Readers,
From the below is an illustration of RCS contributors on a complex body:
- Scattered waves: Summation from all possible scatter points (generators).
- Specular reflections: Immediate upon impact. This occurs even if the surface is an absorber, albeit at a lesser energy level.
- Surface waves: Occurs when the incident angle is less than perpendicular (normal). As incident angle departs from perpendicular and approaches parallel the surface become an inducer to the creation of the surface wave phenomena and a conductor to the same. At this point, the surface's expanse is called 'the electrical path'. In a dynamic situation, as in a maneuvering aircraft, surface waves are not constant.
- Diffraction signals: Occurs when there is an abrupt interruption of the surface wave's travel path. Infinite surfaces are theoretical. Practical surfaces are finite and will always produces diffraction signals.
- Traveling waves: A category of surface waves. Usually occurs on the gradual curvature of a surface. A conducting flat surface can also induce the traveling wave effect. Same for a wire. For an aircraft, traveling wave effects are more prominent at nose-on illumination by the seeking radar.
- Creeping waves: Occurs when a surface wave has sufficient energy to travel into the 'shadow region' of a curvature. Best illustration of this is the sphere or a cylinder. At the radar signal's impact point on the sphere or the cylinder, the opposite side is 'the shadow region'. There is a rule called the 'ten lambda' rule that may not allow the creation of the creeping wave effect. Lambda is the symbol for wavelength. For a wavelenght at a fixed distance, if the diameter of the sphere or cylinder is greater than ten lambda, as the surface wave travels, 'leaky waves' radiation will be the primary loss mechanism and eventually will dissipate the surface wave completely. If the diameter of the sphere or cylinder is ten lambda or less, the creeping wave effect will occur and as the wave emerge from 'the shadow region' into the impact side, constructive interference with the specular reflection will enhance the body's detectability.
- Trapped waves: Occurs in cavities such as a tube or a pocket of any shape of a volume. For an aircraft, the cockpit and the engine inlet tunnels are trapped waves creators. Also called 'guided waves'. Depending on length and diameter, a tube with trapped waves from pulses may create continuous wave (CW) emissions that ring (time domain) or resonate (freq domain) and can escape the cavity through the same entry point.
- Ducted waves: Occurs in longitudinal depressions such as between the engines on the aircraft's underside or longitudinal depressions for missile attachments.
The concept of radar cross section (RCS) is not new. But what is 'new' or at least continually being updated it what kind of data are we receiving and
CAPABLE of processing. Since the early days of radar detection,
ALL of the above have been with us but we were not capable of processing them as well as we knew of them and as well as we would like.
What APA did recently for the J-20 was perform Physical Optics (PO) measurements
ONLY and PO is for item two: specular reflections. And I have shown everyone here via post 306 with decades of historical evidences that PO is an inappropriate tool for a complex body. It was not technically wrong because the PO tool can be used any where on a complex body, but it was clearly an inappropriate one. Given the facts that radar detection is essentially a statistical process and that against a neutral background contributors on a complex body will naturally cluster, targets are categorized into two main states: Steady and Dynamic.
- Steady: Geographical features like mountains. Man made features like buildings or towers. These items have a uniform distribution. No further discussions needed for them.
- Dynamic slow: A ship is an example of this. The ship's superstructure is a dominant contributor where lesser structural contributors are clustered around the dominant contributor. In the time domain, lesser contributors may disappear but when they do reappear and even in different aspect angles to the seeking radar, they still cluster around said dominant contributor. To correlate is to examine a detail under different conditions and record the results. Such a slow moving target will require several seconds of target focus to process all major and minor contributors before we can assure ourselves that we are looking at an oil tanker with an aft superstructure, or an aircraft carrier with a center superstructure, or a submarine that is on the sea surface, or a float plane.
- Dynamic fast: A maneuvering aircraft is an example of this. Before the F-117, all aircrafts have a dominant contributor: the corner reflector. As seen below...
Even though the airliner is not a high-g fighter, it still qualify as a 'dynamic fast' target because its operating environment require it to travel at several hundreds km/h.
The airliner has a dominant contributor: the tail assembly corner reflector as seen by the large spike. Then many lesser contributors clustered
WITH (not necessarily around) the tail assembly. Even if we treat these lesser contributors, the tail assembly reflector as the dominant contributor will reveal the aircraft any way. Going back to the 'untreated' airliner, to correlate and therefore assure ourselves that we are looking at a valid target, because of the airliner's speed that will quickly recreate contributors (spikes) that were destroyed earlier, the correlation process will take mere milliseconds. The dominant contributor give us one target characteristic. The lesser contributors, based upon their spatial distribution, give us another target characteristic because its (air)speed over time continually destroy and create new spikes and they all clustered. Hence, it takes only milliseconds or faster to correlate a 'dynamic fast' target.
Things changed radically for military aviation after the F-117. Now despite the fact that we have a 'dynamic fast' body that supposedly need only milliseconds to correlate we have no dominant contributor. The twin canted vertical stabs deny us the corner reflector. Enclosed weapons further deny us the corner reflectors created by missile fins. Treated cockpit canopy deny us the corner reflectors inside the cockpit. And so on. As for the lesser contributors, they are treated in ways that make it difficult for us to find them as a cluster in the first place. So for our newly design 'stealth' aircraft, even though it is true that specular signals are dominant, we bodyshaped the aircraft and made them so low, hopefully into the clutter rejection threshold, that we have no choice but to include tools, and combinations of tools, far more sophisticated than Physical Optics (PO) to capture as much as possible our model's RCS contributors before we can declare that we achieved our goal. That is the honest way to go.
This is why it is totally inappropriate for APA, a third party that has no access to the J-20, to confine their measurements to just Physical Optics (PO) in their J-20 'analysis', favorably downplay those specular reflections, and simply hand-waved away other contributing mechanisms to support their preconceived ideas on how the J-20 should be. This is seriously flawed because the concept of a radar cross section (RCS) is
INCLUSIVE of all radiating mechanisms. It is the degrees of contributorship, based upon comparisons to each other, that will determine if a particular contributor is of interest for further attempts at reduction measures. For all we know, it could be those non-specular contributors that will provide us with sufficient energy level and spatial distribution to make the J-20 detectable. By performing only Physical Optics (PO) and dismissing away other radiating mechanisms, APA is effectively redefining the concept of RCS to suit their pet 'du jour'.
How would any Chinese member like it if I go to China and only after one month, I start making declarations about China's agriculture and the Chinese people based upon...aaahhh...18th century Buddhist pagoda architecture? Would you be offended? Sometimes the group does not need to be vocal in its disapproval of a wrong committed by a person. What APA did was the equivalent of a loudmouthed fool in the company of silent sages.
