German radar vendor says it tracked the F-35 jet in 2018

[marauder2048]

Should also be noted that a Luneburg Lens can have a fairly large bi-static RCS across many degrees
(+/- 10 degrees is common) off the transmitter boresight.

[SpudmanWP]

On the other hand, intentionally attack civilian infrastructure is war crime.

Sometimes I really wonder where people come up with their "facts".

I blame public education

and social media.

[ricnunes]

Nope, you are ignoring what happens to EM energy when it hits a stealth fighter and how a Luneberg Lens works. The vast majority of "stealth" comes from shaping, not from absorption. Radar energy is reflected in directions away from the transmission source as opposed to nullified or cancelled out.

You would be correct if you said that the majority of "stealth" comes from shaping, not from absorption, however I believe that you're not correct by saying that the vast majority of "stealth" comes from shaping, not from absorption. (note the emphasis on "vast").
While this is extremely classified info and no one can know for sure it is suspected/reported that the "stealth" of a "stealth aircraft" comes 60% to 70% from the shape (Radar/radio energy is reflected away from the emitter) while 30% to 40% comes from RAM (Radio/Radiation Absorbent Material) coating or "more precisely" from absorption. I would say that for a F-35 which uses extensive and advanced RAM coating it is feasible that 40% of its stealth comes by absorbing the incoming radar/radio waves which by no mean represents a "vast minority" of it's stealth (although being a "minority").

So, Luneberg lens are passive, they do not amplify the energy, merely make it more concentrated. Essentially it acts like a corner reflector, reflecting more of the signal back to the radar. The total energy reflected is the same, just more is concentrated back toward the transmitter. If anything, use of a Luneberg Lens will decrease the energy reflected in other directions.

Well, sorry but I don't follow that logic above. Lets imagine a certain radar which is detecting for example a F-16 and a Boeing 747. That same radar will inevitably detect the 747 at a much longer range than it can detect the F-16, right? And why? Because and since the 747 presents a larger reflecting area (what we call larger RCS) it means that more energy is reflected/bounced back to the receiver this when compared for example with the F-16 thus enabling the radar to detect the 747 further compared to detecting the F-16.

Lets imagine another example:
1- Imagine that you have a 5 liter bucket filled with water and then you throw all the water against a 1 meter diameter sphere in front of you - The result will be that a certain amount of water will splatter back and away from the sphere.
2- Now imagine doing the same against a 0.5 meter diameter sphere in front of you, what will be the result? The amount of water splattered back and away from the sphere will inevitably be much less compared to the previous case 1- this despite the amount of water being thrown in is the same.
This is the principle behind radar/radio waves.

And what does the Luneberg Lens have to do with this? The Luneberg Lens will increase the area to which the F-35 (or any other Stealth aircraft) can reflect incoming radar/radio waves back so more energy will inevitably be reflected away from a F-35 with Luneberg Lens compared with one that does not have them.

But even that is not important. The system in question uses FM signals in the VHF Radio/TV range, so 54-216MHZ while a Luneberg Lens is designed to work in the 2.7-2.9GHZ range used in air traffic control radars. At the higher frequency, the reflectors amplify the signal but at the lower FM frequencies the amplification is much less (if at all) since it is the entire aircraft that reflects the signal.

This is also strange for me to follow. So you're saying that Luneberg Lenses reflects back radio waves in the 2.7-2.9GHZ range but not radio waves in the 54-216MHZ range?
For me this would mean that Luneberg Lenses and whatever material their made are completely stealth against 54-216MHZ range radio waves which would be very strange since this wouldn't be a matter of having lower RCS against such waves but instead a "total and complete" invisibility/stealth against the same wave range.

So and in case I'm missing something here, could you care to explain why would something passive like a Luneberg Lense be invisible to 54-216MHZ radio waves?

