Fundamentals of Stealth Design & Concepts of RCS Reduction

[gambit]

stealth can be detected by HF bands radars then what is their point ?

This subject have been discussed before. It is not as if no one knew and am not talking about people on this forum.

In radar detection, NOTHING is 'invisible' and the US never claimed the F-117 and others to be 'invisible'. The issue/problem is the freq and intention combination.

In radar detection, these are the most vital target resolutions:

- Altitude
- Speed
- Heading
- Aspect angle

All freqs can produce these resolutions but the issue/problem is how FINE are they. The higher the freq, the more fine or precise those resolutions OVER TIME. The more precise those resolutions, the better the PREDICTIVE algorithms inside weapons systems. When there is a target traveling at several hundreds km/hr or even at Mach, low freqs like the HF bands are practically no good at providing those resolutions.

There are uses for these low freqs, which is for long range volume search.

- Detection
- Tracking
- Targeting

Detection is simply looking to see if there is anything out there. Low freqs or long wavelengths are useful for this. Wavelengths such as the meters lengths bands.

Tracking is when an object is designated as an 'interest' item. Higher freqs such as bands in the hundreds cm are used.

Targeting is when an object requires constant attention and higher freqs such as the low centimetric bands are used.

So if an HF freq is used to detect 'stealth', you can see it but you cannot shoot at it. What good is it then ?

 

[giger]

guiding missile by a UHF band radar to an approximate address of stealth and then missile use it's own infra red sensors to reach target can be useful .

 

[gambit]

guiding missile by a UHF band radar to an approximate address of stealth and then missile use it's own infra red sensors to reach target can be useful .

Now you are talking about a tactical situation. Such a system would have a large antenna array, high energy output, a few minutes to set up, about an hr or two to run to find anything, and vulnerable to anti radiation missiles. Everything have trade offs.

 

[mil-avia]

Radar cross section measurement facilities / RCS ranges outside of U.S.A. :
BAe Warton, EADS' Manching, SOLANGE, Linkoping, Tver, Voronezh, Beijing & Sao Jose dos Campos.

 

[gambit]

A repeat question was asked and here is the summary of it: What is the final goal of 'stealth' ?

Answer: The final goal of a radar low observable, aka 'stealth', attacker is to reduce the reaction time of the (radar) defender, whether he is ground or airborne.

The above illustrate the radar detection and responses 'balloons' or zones, as some would prefer. An 'attacker' or 'penetrator' could be a civilian airliner against a civilian airport air traffic controller, or a military aircraft on a hostile mission against a defender, and a radar would be at the center of those zones.

For the civilian air traffic controller, an airliner could be passing through the 'Surveillance' zone and all that is required of the transit aircraft is a cooperative ID.

If the target enters the 'Planning and Coordination Time' zone, an ID should be established before time of entry of this zone, and once the target is inside this zone, both sides would be actively working on items such as safe altitude separation with other aircrafts, landing priority, emergency routes, etc.

Once the target enters the '80% Weapon Employment Range' zone, the civilian equivalent would be for the air traffic controller to assign landing queue position, runway assignment, etc.

Then once the target is inside the 'Terminal' zone, essentially the airliner have its landing gears down and ready to land.

For the military version, each zone represent an increasing threat level and appropriate defensive response. The final goal of the 'stealth' attacker is to deny the radar defender the opportunity to formulate any responses appropriate to each zone simply by being 'invisible', to casually use the word, in each zone. Given the laws of physics, once the 'stealth' attacker is inside the 'Terminal' zone, odds are good that he is detected, hopefully not tracked, but then it would be too late for the defender anyway.

The weapons employment zone is a double edge sword. This zone is where both attacker and defender can unload their weapons. For the attacker, he want to release his weapons as far from the defender as possible, which would make it the edge of that zone. For the defender, he want to destroy the attacker at as far away from his position as possible.

 

[Donatello]

A repeat question was asked and here is the summary of it: What is the final goal of 'stealth' ?

Answer: The final goal of a radar low observable, aka 'stealth', attacker is to reduce the reaction time of the (radar) defender, whether he is ground or airborne.

View attachment 148937

The above illustrate the radar detection and responses 'balloons' or zones, as some would prefer. An 'attacker' or 'penetrator' could be a civilian airliner against a civilian airport air traffic controller, or a military aircraft on a hostile mission against a defender, and a radar would be at the center of those zones.

For the civilian air traffic controller, an airliner could be passing through the 'Surveillance' zone and all that is required of the transit aircraft is a cooperative ID.

