Now lets come back to the J-20, I am sure that the J-20 went through a similar procedure. The designers of the J-20 might have(as we calculate a guess) seen similar red areas for the aircraft with the cockpit, the intakes and the Canards over a full 360 degree sweep. They would then start looking for design ideas(compromises) that reduced those red areas; they could coat the cockpit with RAM, they would change intake shape and coatings, the would design algorithms built into the FLCS that manages the Canards to change position to minimize its energy return and avoid red spikes in favour of yellow or light yellow ones. The same sort of compromises go into aircraft such as the F-22, F-35, PAK-FA .. and are a reference to all those trying to come up with ideas to reduce RCS of conventional aircraft such as the F-15(F-15SE) or the JF-17.
Another problem is that people believes that 'stealth' is a definitive line when in reality, radar engineers do not care for that word. In their view, there is only one thing: distance of detection.
We can 'massage' existing aircrafts, re F-15SE, and we will gain some benefits, as in reducing the detection distance, but as far as radar engineers are concerned, a true radar low observable aircraft must be designed from paper.
There is a legitimate tactical reason on why all major aircraft manufacturers are pushing on the 'massaging' method. No, it is less about money and exploiting the 'stealth' aspect, although money is a part of it. Am going to have to tread carefully here.
There are enough sophisticated radar systems that contains signature libraries, that while are not exact representation of all the major combat vehicles, they are well within statistical range of what is an F-15 or F-16 or even a ship as the radar system analyzes the target. Each signature for each aircraft is compiled from intelligence gathering efforts that ranges from scientific analyses of third hand sources to direct EM observations from events such as airshows or visiting air forces. For exported fighters like the F-15 and F-16, the libraries for their signatures contains true physical dimensions.
So assuming a radar system that is sophisticated enough, also meaning costly enough, if an incoming threat have a flight profile and radar return that matches a signature inside the libraries, the defense can better formulate a response if they know that at so-and-so distance, a target or cluster of targets is likely to be a flight of so-and-so fighters, which inevitably leads to reasonably accurate estimates of what kind of weapons the defense can expect to face.
An F-15 or J-17 that is 'massaged' to have a reduced RCS, thereby reducing the detection distance, can (not will) shift the immediate tactical situation to the attacker's favor. Whatever algorithms the radar are using now reduces the statistical probability of whether the incoming threats are F-15 strike fighters or F-15s flying CAP for other strike fighters that are not yet detected. Back in Desert Storm, Allied fighters often spoofed Iraqi air defense radars by flying at altitudes and speeds that are assumed to be common to certain other aircrafts, and yes, the Iraqi air defense was smart enough to differentiate between different aircrafts. This is a combination of technical and human experience and a good air defense commander is worth his weight in gold, even though we usually pay them far less and his job description is not as glamorous. Human experience is difficult to quantify and preserved and that is why we had to go the route of creating these signature libraries.
Here is an example of what we do to create a signature of a complex body with known and fixed physical features...
http://www.dtic.mil/dtic/tr/fulltext/u2/a461958.pdf
The high-resolution full-polarimetric Ka-Band turntable signatures of the MBTs were acquired over a two-week period in April of 1999. The signature data was acquired at five elevations spanning 5° to 60° for a T-72M1, T-72B, M1, M60-A3 and one classified vehicle.
To 'massage' an existing aircraft to reduce its RCS is not an easy endeavor, and an endeavor it is, not merely a task. It will cost money and while not as much as designing a radar low observable fighter from paper, it remains out of reach for most companies and countries out there, especially the ones that have to import their defense.
