You joined in 2009 and BEHAVED NICELY...
Good...Then we can call you a liar for saying that I 'mistreated everyone' over at your playground.
Moving on...
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Readers,
Recently some supposedly 'evidences' were presented to support the argument that we can be conclusive about the J-20's radar reflective properties. Said 'evidences' are in the form of physical optics measurement performed by an Australian group =>
Air Power Australia - Home Page <= APA does have some excellent articles with credible technical analysis worthy of being references themselves. But APA dropped the ball on this one...
What APA did was performed a physical optics (PO) simulation on the J-20 regarding its radar reflectivity. They
DID NOT performed said measurements with established physical dimensions commonly known from profile views: top, bottom, front, and sides. In other words, any physical dimensions input into the simulation were of inferences from photographs. As if that was not bad enough, the photographic experience was out of their control. It is very feasible to have very accurate physical dimensions of any complex body from photographic experience. We do that regularly with satellite imagery. But the difference here is that we have ownership of said photographic experience and whenever we do not we made it clear to our audience, be it the public or the President himself, that because we did not have control of the photographic experience, the physical dimensions of said body should be taken with a large margin of error. In efffect, APA guessed what the J-20's physical dimensions could be and inserted them into a PO simulation. The computing adage 'Garbage in. Garbage out.' is very true and applicable here. APA could be as lucky as we have from satellite imagery and what they inferred are true. But hope is insufficient for any critical thinking person to accept as part of any declaration of any argument he is willing to stake his intellect upon and defend.
Here are the admitted flaws in APA's simulation of the J-20's radar reflectivity...
1- The simulator at this time
does not model backscatter from edge diffraction effects, although the resulting error will be mitigated by the reality that in a mature production design these RCS contributions are reduced by edge treatments;
2- The simulator at this time
does not model backscatter from surface travelling wave effects. In the forward and aft hemispheres these can be dominant scattering sources where specular contributions are low. The magnitude of these RCS contributions is reduced by edge treatments, lossy surface coatings, gap treatments, and panel serrations;
3- The simulator at this time
does not model backscatter from the AESA bay in the passband of a bandpass radome, due to the absence of any data on the intended design of same, the resulting error will be mitigated by the reality that in a mature production design much effort will be expended in suppressing passband RCS contributions;
4- The simulator at this time
does not model backscatter from the engine inlet tunnels or engine exhaust tailpipes, due to the absence of any data on the intended design of same. In the forward and aft hemispheres these can be dominant scattering sources where specular contributions are low. The magnitude of these RCS contributions is reduced by suppressing these RCS contributions with absorbers, and in the case of inlet tunnels, by introducing a serpentine geometry to increase the number of bounces;
5- The simulator at this time
does not model structural mode RCS contributions from antenna and EO apertures, panel joins, panel and door gaps, fasteners and other minor contributors; although the resulting error will be mitigated by the reality that in a mature production design these RCS contributions are reduced by RCS reduction treatments.
All the highlighted items that APA's PO simulation did not collect data are what make a complex body electronically visible on a 'radar scope'...As illustrated below...
In all five items, APA essentially hope or gamble that the J-20's manufacturer will successfully compensate. The J-20's supporters then gleefully leap over the APA statement that theirs is a 'Preliminary Assessment' and ignore the APA admitted 6 important shortcomings, five outlined above, and demand that everyone, including those who have relevant experience in the radar detection, to accept blindly that the J-20 has a conclusive RCS. It is possible that APA is correct. But that is not how critical thinking people work.
For the ignorant but curious and critical thinking lay person, those five points above would be enough to cast a healthy measure of doubt on APA's methodology. But the J-20's supporters would counter via the 'appeal to authority' argument by laying out the credentials of the simulation's author: Peter Goon =>
CV - Peter Goon - APA Co-founder . No offense to Mr. Goon, presumably a very nice man, but his
curriculum vitae seems to involve more of test flights than of radar detection. For the ignorant but curious and critical thinking lay person, he is now being beaten over the head and hopefully into intellectual submission via Mr. Goon's credentials.
It is quite unfair to the lay readers to see paywalled sources from an advocate. For all we know, the advocate is being selective with his sources to deceive. But considering someone's credentials are being used as an 'appeal to authority' argument club, it is only reasonable that the lay readers have a shield so they can resume to exercise that which is critical for any thinking person: objectivity.
So here comes that shield...
In the radar community, there are very very very few personalities that has the technical gravitas as that of Merril Sknolnik or Eugene Knott. Their works are required reading. Combined, the pair alone have decades of technical knowledge, experience, accomplishments, and published works that set the standards in the community. Any engineer who claim to specialize in radar detection better have their most famous works on his/her bookshelf.
Regarding the above, it is a 1985 summary/abstract of Knott's paper about the techniques used to predict and model a complex body under radar measurement. An abstract is usually written by a reviewer, rarely by the paper's author. In this paper, Knott outlined the strengths and weaknesses of the major methods and what he said about physical optics (PO), as the reviewer pointed out, is significant...
Physical optics does yield results in those cases. but fails by progressively wider margins as the scattering directions swings farther from the specular direction.
Thirty years later, we have this...
The readers should note that the paper's authors are:
IRANIANS. Look who they referenced for their submission: E F Knott. And look at what the Iranians said about PO...
As is observed, PO give erroneous results at wide angles.
Physical Optics (PO) is a common EM analysis tool on which many efficient computer codes are based. Despite the method's efficiency in case of surface structures and in specular regions, it looses its significance when applied to objects having edges or discontinuities.
To their credit, APA did admitted the shortcomings of their analysis and did called it a 'Preliminary Assessment'. We should take it no more than that and based no assertions upon it. If we have a bias or being an advocate, then we should be intellectually honest enough to state that it is only our own opinions and not demand that others accept those opinions as gospel truth. Thirty years passed and the technique passed through countless hands but the conclusion is still the same: That Physical Optics (PO) is
NOT the correct tool for a complex body and APA stated why in the 6th caveat...
6- The PO computational algorithm performs most accurately at broadside or near normal angles of incidence, with decreasing accuracy at increasingly shallow angles of incidence, reflecting the limitions of PO modelling. The simulator does not implement the Mitzner/Ufimtsev corrections for edge currents. While a number of test runs with basic shapes showed good agreement between the PO simulation and backscatter peaks in third party test sample measurements, even at incidence angles below 10°, characteristically PO will underestimate backscatter in nulls. This limitation must be considered when assessing results for the nose and tail aspects, where most specular RCS contributions arise at very shallow angles39.
That does not mean that APA is absolutely wrong but what it mean is that if an incorrect tool was used, or must be used because of some reasons, then an objective person should take a neutral stance and wait for when the target is available so we can impose the correct tool or correct combinations of tools to have a more honest analysis. This should have nothing to do with one's own wishes out of nationalistic pride. This is about common sense regardless of whether APA may be correct or incorrect.
Physical Optics (PO) is best for individual RCS contributors in a complex body. Not on the total assembly itself. If the Iranians recognized PO's limitations, odds are very good that Chinese engineers have as well. And they have Skolnik's and Knott's works -- real physics -- on their bookshelves.