If the target enters the 'Planning and Coordination Time' zone, an ID should be established before time of entry of this zone, and once the target is inside this zone, both sides would be actively working on items such as safe altitude separation with other aircrafts, landing priority, emergency routes, etc.

Once the target enters the '80% Weapon Employment Range' zone, the civilian equivalent would be for the air traffic controller to assign landing queue position, runway assignment, etc.

Then once the target is inside the 'Terminal' zone, essentially the airliner have its landing gears down and ready to land.

For the military version, each zone represent an increasing threat level and appropriate defensive response. The final goal of the 'stealth' attacker is to deny the radar defender the opportunity to formulate any responses appropriate to each zone simply by being 'invisible', to casually use the word, in each zone. Given the laws of physics, once the 'stealth' attacker is inside the 'Terminal' zone, odds are good that he is detected, hopefully not tracked, but then it would be too late for the defender anyway.

The weapons employment zone is a double edge sword. This zone is where both attacker and defender can unload their weapons. For the attacker, he want to release his weapons as far from the defender as possible, which would make it the edge of that zone. For the defender, he want to destroy the attacker at as far away from his position as possible.

Do the stealth planes, like F-117, B-2, F/22 etc, carry transponders, like in civilian planes, to let their position known in civilian airspace while travelling? I know it is counter intuitive, but a fast moving, agile object in the sky, isn't all safe. So how do they manage the ferry/combat flights?

 

[gambit]

Do the stealth planes, like F-117, B-2, F/22 etc, carry transponders, like in civilian planes, to let their position known in civilian airspace while travelling? I know it is counter intuitive, but a fast moving, agile object in the sky, isn't all safe. So how do they manage the ferry/combat flights?

Not transponders, but a device called a 'luneberg' lens.

radar-reflector.comRCS Radar Cross Section, Lüneberg Reflector lensref - Luneburg radar

The Luneberg reflector significantly increases the Radar Cross Section (RCS) of any system which has little or none at all.

Its Radar Cross Section is several hundred times the RCS of a metallic sphere of same diameter.

The Luneberg reflector gives an homogeneous response inside a wide angle. It is an ideal passive responder, perfect for highlighting, and eventually monitoring the radar target to which it is attached, with a high level of security.

The Luneberg lens is the most efficient passive radar reflector available.

The Luneberg reflector requires no power supply nor maintenance.

The word 'transponder' is a composite of 'transmission' and 'responder'. The device will activate to an electronic query and when triggered by that query, it respond with certain flight information. Airliners carry transponders and they are considered radar 'cooperative' targets.

A radar enhancer like a luneberg lens device is completely passive. The device's internal construction consists of coils of a conductor material precisely shaped to concentrate any quantity of received EM signals and to 'reflect' or 'broadcast' them.

For the F-22, an enhancer is usually attached to the underside...

Under civilian air traffic control, the F-22 would not be classified as a radar cooperative target but as an unidentifed target. The word 'cooperative' in the context of civilian air traffic control includes at least: identification, purpose, and destination. Absent any of them, and the target is considered 'suspicious' and not identified. All three items can be compiled into an automatic response like how airliners does it. Or in the case of an F-22, the controller would verbally query this unidentified target and essentially 'hand written' those three items on the tracking board.

The luneberg lens is used because it is compact and the refocusing ability is better than other EM enhancing devices. The device is often used in bodies that requires radar monitoring.

Tri-Lens Radar Reflectors - Radar Reflectors - Safety & Regulations - Downwind Marine

The Large Tri-Lens is for that sailor who wants the best for his yacht. Designed for use on boats 30 feet and longer, the Large Tri-Lens performance is second to none with its specifications far exceeding the ISO 8729 standards. Similarly sized Luneburg lenses have been used by the U.S. Navy in minesweeping operations in the world's oceans. Navy ships sweep an area, then drop a buoy with a Large Tri-Lens on it. This enables them to see on radar where they have been.

Let us say that you want to radar track your new cruise missile under 'real world' environmental conditions. You could install an enhancer -- of any design -- on your cruise missile. Under your testing regime, you can begin to track the cruise missile's flight under clear sky, through a rain, or through a snowstorm. Any problem and you can real time detect any flight anomaly and that would help narrow your troubleshooting processes as far as meteorological phenomena are concerned. The final test would have you remove the enhancer.

 

[gambit]

I am wondering why landing gear cover needs RCS too? You usually take off from friendly airfield and its only when you enter enemy aerial area then you need RCS which is when you have long retracted the landing gear and the door close.

