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RCS OF Different Fighters

A low frontal RCS is important but how can it be achieved with a big radar antenna in the nose radome which is transparent to radar waves? While special techniques have been adopted for stealth fighters, others still lack a low frontal RCS. Sometimes it is shown that AESA radars antennae are mounted at an angle in the nose to reduce frontal RCS but it still not sufficient to achieve low-observability in front.

A different approach to reduce frontal RCS seems to have been adopted in YF-23 and Boeing X-32 with very slender noses which apparently can't house a medium-sized airborne radar. It is apparent in the below shots,

dvd-17-01.jpg


YF-23_side.jpg



To reduce frontal RCS, it is logical that adversary's radar waves should not enter into the nose radome and get reflected back from a flat plate like the antenna of an airborne radar.
 
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Is such a huge nose a requirement with the upcoming AESA systems??
 
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Is such a huge nose a requirement with the upcoming AESA systems??

Thats the point. A huge nose is not at all required with AESA or even Phased Array Antennae with electronic scanning of the radar beam. Lets go a little backwards in 1970s when it was necessary to house a radar with its antenna in the nose of a fighter. The nose radome being transparent to radar waves, it gave a good frontal RCS.

Then came the electronic scanning of Phased Array radars, which rendered useless the mechanical scanning antennae. A good example is the radar of the Patriot SAM battery.

Now the problem that aircraft designers face is that they need a radar in the aircraft with boresight as the preferred direction BUT they want to reduce the frontal RCS too. Reduction of RCS means that radar waves of adversary should not enter the nose section if radar is housed there with its antenna. So what are the options?

Electronic scanning gives new options and logical solution would be to make the nose cone (or part of it) itself an antenna of the radar instead of housing the antenna inside the cone. In these cases, special care must be taken to protect nose cone from weather conditions etc as it is also the antenna. As the nose cone is antenna, it is NOT transparent to the radar waves and your minimum frontal RCS dream is realized.

But thats not all. Electronic scanning enables the placement of sideways looking radars in the aircrafts and even backwards looking to cover the rear.
 
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An exchange of exhaust shaping goes this way...

Is the design stealthy? Anyone?
I don't think so. You need saw-toothed edges to redirect away the radar energy. The J-20, F-22, and F-35 all have saw-toothed edges on the engine nozzles and the engine pod interface with the fuselage.
Nope. It's round.

The correct answer is not because "It's round." but because of its lack of edge diffraction control measures.

To return to the most basic radar signal behaviors...

direct_sing_refl.jpg


direct_corner_refl.jpg


Two structures or even lines joined to create a 90 deg corner reflector is to be avoided. However, any corner reflector that is either greater or less than 90 deg will reduce, not completely eliminate, the amount of diffracted energy back to the seeking radar.

f-35_j-20_exh.jpg


In the above image, if a radar is looking straight at the exhausts of both aircrafts, no RCS control measures other than active cancellation will save the aircrafts. The seeking radar will be looking directly at the engines themselves.

However, if the image is seen as from a radar off angle view, then those corner reflectors, which can be safely assume not to be 90 deg, will be effective in reducing the amount of diffraction energy back to the seeking radar.
 
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This is a supplement to post 105 => http://www.defence.pk/forums/military-aviation/20908-rcs-different-fighters-7.html#post2107861

An issue was raised that given the range of frequencies that can be used for radar detection, from mhz meters length HF/VHF/UHF to the ghz centimetric and millimetric bands, and given that active cancellation is still confined to the laboratory, what bands should be targeted by RCS control methods. Post 105 gave X as the targeted band and did it in principle. This supplement will explain why in the engineering.

It is known that there are several characteristics of a radar transmission that directly affect the quality of target resolutions, the main ones are:

- Beamwidth
- Antenna shape
- Antenna dimension
- Frequency
- Power

The last item 'Power' affect distance. The other items have complex relationships to each other.

radar_resol_cell.jpg


Definition: radar resolution cell
radar resolution cell: The volume of space that is occupied by a radar pulse and that is determined by the pulse duration and the horizontal and vertical beamwidths of the transmitting radar. Note: The radar cannot distinguish between two separate objects that lie within the same resolution cell.

The above illustration is about radar resolution cell. In most cases, a narrow beamwidth in both the horizontal and vertical planes are desirable, as in the left situation. When volume search is the mission, the wider the beam the greater the volume that can be scanned per sweep, however, it will be at the expense of target discrimination in a multiple targets environment.

Repair and Maintenance Manuals - Integrated Publishing
Beamwidth varies directly withwavelength and inversely with antenna size. Radarsystems that produce relatively small beam widths generally provide greater target resolution.

Basically, the larger the antenna the smaller the beamwidth, however, the lower the frequency employed, the larger the beamwidth for the same antenna.

Another consideration is beam broadening.

RADAR BEAM CHARACTERISTICS
As pulses travel away from the antenna, the beam takes on a cone-like appearance and expands in all directions. This expansion or beam broadening increases pulse volume, resulting in decreased signal strength (fig. 2-11). Distant targets appear distorted, in fact, they may not be seen at all. Beam broadening also causes "partial beam filling," which implies that distant targets occupy proportionally less of anexpanded beam. Thus, the true characteristics of a target may be hidden or altered during display.

Beam broadening reduces azimuthal resolutionand produces a form of radar nearsightedness. As the beam diameter increases with distance, closely spaced targets may occupy the beam simultaneously and appear as one echo. In short, multiple targets at a distance are difficult to see correctly.