However, the article illustrates the difficulty of using a passive radar system, especially one in the FM TV Band. First you need multiple receivers all linked together. Look at the receivers, no way you have any vertical beam forming on those and there is no way they are accurate enough horizontally to track the signals by direction with any fidelity. So they are basically working GPS is reverse to use time delay from the signal at multiple points to track a reflected source. That is really hard to do, especially the detection part, over tactically relevant ranges. The German company clearly indicated that they had to detect and track the F-35s on ADS-B before they could track them with the passive radar. Because sorting out the hundreds of thousands of targets (note that every moving item such as planes, cars, trains,etc also reflects the signal and you can't use Doppler to sort them out for obvious reasons) is hard as well as the fact that any pair of signals could be the target, so the numbers go up exponentially.

And of course, it is almost ridiculously easy to spoof the signal and/or eliminate the transmitter, especially given that things like TV stations and big radios are both prime targets and easily located.

So passive radar is not that big of a deal yet (as opposed to detecting emissions from the aircraft itself, like radio). But the fact that the aircraft had Luneberg lenses, IFF, or ADS-B does not invalidate the test. But it is unlikely any foe would have their receivers near the end of the runway either.

Regarding the rest of your post, I fully agree with you.

[jetblast16]

This all kinda of reminds me of the early warning network illegally constructed by the Chinese in the South China Sea to detect the approach of western (US particularly) stealth planes and bombers. I suppose it is conceivable with the right equipment and techniques to lay radar "trip wires" across a vast chunk of airspace with dispersed receivers to collect the faint (scattered/ redirected) returns from stealth aircraft. Perhaps they can't get an exact lock, but if the techniques are good enough and the air frame shaping/ electronic warfare techniques are not as good as they ought to be, it may be possible to get hints that there are aircraft operating in a certain area, and then use other sensors, like long-range IR to locate the intruding aircraft. Fortunately, the US Air Force is not stupid, and does not solely rely upon stealth only, but strives for a more holistic approach, with an emphasis placed on advanced electronic warfare techniques. And you can bet that the new B-21 will be jam-packed with them.

[swiss]

As usnvo pointed out, this seems to be the most importent sentence from this report.

Camped out amid equines, engineers got word from the Schönefeld tower about when the F-35s were slated to take off. Once the planes were airborne, the company says it started tracking them and collecting data, using signals from the planes’ ADS-B transponders to correlate the passive sensor readings.

[zerion]

Saw an interview with an engineer who said the ratio was about 80 to 20, these guys say it is 90 to 10.

https://www.nextbigfuture.com/2016/11/t ... weden.html

Not sure where they got it from though.

[hornetfinn]

Saw an interview with an engineer who said the ratio was about 80 to 20, these guys say it is 90 to 10.

https://www.nextbigfuture.com/2016/11/t ... weden.html

Not sure where they got it from though.

OMG, that was pretty horrible article. However, I agree that shaping is the most important thing with RAM and RAS needed to address certain areas. Basically F-15 can't be made very stealthy with RAM/RAS as the shaping is so bad for RCS. F-22 or F-35 without RAM would still have rather small RCS, although significantly higher than with it. Still likely better than any 4th gen fighter with RAM.

[tank-top]

Not to get off on a rabbit trail but I have two questions, the second is a bit nebulous.

It’s not a question of if but when will it be technologically possible to have a complete picture of the sky using background radiation and multiple passive sensors? Seems you just need enough processing power and the right algorithms, any additional RF energy would just be welcome a addition.

The second question and obviously I don’t understand the science (most don’t) but will it ever be possible to “see” the RF radiation itself and not just it’s reflection at a sensor. In other words, we don’t see light rays, just the reflection off a physical surface and the reflected wavelength. We can’t see radiation in any wavelength, only it’s interaction with matter, is it physically possible to actually see it?

[swiss]

For me is the primary question is. Is this System superior to a top notch Radarsystem? They did know when the F-35 had take off, the Flight direction and could locate the Lightning thanks also to the ADS-B transponders.

A modern und big Radarsystem, with the same information, can do this also on a short Distance, even without transbonder and a Luneberg Lens .

[ricnunes]

Saw an interview with an engineer who said the ratio was about 80 to 20, these guys say it is 90 to 10.

https://www.nextbigfuture.com/2016/11/t ... weden.html

Not sure where they got it from though.

Wow (on the negative side), I fully agree with hornetfinn - what a horrible (actually BS) article!