If you look closely you'll see all the surface joints are at at an angle. Even though the landing gear doors remain shut and flush to the fusealarge, no seal is 100% so all stealth aircraft use serated edges where possible to redirect the refelected beam away from the source.

The question is technically legitimate. Essentially, why are there physically serrated patterning on various panels, most notably on the landing gear access panels. The answer lies in edge diffraction behaviors and those behaviors are mathematically quantifiable. If something is quantifiable, it can be predictable, and if it can be predictable, there will be attempts to control such behaviors.

The landing gears areas are considered frequently access areas, same with the air refueling port, same with the cockpit canopy, and same with the many components under the surface that must be maintenance accessible. As such, the doors covering them must be physically separable from the aircraft, and because they are physically distinct structures, their connections to the aircraft will have physically distinct gaps.

Under edge diffraction behaviors, those gaps are EM liabilities, meaning they will contribute to any quantity of energy that may be received by any seeking radar, which in turn will increase the total RCS value.

The above is just ONE of the many possible edge diffraction behaviors, depending on the approach angle of the radar signal and the physical dimension of the gap. That gap could be from a fastener head or a landing gear door.

Since currently it is not possible to completely eliminate all of these behaviors, the solution is to control them, as in mathematically predict, considering as many variations as possible of how that particular gap may encounter a radar signal, reorient the gap, then physically verify the proposed solution thru actual radar bombardment. With sufficiently powerful computer simulations, only the final step, actual radar bombardment, is required.

However, because the panel is in a physical relationship with the aircraft and that connection have limited travel directions: up/down away from the aircraft or on/off the aircraft, it is not possible to absolutely control all diffraction behaviors off the gaps. If the aircraft just happens to orient itself to the seeking radar in the most fortunate viewing angle, SOME of edge diffracted signals WILL make it back to the seeking radar, serrated patterning does not matter.

 

[gambit]

How legitimate is the often raised question/criticism: 'The Northrop designed YF-23 have a lower radar observability factor, aka 'stealth', than the Lockheed F-22, which the USAF bought.'

Answer: No legitimacy at all.

In the PUBLIC absence of hard information, such as:

- Test pilots' testimonies.
- Program managers' admissions.
- Radar experts commentaries.
- A concessionary statement from Lockheed.

There is no reason to place any level of credibility on the assertion that the Northrop YF-23 is 'stealthier' than the Lockheed F-22.

The above is a graphical representation of how the radar data processor sees ANY body, whether that body is a human body, a box, an automobile, a bird, a raindrop, the Space Shuttle, or even the Moon -- a voltage spike or a cluster of spikes.

Depending on the operating freq, the above could represent a cluster of individual aircrafts flying in a formation. In this case, those spikes represents major physical structures on the L-1011, such as engines, wings, and the fuselage, which is that chain of spikes in the center of the cluster. The highest spike came from the starboard engine, which would mean the seeking radar, that produced this graph, is slightly off center, therefore it sees more of the starboard engine than of the port engine. And it is not that difficult to recognize the rudder and horizontal stabilizers complex physical arrangement common to airliners.

The graph's baseline, or 'floor', is significant. It represents a line where there are data that the radar processor deemed statistically insignificant to display. In radar detection, NOTHING is invisible. The radar sees all. But the more the radar processor is forced to format what it sees and display it for human consumption, not only will the hardware requirement be financially prohibitive, the human would literally be overwhelmed by the amount of data, represents by the common symbol that is the 'blip' on the radar display scope. There would be far too many blips of diverse colors.

So what developed was the 'clutter rejection' threshold. Clutter is not 'bad' data but data that matches a library of levels of voltage spikes that were determined to be inappropriate for the radar's design.

- If the radar is designed for meteorological missions, then the threshold would be lowered enough to detect hydrometeors, a descriptor for water based bodies like raindrops, snowflakes, and even fog.

- If the radar is designed to track bird migration, a scientific mission, the threshold could be at a level to detect individual birds, or at a level to detect only flocks, meaning when there are enough birds flying together to form a reasonably coherent mass, but if one bird leave the flock, that bird would not be detected. This radar would be designed to detect 'volumetric' targets.

