Clutter rejection is largely a function of the signal processing filter of which you mention a few such examples yourself.
Correct, that's why the lower threshold would make processing overwhelming and need a supercomputer.
My use of the word "clutter" was obviously in reference to radar returns that are processed and found to not be targets, aka. processed radar returns to be ignored. Why else would I refer to the need for a supercomputer to wade through this "clutter"?
Even if we grant you this latitude, it would be conditional, here is why...
A priori and a posteriori - Wikipedia, the free encyclopedia
The phrases "a priori" and "a posteriori" are Latin for "from what comes before" and "from what comes later"...
1- Say you are told to look for a diamond ring in a playroom. Immediately you are loaded with the relevant
a priori knowledge about 'toy', 'diamond', and 'ring'. When you enter the playroom, your vision is filled with toys and you know that you are safe in mentally discarding toys to search for a diamond ring. The patterns for what is a 'toy' and a 'diamond ring' are known to you.
2- Say you are told to look for an object that does not belong in a playroom. Immediately you are loaded with relevant
a priori knowledge of what is a 'toy' but nothing about this object. Your problem is now much more difficult in magnitudes. Even though you have the
a priori knowledge of what constitute objects that are of entertainment nature, each object slightly differs from its companion in this room. Were you are given any
a priori knowledge of the
DEGREE of difference(s) of this unknown object against the toys? The less
a priori knowledge you have of the degree of difference(s), the greater the burden upon you to inspect, catalog the physical characteristics of
EACH toy, correlate what are the same, correlate what are similar, discard the correlated, and finally discern the object that supposedly does not belong in a playroom. If anything, it may be a diamond ring but you would not know what it is anyway. All you know is that based upon
posteriori knowledge after all that work, you finally found this <something> that does not belong.
3- Say you are told to find an object that does not belong in a room. Not a 'playroom' but simply a room. As the Americans would say: Now you are in a world of sh!t. You have no
a priori knowledge of anything in this room. You do not recognize what each item is and what it does. For all you know, the first thing you discarded was the diamond ring, except you did not know what it was and how valuable it is.
Situation 2 gave you an advantage in that after a certain amount of time and physical structures you inspected, if you come across the diamond ring, its physical structures and characteristics would be sufficiently at odds with the
posteriori knowledge you have gathered that you can reasonably be assured that you have found the object that does not belong.
Situation 3, the diamond ring may be the first or 10th or 20th object you inspected, memorized, and discarded. Only after a while when you have finally gathered enough
posteriori knowledge of what is a 'toy', although you may not call it that, can your memory triggered you that you have discarded the 1st, or 10th, or 20th item that have gross physical structural differences from the 'toys'. Now you have to return to the spot where you discarded the diamond ring and examine it again. Note: The word 'discard' here does not mean thrown away, simply mean not targeted or focused upon.
In situation 1, the clutter rejection threshold is high and can be safely set as high based upon
a priori knowledge of
BOTH types of objects.
In situation 2, even though you have no
a priori knowledge of one type of object, you have plenty of
a priori knowledge of the items in the room so while you can categorize them all as 'clutter' you cannot really discard them below any threshold because you still need to examine each to see if you can find something -- anything -- that does not belong. You may find the diamond ring under the 5th or 10th object you overturned.
An F-117 class body belongs in situations 2 and 3 as
THE unknown object or a type of object that must be studied before any categorization.
One may argue/ask: How can that be when the F-117 is still made up of metal and have visually recognizable structures that cry out 'aircraft'?
The problem and counterargument is that radar detection is based upon feedback, as in real time target feedback, and they are passed through filters that contains
KNOWN factors that make up 'aircraft'. To put it another way, plenty of
posteriori knowledge is carried by each radar system in order to form criteria of
a priori knowledge that the system can apply against any target in real time.
We have no
posteriori knowledge of low radar observable aircrafts and how they generally look under radar bombardment. No current radar system carries that
posteriori knowledge to apply an
a priori criteria in real time against an unknown body buried in what used to be rejected.
When I say 'we' I mean in general principle and do not include US. Am sure you and the more intelligent people here will understand why.