Of course that with all the BS in that article I find hard to believe in the 90% to 10% shape/RAM ratio for stealth aircraft.

You heard 80% to 20%, well I certainly won't say that those couldn't be feasible values. They certainly can!
Like I previously mentioned, I personally heard 70% to 30%.
Over a discussion here in this same forum, I "heard" someone saying 60% to 40%.

I would even say that all those values above have a chance to be correct. Perhaps for the stealth aircraft X (lets say a B-2 for example) it could be 80% to 20% like you said while and perhaps for the stealth aircraft Y (lets say a F-35 for example) it could be 70% to 30% or even 60% to 40% since the F-35 has certainly and by far the most advanced RAM coating (a rubber like RAM material which If I'm not mistaken covers most part of the aircraft).

But yes, I agree with you, usnvo and hornetfinn that the majority of stealth come from shape (as opposed to RAM). I never mentioned otherwise and I hope I haven't implied that.

[ricnunes]

For me is the primary question is. Is this System superior to a top notch Radarsystem? They did know when the F-35 had take off, the Flight direction and could locate the Lightning thanks also to the ADS-B transponders.

A modern und big Radarsystem, with the same information, can do this also on a short Distance, even without transbonder and a Luneberg Lens .

Yes, I agree with you swiss. IMO, there are active radar systems which would fare quite better against stealth aircraft like the F-35 than the previously mentioned passive radar or any other passive radar for that matter. One of such example would be the NEBO-M AESA VHF Radar. In theory this radar could detect a F-35 at a range up to or close to 90/100km and doesn't have to rely on other transmitters (TV, etc...)

This being said, this obviously isn't a stealth killer solution (actually there aren't any single stealth killer solutions) because of lots of limitations such as: not being able to guide weapons (such as the passive radars of above), the F-35 can still employ weapons that can and will destroy it, this outside the radar's detection range (against the F-35) and above all, if that radar can/could detect a F-35 at 100km than I would say that it will detect a 4.5th gen fighter aircraft at 400km or so. IMO, some people for some odd reasons trend to forget this last point.

[usnvo]

Saw an interview with an engineer who said the ratio was about 80 to 20, these guys say it is 90 to 10.

https://www.nextbigfuture.com/2016/11/t ... weden.html

Not sure where they got it from though.

Wow (on the negative side), I fully agree with hornetfinn - what a horrible (actually BS) article!

Of course that with all the BS in that article I find hard to believe in the 90% to 10% shape/RAM ratio for stealth aircraft.

You heard 80% to 20%, well I certainly won't say that those couldn't be feasible values. They certainly can!
Like I previously mentioned, I personally heard 70% to 30%.
Over a discussion here in this same forum, I "heard" someone saying 60% to 40%.

I would even say that all those values above have a chance to be correct. Perhaps for the stealth aircraft X (lets say a B-2 for example) it could be 80% to 20% like you said while and perhaps for the stealth aircraft Y (lets say a F-35 for example) it could be 70% to 30% or even 60% to 40% since the F-35 has certainly and by far the most advanced RAM coating (a rubber like RAM material which If I'm not mistaken covers most part of the aircraft).

But yes, I agree with you, usnvo and hornetfinn that the majority of stealth come from shape (as opposed to RAM). I never mentioned otherwise and I hope I haven't implied that.

Actually the F-35 probably has a much lower percentage of signature reduction coming from RAM than the B-2, especially at the lower frequencies which require thicker RAM to be effective. There is just not enough payload available in a tactical fighter to carry meaningful amounts of RAM. Although I have no direct knowledge, it is widely reported that he B-2 has RAM that is feet thick in places, no way to do that in a tactical fighter sized aircraft. Frequency matters, at 10GHZ the wavelength is 3cm, at 2.5GHZ it is 12cm, but at 200MHZ it is 1.5m!

[usnvo]

This is also strange for me to follow. So you're saying that Luneberg Lenses reflects back radio waves in the 2.7-2.9GHZ range but not radio waves in the 54-216MHZ range?
For me this would mean that Luneberg Lenses and whatever material their made are completely stealth against 54-216MHZ range radio waves which would be very strange since this wouldn't be a matter of having lower RCS against such waves but instead a "total and complete" invisibility/stealth against the same wave range.