- If the radar is designed to track airliners, traffic management, then the threshold would be raised high enough to reject, as in do not display after processing, bodies like birds or flocks of birds, insect clouds, geographical features like tall mountains, and even weather related bodies such as storm clouds. A major airport would have different types of radars designed for different missions. An air traffic controller does not need to see a storm cloud because another specialist can use a specific radar to track local weather and advises the traffic controller of any interest. The controller needs to see only blips that represents aircrafts, their altitudes, their speeds, and their headings.

Most of the world's radars, civilian and military, operates in the centimetric regions, meaning the operating freqs literally have wavelengths measures in centimeters. The X-band is among the most popular because this region offers the most useful target information, or resolutions:

- Altitude
- Airspeed
- Heading
- Aspect angle

The lower the clutter rejection threshold, the more the X-band radar would display -- after processing -- of what it sees. Remember, in radar detection, nothing is 'invisible'. The higher the rejection threshold, the less populated the display will be. In sum, clutter is not 'bad' data but only data that is deemed statistically insignificant and/or mission inappropriate for display.

What 'stealth' does is to insert the aircraft into the clutter rejection threshold that is common to most radars, civilian and military. In other words, if this threshold is lowered enough, the display would be so populated that the human operator would have a difficult and long time to distinguish any target of importance. The 'stealth' aircraft would be among flocks of birds and/or clouds of insects and/or storm clouds. The threshold would have to raised in order to be useful according to mission intentions, which leads back to what 'stealth' is supposed to do in the first place.

From this perspective, Northrop and Lockheed succeeded with their respective products. If the clutter rejected region is examined, both aircrafts would be 'visible', in a manner of speaking, and it would be possible to say definitively which is 'stealthier' than the other. This is where hard radar measurement data is necessary because inside the amount of data are the variety of clutter rejection thresholds (plural) and the electronic locations of both aircrafts in these regions.

It is possible that even when the clutter threshold have been lowered enough to detect insects and tree leaves, both aircrafts are even lowered than what insects and leaves returned, in other words, the voltage spikes that are the YF-23 and F-22 are even less than the spikes that came from insect clouds and trees. But it is unlikely that the USAF, Northrop, and Lockheed will release these measurement data -- for a very long time.

So then it begs the question of why is there a need to criticize the purchase of the F-22 over the YF-23's alleged lower radar observability -- ALLEGED ?

Answer: The need to criticize is irrational because both aircrafts are already automatically rejected by %90 of the world's radar systems, civilian and military.

Even if the YF-23 is 10+ decibels 'stealthier' than the F-22, it would still be irrelevant because if the clutter threshold is lowered enough to display the F-22 while the YF-23 remains masked, the human operator could be looking at the F-22 among the mass of blips on his display and not recognize the aircraft.

So until there are PUBLIC sources of credible hard data about the testing regimes of both aircrafts, anyone who claimed to know definitively either way is essentially full of shit.

 

[gambit]

A question was asked on how bad is the corner reflector structure in trying to design a low radar observable figher.

A: Really bad.

Actually, really really really really really really bad. Almost evil.

The primary rules for designing a low radar observable body are control of:

- Quantity of radiators
- Array of radiators
- Modes of radiation

The rules regarding the corner reflector are:

- Avoid the corner reflector
- If not possible, avoid the 90 deg type.

The corner reflector is a complex structure and as with any complex structure, there will be complex multi-facets reflections. Often it is said that if something is quantifiable and predictable, it can be controlled. While these reflections are mathematically quantifiable and predictable, aerodynamic necessities often prevent control methods. The most obvious corner reflector structure on any aircraft is the vertical-horizontal stabilators configuration and usually it is of the 90 deg type.

Consider the typical right angle, aka 90 deg, example below.

The structure can be broken down into discrete components, which for simplicity's sake, there are five. Upon radar bombardment each component becomes radiator R. Whether the structure is simulated measured or physically measured, the math of the interactions among radiators must be outlined.

For the above illustration...

The first example, in order of occurrence of radiation, could be 'R1R5' or 'R5R1'. Double edge diffractions.

The second example could be 'R2R4' or 'R4R2'. Double reflections.

The third example could be 'R5R1R3' or 'R3R1R5'. Double edge diffractions and single reflection. This example involves surface wave that travels from the single point of an edge diffraction to the corner of the two plates. Surface traveling waves have their own radiation patterns that will reflect off any nearby structure, but for simplicity's sake, those signals are not counted in this example.

The combinations of radiators interacting with each other will be compounded exponentially if the structure is rotated in a single axis, and even greater if the structure is in motion in 3D space, due to different angle of approaches to each component.

