Schneier on Security

Clive Robinson • November 22, 2009 6:17 PM

@ neill,

“there was some other research on how to detect stealth planes by detecting them blocking satellite signals”

It is now no great secret and is fairly obvious if you thing about it.

To be fully stealthy you need to do two things,

1, Stop reflections.
2, Allow transmission.

That is in a black room with just a single candle lighting it you can hide by wearing black (absorbant) material with a mat surface (no reflection).

However walk between the candle and an observer then you will be seen by your shadow (transmission loss).

Then there is the question of if your black suit is the same black as the backdrop across the entire EM spectrum.

The current state of practical stealth technology is “absorb” and minimise “reflection” back along the EM source path.

The UK’s manufacturer of the rapier missile system developed a number of working systems to detect the US stealth fighter/bomber and gave a working demonstrated at an international trade show.

There are infact several currently known ways that stealth technology can be defeated.

The mechanical design of a stealth aircraft is such that any reflections should be minimal back towards the EM source (no curves, or 90 degree angles and plane surfaces not normal to the likely direction of an EM source). However “offset” or space diversity EM systems with multiple radiators and receivers will pick up reflections off of plane surfaces that are not reflected normal to the plane surface (ie 90 degrees to the plane thus 180 degree reflection back to the radiators).

To limit this the aitcraft surfaces are coated with EM absorbing materials. However they are by no means perfect in the same way matt black paint is not perfect.

Firstly the abosrbtion is not uniform across the full EM spectrum. And secondly an absorber will actually reflect at certain angles, you can see this with a flat board painted matt black as you tip it through the light at angles close to it’s plane it appears shiney.

Thus even in the part of the EM spectrum where the absorber works it will still have angles at which it reflects. Obviously by grading the surface (as in the glass of optical cables) you can minimise this reflection. But doing this is a little like having air bubbles under freshly put up wallpaper where you try to push it down it makes it worse somewhere else (worse a graded surface can act like one of those edge lit emergancy exit signs that give illuminated words in the middle of a pane of glass.

Importantly any reflecting surface changes the phase of a coherant EM signal near it’s surface, this produces peaks and nulls in the field where the non incident EM radiation and reflection co exist. If the reflecting object moves with respect to the EM source then the phase will change, this causes incidental FM modulation of the field and then there is also the doppler effect as well.

Also the peaks and troughs move making the signal AM modulated (think moon light reflecting off of rippling water). Thus all three modulation components are related to each other which alows cohearent demodulation of the reflected signal.

Further the combined effect is to widen the bandwidth of the combined EM signal at the receiver. Now by using some fairly well known techniques you can “null out” the direct path EM signal from the source and leave the modulated reflected signal.

Combined, space diversity systems and advanced signal processing techniques can significantly reduce the level of stealth the curently implemented stealth technology produces.

One of the curious things that happens at the edges of an object when it is illuminated from behind or where the EM source is not directly visable to the receiver, is what is sometimes called the “edge effect” or “skirt effect” which effectivly bends the EM waves and enables the receiver to pick up the EM source. Usually the sharper the edge the more pronounced the effect, however “sharp” is relative to the EM wavelength.

There are other issues to do with turbulance coming off of an aircrafts wings and other surfaces. Under certain conditions this turbulance can be detected by the effects it has on parts of the EM spectrum (in a vaguly similar way to the turbulance seen under water from a propeller or impeller, or in clouds with certain type of weather radar).

This is all before transmission loss effects (hand infront of your face / shadow). And this is the real problem with current practical stealth systems they are the equivelant of the “black Ninja suits” not a mythical “cloak of invisability”.

To do “invisability” you have to solve the transmission loss problem by being either losslessly transparent across the whole EM spectrum or cause it to wrap around you coherantly in all directions with zero loss and distortion. Neither is much of a proposition currently (which is why stealth aircraft fly at night 😉

Which means any object that gets between an EM source and a receiver is going to be detected by a loss in signal at the receiver. But also it will throw a shadow onto a back drop which can be detected. So in theory you could put suitable EM sources and receivers on a geo stationary satellite system with a tight focus covering an area of the earths surface. Two or more satellites monitoring the earths surface from sufficient angle would observe the shadows made by all aircraft. Is it practical as a system well… Let’s just say we have the knowledge and technology to do it, if we can get the required CPU power to make it practical 😉