student I worked with mentioned way down the line the possibility for essentially infinite resolution displays, although how that'd work isn't quite clear.

Dazzle Camouflage

[rednip]

how would that look as you were driving down the street :)

Since this technology will only cover "long-wavelength radiation such as microwaves". Perhaps you should consider using the same method as many of the battleships of WWI. They called it "Dazzle", the idea was to visually break up the shapes of the ship so that they would be harder to see. A Ghillie Suit suit also works the same way.

Re:Dazzle Camouflage

[FuturePastNow]

Razzle Dazzle camo made no attempt to hide a ship or make it harder to see. As the linked article says, the effect is to make it hard to identify the type of ship (by breaking up its lines) and to make it difficult to determine the direction of movement ("Which end is the bow?"). All of this was a defense against U-Boats, IIRC, Dazzle was eventually discontinued as camouflage because it actually made ships easier to spot from the air. The "haze grey" color of modern ships may not look like camouflage, but it was chosen because it makes them harder for airplanes to see.

Re:Restrictions far too great

[Anonymous Coward]

Errr, there's a big difference between "see" and "resolve". You can use radio waves to detect aircraft (we call it radar) even though the wavelength of the radiation used is much much bigger than the plane.

That's not even close to true. The wavelengths used are much smaller than an the plane. Usually it's microwaves. You can't get a reflection off an object smaller than a wavelength, really.

Re:Everybody knows

[Jan Brunner]

I don't know more about the phenomenon (yet), I just wanted to confirm it and the story you told.

I'm studying nanoscale science (still at the beginning) and we visited the institute where the guy you mentioned is working. He introduced us to the institute and some technical aspects and we were led through the laboratories, which was very interesting.

Of course he told us the story about the misunderstanding, too. Here's the guy: http://www-isis.u-strasbg.fr/nano/ Here are several publications about the phenomenon and other topics: http://www-isis.u-strasbg.fr/nano/pub.html