Knowing the order of occurrences is necessary. For example, if the radar threat is not going to affect the entire corner reflector structure, then perhaps absorber treatment is needed only at strategic locations instead of treatment for all the individual components. Another reason why is that different modes of radiation require different predictive/measurement techniques such as Physical Optics (PO) or Geometrical Optics (GO) or Methods of Moments (MOM). Knowning the exact characteristics of a particular signal may result in destructive interference which reduces contribution to final RCS.

This example does not factor in polarizations, whose signals will change upon reflection and/or diffraction, which will affect final RCS.

The complex interactions between corner reflector components are why missiles and bombs must be enclosed. A cluster of munitions is composed of many corner reflectors in close proximity with each other, making their contribution to final RCS the equivalent of holding a torch at night.

For the above example of a cluster of JDAM GBU30, each bomb have a tail assembly that contains 4 corner reflectors. Bomb no. 1 could be B1. The first corner reflector could be B1C1, then for the second corner reflector it would be B1C2, and so on. The naming convention must be uniform throughout the cluster. So for bomb no. 1 with the first corner reflector, a simplfied expression for an event that contains multiple diffractions and/or reflections could be: B1C1R2R4 or B1C1(R5R1R4R2) or B1(C1[R5R1R4R2]). Note the brackets and parentheses.

For bomb no.2 with the second corner reflector, one possible expression could be: B2(C2(R2R3R5R1)).

Because there is a cluster of corner reflectors, there would be an expression for interactions similar to this: B1(C1[B2(C2)]). This mean bomb 1 corner reflector 1 interacts with signals from bomb 2 corner reflector 2. There are no details of any signals from any radiator component (R) from any corner reflector. This is either an extremely coarse signal expression or a summary of interactions after all the radiator components have been factored. Not all corner reflectors faces each other so attention must be paid on how the individual bombs are arrayed in the cluster.

For the math and presumably at least one supercomputer that are being used to estimate the bomb cluster's own RCS, the more detailed (longer) the chain of radiators involved, the greater the precision of that estimation. There are no standardized expression formats simply because this is not a common thing to do in the industry. There are trade secrets, military secrets, and national security considerations in play. Diffraction points can be odd numbered 1, 3, 5, 7, and so on. Reflection surfaces or plates can be even numbered 2, 4, 6, 8, and so on. It is possible to even ignore the sources, which bomb and which corner reflector, of these signals and simply assign odd/even numbers to all known sources and go from there. But no matter the convention, whoever custom designed the math must be consistent and if a computer is used, the predictive/measurement software must be modified to use the custom math.

When the F-117 was under design, a computer with the performance of today's department store consumer personal computer was a luxury confined to engineering team leaders and project managers. RCS estimation of complex structures, from major flight control surfaces to access panel alignments to cockpit area, were done with long hand math much more complex and most engineers had their mechanical slide rulers.

Again...Keep in mind that this example is grossly simplified but even so, it already revealed the initial complexity produced by first-order reflections and first-order diffractions, not yet second- and third-, and certainly not a cluster of corner reflectors like in a bomb rack.

Currently, fighter designs need the vertical stabilator. Designs that have a single vertical stab will have the worst possible 'array of radiators' (rule 2) of the primary rules for designing a low radar observable body and of the secondary rules concerning corner reflectors in terms of contribution to final RCS. That is why for designs that must have the vertical stab for yaw axis stability and control, twin canted vertical stabs are used and they produce acute -- less than 90 deg -- angles with the rear horizontal stabs. Acute angles from canted vertical twin stabs greatly reduces the corner reflector's contribution to final RCS, but does not eliminate such contribution.

For the above F-22 profile, the rear horizontal stabs are not visible, but they are there. A single vertical stab produces two right angles. Twin canted vertical stabs produces three angles: two acute and one large obtuse. Initial assumption would be that since they are not right angles, the efforts to estimate their contribution to final RCS are not as important as right angles. This assumption is wrong.

Taken from the beginning of this explanation, two rules must be considered:

- Control of QUANTITY of radiators.
- Avoid the corner reflector.

Twin canted vertical stabs do not so much 'violate' those rules as they are LESS OBEDIENT to them.

An aircraft is a finite body that is also an assembly of many smaller finite bodies. A radar signal cannot stay on a finite body forever, hence there is the first-order diffraction signal off the edges of a finite body. Twin canted vertical stabs increases the quantity of radiators, which increases the level of first-order diffraction signals. A radar signal is not as straight as a simple arrow often used in illustrations, rather, a radar signal is a cone that expands in relation to distance, aka 'beam spreading'.