The Chinese fellas may have mocked what I say about clutter but that is because they are ignorant of the fact that radar detection is based upon information theory. Not 'highly' or 'quite' but simple
IS based upon information theory. Without it, there would be no radar detection at all. There are plenty of retired engineers who lives comfortably on their houseboats in Florida who spent their entire careers on practically nothing but clutter processing.
Clutter is an enormous challenge in radar detection. In the air, insects and birds are known as 'volumetric' RCS, meaning a flock of birds that created an RCS can do so only if all birds are sufficiently close to each other where radar signals that bounced off each bird interacts with its companions and this constructive interference or amplification create an RCS. If one bird depart, the radar will not pick up this bird and the flock's RCS decreases a little. Same for insects or hydrometeors, fancy word for rain and snow...
Ruoskanen: Tools and Methods for Radar Performance Evaluation and Enhancement, ISBN 951-22-8144-9
For example, for rain clutter volumetric backscattering...
Birds and insects have been known to fool radars as 'angels'...
Detection of Bird Migration by Centimetric Radar-- a Cause of Radar 'Angels'
High-power centimetric radar at times records random scatterings and occasionally dense displays of small blobs of echo, which have been called radar 'angels'.
It is shown that these properties can be satisfactorily explained on the assumption that the echoes are received from birds on migration.
On the ground, vegetation from fields of grass to lush forests create their own volumetric RCS except that their characteristics are predictable, such as swaying in the wind, and that rhythmic motion can be factored in as a discriminant.
Man made clutter such as urban environment are particularly hostile to airborne radars but friendly to 'stealth'.
See this document...
www.ifp.uni-stuttgart.de/publications/phowo07/180Stilla.pdf
Figures 2-9 illustrate the difference between a photograph of an urban environment versus a radar derived imaging. For the radar, the urban environment is filled with 'flashes' and figure 12 explains why: Corner reflectors. That structure have been discussed here before. Any airborne radar looking down at a city would most likely miss a flight of F-35s even if they flew across his radar view.
Correct, that's why a supercomputer could potentially improve LO aircraft detection range.
Bottom line is, the more processing power is available the more easily a LO aircraft could be detected.
As I said earlier, the manned fighter could sit back silently as the backup attacker while interlinked UAVs did the target illumination and attack in conjunction with the radar from AWACS and/or onboard the UAV or perhaps a multi-modal radar.
There is another problem regarding clutter when dealing with 'stealth' aircrafts and that is the signal-to-clutter (SCR) ratio...
Filters for Detection of Small Radar Signals in Clutter | Browse Journal - Journal of Applied Physics
Radar clutter is distinguished from thermal noise by being caused by random reflection of transmitted electromagnetic energy.
Defining the signal‐to‐clutter ratio as the ratio of the peak signal to the rms value of the clutter, the optimum linear filter is derived for enhancing this ratio.
IEEE Xplore - CMP Antenna Array GPR and Signal-to-Clutter Ratio Improvement
CMP Antenna Array GPR and Signal-to-Clutter Ratio Improvement
Xuan Feng; Sato, M.; Yan Zhang; Cai Liu; Fusheng Shi; Yonghui Zhao; Coll. of Geoexploration Sci. & Technol., Jilin Univ., Changchun
Ground-penetrating radar (GPR) is recognized as a promising sensor for detecting buried landmines.
The result shows the signal-to-clutter ratio was dramatically improved.
For the Chinese boys here, once again do note the names of my second source lest you accuse me of using 'biased' sources.
Anyway, clutter is distinct from noise in that noise is internally generated while clutter is based upon external response. Clutter varies with range and direction. Noise is constant except for temperature. Clutter + Noise = Total interference. Somewhere in Clutter is a bunch of F-22s and F-35s. If the radar's avionics is crappy enough its own noise can be greater than clutter, although that possibility is quite remote today. Noise, if recognized, will be suppressed but if clutter level is the same as noise, then both will be suppressed, so the goal is to have avionics that are lower in internal noise than from external clutter. If not possible, then noise must not be suppressed but factored in into the filters, making said filters more complex and increase odds of missing a target.
So against low radar observable targets, two things are important: A flying supercomputer and previous knowledge of what a 'stealth' target look like. The US have both.