Hi there! Recently I googled "stealth defeating radar" or something and got a link to an old post from this forum. The posts focused on different ways to detect and defeat modern stealth aircraft like the f-22 and b-2, but I have a few questions.
The techniques and systems that drew my eye were passive systems and "JORN" type systems (if JORN is passive I'm sorry).
It's my understanding that the JORN uses the ionosphere to bounce EMEnergy back toward the surface of the Earth, but it wasn't made clear whether the system then looks for "shadows" of this EME or if it's looking for reflections of that EM off the aircraft or if it's looking for that EME to, once it hits the top of the aircraft, to then be bounced yet again off the ionosphere then back to Earth?
1) For JORN, if the system looks for reflections from the aircraft would designers need only to make the topography of the top of the plane just as properly angled as the bottom?
2) If JORNs look for shadows in the ionosphere would an aircraft simply need to detect the above radiation and transmit an identical signal from the bottom of the plane downward, sort of like shining a flashlight on a shadow to make it disappear?
3) For passive radars (radars that look for either emissions from aircraft or look for EM displacement by aircraft of natural or manmade signals) would the solution I asked about in question 2 work? Or do those systems look less for shadows and more for bendings of natural emissions around the planes or the subsequent warping of the signals due to it?
I hope this post makes sense, and thank you in advance for humoring my ignorance
Well, I'll venture a try:
A radar works by emiting/propagading an electromagnetic field.
An electromagnetic field is a strange thing, but it can be detected because it will excert a force on a charged particle (f.ex. the field will make the charged particle accelerate).
The field has a frequvency and a propagading speed, which is the speed of light (note that stating the frequvency gives you the wavelenght, via the wave equation, since the speed of light is a constant). So a charged particle (that could f.ex. be an electron) will start oscillating with the frequvency of the field that exerts a force on it.
When a charged particle is accelerated in this way, it emits/propagade yet another, slightly out of phaze (if I rember my physics correctly), EM-field, and it, in this case, has the same frequvency as the inducing field.
So a radar-antenna propagades an EM-field, causing objects in the field to emit secondary EM-fields (because all the electrons are accelerated etc. - just like a radar antenna), the radar reciver picks up these secondary fields, and it can deduce the direction and the distance and you can fix the position of the objects relative to the radar (or about that).
A good antenna is partly an antenna that directs the strongest possible EM-field in the direction that you want it to. And making a (radar) stealthy object is basically the reverse of making a good antenna; Making an antenna (the object) that directs the the smallest possible EM-field anywhere, but in a given direction (namely the direction of the inducing field, which is the direction of the radar that you wish to be stealthy against).
Antenna design is complicated, but one property, amoung many, is the geometry. The geometry/dimensions of the good antenna has to "fit" in a specific way to the frequvency/wavelenght of the EM-field. You f.e.x. can imagine a transmitting antenna as covered by small "ribbles" (the "waves" of in the field) if these "ribbles" can get "trapped" and move back and forth across a surface, they can create interferrence, and if the conditions are right; ressonance, producing a single powerfull signal out of many small signals (the physics are more complicated than this, but it will do).
The geometry also has significance for the direction in which the field is emitted, but this takes us too far (and quickly beyound my knowledge), though f.ex. a rhombe shape is basic shape that has a tendency to direct the emitted field away from the direction of the incident field (see the f117).
In making a stealthy object (a bad antenna) you would like to avoid constructive interference and ressonance and you would like to direct the field, that is emitted, away from the incident field.
If you look af the F22 or B2 you will notice that it's featureless on a millimeter scale (the wavelength of a military radar). There is no corners or sharp edges that can assist in f.ex. (radar) ressonance on the relevant millimeter scale - Ofcourse there are larger features, like "a wing" though that's on a meterscale.
And while I can't explain you "how", the shape is designed in such a way as to emit it's field away from the incident field (the direction of the radar). This likely works better from some directions than other directions. F.ex. the top of the B2 might not be so good at directing the field as the front section.
The RAM coating is basically a material that is specifically bad material for an antenna (I am not sure, but I believe that the mechanismen has to do with the molecular structure, which dampens the movements/acceleration of the electrons that create the emitted field, this damping is/should be closely tied to the frequvency of the electrons (that'll be the frequvency of the field).
Your J.O.R.N radar is, as I understand it, a high frequvency radar (with a higher frequvency than the millimeter radars that "some" say that current stealth is optimised against) Hence a B2 might be a much better "antenna" at that frequvency (and hence less stealthy). Maybe the RAM coating doesn't work at high frequvencies/short(er) wavelengths, maybe the geometry will loose some of its "re-direction" abilities and supression of radar ressonance and more? - I don't know.
Also, I have read, the radar works by reflecting (or by re-emition of) it's signal/field on the ionosphere. So the radar "shoots" upwards, the signal is reflected/emitted downwards, "hits" the top part of an object, and the return signal is emitted, some of it upwards, and reflects/emits via the ionosphere back to the reciver.
If the top part of the object isn't stealthy, you would expect a (perhaps strong) return signal.
So
1) yes,
2) &3) I don't think it works like that.