JetStream MAX - Doppler Radar Beams

The width of the beam expands at a rate of almost 1000 feet for every 10 miles of travel. At 30 miles from the radar, the beam is approximately 3,000 feet wide. At 60 miles, the beam is about 6,000 feet wide. At 120 miles the beam is nearly 12,000 feet or over two miles wide.

A weather radar beam is not designed/shaped/sharpened like a radar in a fighter jet but the core effect of beam spreading over distance is still the same. A radar beam is also NOT unitary but instead composed of many lobes. A radar beam can be visualized/graphed and it would look like this...

The center/main lobe is where the seeking radar derive most of its information about the target. The side lobes are where corner reflectors with non-right angles can still contribute to final RCS. Their angles of approaches are different from the main lobe, hence they deserve their own first-, second-, and third- orders calculations and estimation. In other words, for a single corner reflector of ANY angle, each lobe inside a radar beam warrant own investigation. Try to plug the main lobe somewhere into this expression: B1(C2(R2R3R5R1)). Do the same for all the other lobes. Then keep it uniform throughout the computer software. If there is no solid chain of communication between the design engineer and the measurement engineer, then the potential for an erroneous design increases.

This is why the first rule regarding the corner reflector is: Avoid the corner reflector. Twin canted vertical stabs are much more preferable than single vertical stab, but their physical attributes such as shape, dimensions, material, and arrangement have direct effects on their status as radiators and as such, they need equal diligence as the single vertical stab configuration. An obtuse (greater than 90) angle have the least RCS, an acute (less than 90) have slightly higher RCS, and the right (exactly 90) angle have the highest RCS.

The B-2 is of a flying wing design. The first generation flying wings, such as the Northrop YB-49, has vertical stabs for yaw axis stability and control. Because of advances in avionics, the B-2 is without the vertical stabs. So as far as major structures goes, the B-2 is the most obedient to the rule: Control of QUANTITY of radiators. And that despite its size, its final RCS is so small that it threatens the efficacy of just about every air defense radars on the market.

In sum, the corner reflector is almost an evil in trying to design a radar low observable aircraft. It is aerodynamically necessary in many cases, such as the vertical/horizontal stabs arrangement, can be contained such as weapons enclosures, or hopefully avoided completely like with the B-2. Final note, this example is of a dihedral corner reflector typically found in major visual clues such as the vertical-horizontal stabs configuration. The trihedral corner reflector can be found on fuselage structure connection points. They may be much smaller than the vertical-horizontal stab configuration, but if allow to exist, to a seeking radar, they are are like flying with small lights.

 

[ynm]

Hi gambit,

Why is there a hype that long wavelength radar can detect stealth aircraft?

 

[mil-avia]

Analysis: Expectations for deployment of fifth-generation fighters

 

[IrbiS]

New Radars, IRST Strengthen Stealth-Detection Claims
Counterstealth technologies near service worldwide

Mar 16, 2015 Bill Sweetman Aviation Week & Space Technology




Counterstealth technologies, intended to reduce the effectiveness of radar cross-section (RCS) reduction measures, are proliferating worldwide. Since 2013, multiple new programs have been revealed, producers of radar and infrared search and track (IRST) systems have been more ready to claim counterstealth capability, and some operators—notably the U.S. Navy—have openly conceded that stealth technology is being challenged.

These new systems are designed from the outset for sensor fusion—when different sensors detect and track the same target, the track and identification data are merged automatically. This is intended to overcome a critical problem in engaging stealth targets: Even if the target is detected, the “kill chain” by which a target is tracked, identified and engaged by a weapon can still be broken if any sensor in the chain cannot pick the target up.



Along with the multi-radar, truck-mounted 55Zh6M, NNIRT is offering the trailered, single-unit 55Zh6UME with VHF and UHF antennas mounted back-to-back. Credit: Bill Sweetman/AW&ST


The fact that some stealth configurations may be much less effective against very-high-frequency (VHF) radars than against higher-frequency systems is a matter of electromagnetic physics. A declassified 1985 CIA report correctly predicted that the Soviet Union’s first major counterstealth effort would be to develop new VHF radars that would reduce the disadvantages of long wavelengths: lack of mobility, poor resolution and susceptibility to clutter. Despite the breakup of the Soviet Union, the 55Zh6UE Nebo-U, designed by the Nizhny-Novgorod Research Institute of Radio Engineering (NNIIRT), entered service in the 1990s as the first three-dimensional Russian VHF radar. NNIRT subsequently prototyped the first VHF active electronically scanned array (AESA) systems.