So and in case I'm missing something here, could you care to explain why would something passive like a Luneberg Lense be invisible to 54-216MHZ radio waves?

It has to do with the frequency in question (well actually wave length but they are really the same thing). When the wave length is low compared to the size of the object, the radar energy is reflected from the skin of the object. But when the wave length is similar in size to the object (say a vertical stabilizer), the entire object reflects the energy just like a dipole antenna. Look at Chaff as a parallel to a Luneburg lens (which is just a really cool version of a corner reflector or parabolic dish but I digress). Chaff works by using a large amount of half-dipole length antennas. For X-Band, where missile radars typically work, the wavelength is on the order of 3cm, so the pieces of reflector are cut to a variety of lengths around 1.5cm in length. Works great in X Band but not so well in S-Band (2.5GHZ or so where most civilian and military ATC radars operate), where you need dipoles that are more like 6cm long. Drop down to 100MHZ (3m wave length), and now you need a dipole cut to 1.5m! The X and S band dipoles don't effect it nearly as much because you are working at really high order harmonics and you just don't get much return. Just like Chaff, a Luneburg lens needs to be tuned to the specific frequencies in question and the further you get from said frequency, the less effective they are and frankly, the little ones on a F-35 are just way to small to have a significant effect on the F-35 RCS in the VHF band. This also explains why VHF radars are so large.

[ricnunes]

Actually the F-35 probably has a much lower percentage of signature reduction coming from RAM than the B-2, especially at the lower frequencies which require thicker RAM to be effective. There is just not enough payload available in a tactical fighter to carry meaningful amounts of RAM. Although I have no direct knowledge, it is widely reported that he B-2 has RAM that is feet thick in places, no way to do that in a tactical fighter sized aircraft. Frequency matters, at 10GHZ the wavelength is 3cm, at 2.5GHZ it is 12cm, but at 200MHZ it is 1.5m!

From everything that I read so far about the F-35 RAM coating, it seems that the F-35 RAM coating is much more advanced than the RAM coating on previous stealth aircraft (from F-117 to B-2 and F-22) which is composed by a rubber-like material (which probably reminds a bit of the anechoic coating on submarines) which means that there's a good chance that:
1- This F-35 RAM coating is more effective than previous RAM coating which means you'll need less thickness for the same or better effect.
2- Or, the F-35 RAM coating has the same effect as older RAM coating but being lighter.

I would say that the chances of the above being true are somehow considerable due to the fact that the F-35 is more stealthy (lower RCS) than initially expected.

[ricnunes]

It has to do with the frequency in question (well actually wave length but they are really the same thing). When the wave length is low compared to the size of the object, the radar energy is reflected from the skin of the object. But when the wave length is similar in size to the object (say a vertical stabilizer), the entire object reflects the energy just like a dipole antenna. Look at Chaff as a parallel to a Luneburg lens (which is just a really cool version of a corner reflector or parabolic dish but I digress). Chaff works by using a large amount of half-dipole length antennas. For X-Band, where missile radars typically work, the wavelength is on the order of 3cm, so the pieces of reflector are cut to a variety of lengths around 1.5cm in length. Works great in X Band but not so well in S-Band (2.5GHZ or so where most civilian and military ATC radars operate), where you need dipoles that are more like 6cm long. Drop down to 100MHZ (3m wave length), and now you need a dipole cut to 1.5m! The X and S band dipoles don't effect it nearly as much because you are working at really high order harmonics and you just don't get much return. Just like Chaff, a Luneburg lens needs to be tuned to the specific frequencies in question and the further you get from said frequency, the less effective they are and frankly, the little ones on a F-35 are just way to small to have a significant effect on the F-35 RCS in the VHF band. This also explains why VHF radars are so large.

Ok, your explanation does indeed make sense. Thanks!

But I couldn't help to notice that the example that you gave - Chaff - is "detached" from the aircraft but Luneburg lens aren't and thus they are a "continuity" of the aircraft itself (of some sorts). Shouldn't this change somehow the facts above?

Another question: What's the size of a Luneburg lens fitted on the F-35?