VHF AESA technology has entered production as part of the 55Zh6M Nebo-M multiband radar complex, which passed State tests in 2011 and is in production for Russian air defense forces against a 100-system order. The Nebo-M includes three truck-mounted radar systems, all of them -AESAs: the VHF RLM-M, the RLM-D in L-band (UHF) and the S/X-band RLM-S. (Russian documentation describes them as metric, decimetric and centimetric—that is, each differs from the next by an order of magnitude in frequency.) Each of the radars is equipped with the Orientir location system, comprising three Glonass satellite navigation receivers on a fixed frame, and they are connected via wireless or cable datalink to a ground control vehicle.

One of the classic drawbacks of VHF is slow scan rate. With the RLM-M, electronic scanning is superimposed on mechanical scanning. The radar can scan a 120-deg. sector mechanically, maintaining continuous track through all but the outer 15-deg. sectors. Within the scan area, the scan is virtually instantaneous, allowing energy to be focused on any possible target. It retains the basic advantages of VHF: NNIRT says that the Chinese DF-15 short-range ballistic missile has a 0.002 m2 RCS in X-band, but is 0.6 m2 in VHF.

The principle behind Nebo-M is the fusion of data from the three radars to create a robust kill chain. The VHF system performs initial detection and cues the UHF radar, which in turn can cue the X-band RLM-S. The Orientir system provides accurate azimuth data (which Glonass/GPS on its own does not support), and makes it possible for the three signals to be combined into a single target picture.

The higher-frequency radars are more accurate than VHF, and can concentrate energy on a target to make successful detection and tracking more likely. Using “stop and stare” modes, where the antenna rotation stops and the radar scans electronically over a 90-deg. sector, puts four times as much energy on target as continuous rotation and increases range by 40%.

Saab’s work on its new Giraffe 4A/8A S-band radars points to ways in which AESA technology and advanced processing improve high-band performance against small targets. Module technology is important, maximizing the AESA’s advantages in terms of signal-to-noise ratio. The goal is signal “purity” where most of the energy is concentrated close to the nominal design frequency, which makes it possible to detect very small Doppler shifts in returns from moving targets.

New processing technologies include “multiple hypothesis” tracking in which weak returns are analyzed over time and either declared as tracks or discarded based on their behavior. China is taking a similar approach to Russia, as seen at last November’s Zhuhai air show. Newcomers included the JY-27A Skywatch-V, a large-scale VHF AESA closely comparable to Russia’s RLM-M, developed by East China Research Institute of Electronic Engineering (Ecriee), part of the China Electronics Technology Corp. (CTEC). Two alternative UHF AESAs and a YLC-2V S-band passive electronically scanned array radar were also on show.



The CETC JY-27A Skywatch-V, China’s first VHF AESA, is in production for Chinese air defense units. Credit: Bill Sweetman/AW&ST


CETC exhibits indicated a focus on combining active and passive detection systems, including the flight-line display of a large-area directional, wideband passive receiver system identified as YLC-20. It appears to be used as an adjunct to the CETC DWL-002, which is a three-station passive coherent location (PCL) system similar to the Czech ERA Vera series, using time difference of arrival processing to locate and track targets. Also shown on a wall chart was the JY-50 “passive radar,” which operates in the VHF band.

Previous PCL systems, including Vera, are designed to exploit active emissions from the target. However, by teaming PCL and other passive receivers with active radars, the defender creates bistatic and multistatic detection systems, which may reduce the effectiveness of RCS-reduction measures that are primarily monostatic. For instance, highly swept leading edges are designed to deflect radar signals away from the source, but can create spikes detectable by multistatic systems.

Older and smaller VHF radars such as the NNIRTI’s 1970s-era P-18 are being upgraded by at least five teams: Retia in Czech Republic, Arzenal in Hungary, Ukraine’s Aerotechnica, and organizations in Belorussia and Russia. The -Chinese navy has retained VHF radar on its newest air warfare destroyers such as the Type 52C Luyang II and Type 52D Luyang III. The possibility of a more modern VHF radar appearing on the new, larger Type 055 destroyer cannot be ruled out.

The challenge to stealth posed by lower-frequency radars and other detection means has been acknowledged at higher levels since 2013. U.S. chief of naval operations Adm. Jonathan Greenert has publicly expressed doubt as to whether stealth platforms constitute a complete answer to the developing anti-access/area-denial (A2/D2) threat, and a January 2014 paper by the Center for a New American Security noted, “One recent analysis argued that there has been a revolution in detecting aircraft with low RCS, while there have not been commensurate enhancements in stealth.”

Boeing has promoted the EA-18G Growler’s ability to jam in the VHF band, which is built into the current ALQ-99 low-band pod configuration (the most modern part of the system) and the planned Increment 2 of the Next Generation Jammer system. Increment 2 will likely comprise an upgrade to the current pod—the best solution to emerge from an analysis of alternatives conducted in 2012. A contract should be issued in 2017 with initial operational capability in 2024.

A different kind of radar threat is the very-long-wave over-the-horizon (OTH) radar, typified by Australia’s Jindalee OTH Radar Network (JORN), Russia’s Rezonans-NE, and China’s OTH systems. Again, processing is the key to increasing the accuracy and sensitivity of these systems, typified by the Phase 5 upgrade to JORN.

OTH long-wave radars are inherently “counterstealth” because at very long wavelengths that are close to the physical size of the target, conventional radar cross-section measurement and reduction techniques do not apply. Claims by Jindalee’s original designers that the radar could detect the B-2 were published in the late 1980s and were taken seriously by the U.S. Air Force. At the time, however, the service could argue that OTH’s resolution was so poor that it could not represent the start of a kill chain. Today, however, that low resolution can be mitigated by networking multiple radars, and by using OTH-B to cue high-resolution sensors.

Outside the radio-frequency band, the U.S. Air Force (AW&ST Sept. 22, 2014, p. 42) is the latest convert to the capabilities of IRST. The U.S. Navy’s IRST for the Super Hornet, installed in a modified centerline fuel tank, was approved for low-rate initial production in February, following 2014 tests of an engineering development model system, and the Block I version is due to reach initial operational capability in fiscal 2018. Block I uses the same Lockheed Martin infrared receiver—optics and front end—as is used on F-15Ks in Korea and F-15SGs in Singapore. This subsystem is, in turn, derived from the IRST that was designed in the 1980s for the F-14D.

While the Pentagon’s director of operational test and engineering criticized the Navy system’s track quality, it has clearly impressed the Air Force enough to overcome its long lack of interest in IRST. The Air Force has also gained experience via its F-16 Aggressor units, which have been flying with IRST pods since 2013. The Navy plans to acquire only 60 Block I sensors, followed by 110 Block II systems with a new front end.

The bulk of Western IRST experience is held by Selex-ES, which is the lead contractor on the Typhoon’s Pirate IRST and the supplier of the Skyward-G for Gripen. In the past year, Selex has claimed openly that its IRSTs have been able to detect and track low-RCS targets at subsonic speeds, due to skin friction, heat radiating through the skin from the engine, and the exhaust plume. The U.S. Navy’s Greenert underscored this point in Washington in early February, saying that “if something moves fast through the air, disrupts molecules and puts out heat . . . it’s going to be detectable.”

These detection improvements do not mean the end of stealth, in the view of most industry and government sources, but they do underlie current plans and discussions for the future applications of RCS-reduction and other stealth-related technologies. For example, the long debate over the appropriate level of stealth technology for the U.S. Navy’s Unmanned Carrier-Launched Airborne Surveillance and Strike program has revolved around the development of A2/AD threats. The result is the end of a decades-long misapprehension, widely held in professional as well as public circles, that there is no major difference in stealth performance among various low-observable designs.

 

[SipahSalar]

I heard pierre sprey saying that even WW2 radars can detect stealth fighters, they just have short rangep; but he says Russians have been working on improving them day and night.

So is it true or just a myth or a bit of both?

 

[gambit]

I heard pierre sprey saying that even WW2 radars can detect stealth fighters, they just have short rangep; but he says Russians have been working on improving them day and night.

So is it true or just a myth or a bit of both?

Both.

The lower freqs, aka 'long wavelengths', have been known to detect 'steatlh', but it is myth that said detection is tactically good enough. Any improvement lies in data analyses and not in the physical structure of the radar system itself. The system would still be large and not as mobile as the more popular X-band systems.

Long wavelengths systems are no guarantees against 'stealth'. If they are, Russia, China, and several countries would not work hard in trying to develop their own 'stealth' platforms. Because of the fact that these are meters length pulses, these systems are powerful and as such, quite detectable by warning receivers. These systems will themselves be detected before they can detect any 'stealth' attackers who are aware of their presence.