What this mean -- so far -- is that for a 'fighter' class aircraft where internal volume is already limited, the X-band proved to be the most useful in terms of beamwidth for superior target discrimination in a multiple targets environment.

Radar Cross Section
Raleigh region. If the target is a lot smaller than the wavelength of the radar system, the target is said to be in the Raleigh region. If the target is in the Raleigh region, the radar cross section of the target tends to be smaller than the target's physical size.

Resonance region. If the target is of similar dimension to that of the wavelength, the target is said to be in the resonance region. In the resonance region, the radar cross section of the target may vary a great deal but tends to be larger than the physical size of the target.

Optical region. The optical region occurs when the target is much larger than the operating wavelength of the radar. This is quite often the case with operational radar systems whose wavelengths are normally in the order of centimetres in length. When operating in this region, the radar cross section of the target is similar to its physical size.
The X-band is centimetric (cm) which will have any target in the 'Resonance' or 'Optical' region. An aircraft is meters in length and wingspan. In order to place an aircraft into the 'Raleigh' region, the wavelength would have to be in the mhz meters length HF/VHF/UHF bands. The F-15E antenna is 0.9 m in diameter. If this antenna transmit in the mhz bands, the beamwidth would be so large -- double digits of degrees -- that it would be worthless, whereas the more desirable beamwidth is between 1-5 deg.

Diameter of a Raindrop
"A raindrop may have a maximum diameter of 0.25 centimeter."

"Raindrops generally have a diameter greater than 0.5 mm (0.02 in.). They range in size up to about 3 mm (about 0.13 in.) in diameter."

"The 4 mm maximum diameter of raindrops probably results because raindrops larger than this size tend to break up when colliding with other large raindrops."
The higher the freq the more vulnerable to atmospheric attenuation (loss) and the example above is the reason why. The X-band is centimetric and when a pulse encounter a raindrop the raindrop will be in the 'Raleigh' region. However, if a millimetric freq is used, when this millimetric pulse encounter a raindrop the raindrop will be in either the 'Resonance' or 'Optical' region, resulting in a clutter display of weather phenonmena instead of other intended targets such as armed fighters and bombers.

Add all these factors together and there are those complex relationships that system engineers must take into consideration when designing a system for a specific mission. This is why the X-band proved to be the most useful for the 'fighter' class aircrafts and because of this usefulness, RCS shaping must target this particular threat freq.

These factors and their complex relationships are also the reason why AWACS antennas are relatively 'flat' disks that they are: Antenna shape. A 'vertical' fan is from an antenna shape that is 'long'. A 'horizontal' fan is from an antenna shape that is 'tall'. Basically, the fan is always the opposite of the antenna's orientation and each orientation has its purpose. A 'horizontal' fan sweeps in an up-down motion and is useful as a height finding radar. A 'vertical' fan sweeps either side-side or 360 deg is useful in finding targets' locations in respect to one's own position. The wider the fan the more volume is can cover per sweep cycle so a height finding radar can also find target positions but generally if target position and volume search are the goals then a 'vertical' fan is preferred because of the 360 deg ability with the benefit of altitude information per revolution.
 
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original post -ANTIBODY


one thing that i speculated [maybe wrongly] is the due to the sheer size of the mki , its kill probabilty distance/ no escape zone would be more and would favour sd10 coupled with jft -- so even even the 2 fighters detect each other at the same time.... even if the 2 missiles have same range ... sd10 might be fired earlier by the pilot with relatively better confidence of the firing pilot[not necessarily from the longest possible distance]

in this case please dont take into consideration the jammers etc ... as i want to know who will be able to fire first with better possible results

kind regards



via nabil --the first jft prototype had a frontal rcs of 2.6 m2
Current jft has much less
RCS OF Different Fighters

via chogy --Just eyeballing the airframe... as others have said, I would place it in the F-16 class,
RCS OF Different Fighters

The[dsi] intake reduces one of the three major forward scatters of an aircraft that typically represents between 30%-35% of the RCS of an aircraft.
http://www.grandestrategy.com/2007/1...for-third.html



my questions are -- can the rcs estimate be made roughly by just looking at the fighter? if yes where would you place the rcs of jft? whats is the direct/indirect effects of dsi on rcs reduction , if any


ive asked you questions for 2 different threads now .. in the earlier PM for the ''radar ranges of different fighters'' thread .. amd now for the ''rcs of different fighters'' thread

I am anxoiusly waiting for your replies
kind regards


gambit said:
You took several important variables out of the picture. Nothing wrong with that because from my experience, we do/did that all the time by either altering the physical structures of the targets or by 'handicapping' the radar via software if we cannot alter the physical structures.

If you sort of 'equalize' the fighters in every way, as highlighted below, then the burden of the kill falls ENTIRELY upon the weapon. We have done this in the past when we are faced with physically dissimilar 'adversaries' but we must 'equalize' them somehow. The most commonly used technique is to install radar enhancer on the smaller body to where the estimated RCS is within 5% of its adversary's RCS. To 'hack' a radar's software involved too much time, possible security breaches and worst of all -- copyright related crap.

Anyway...If two fighters detect each other at the same time, and even if one shoot later than his opponent, assuming both fighters know full well the range capability of his missile, then it depends on missile's sophistication such as g-rating, the type of flight controls system, fuel formulation and shapes because they affect thrust and burn duration, missile guidance avionics...

Aerospaceweb.org | Ask Us - Missile Control Systems
NASA Quest > Space Team Online

...In short, everything that we discuss about manned fighters, you can transfer to the missiles because a missile IS an aircraft that have a higher performance envelope because it does not have to worry about keeping a human alive.

Here is the problem for your scenario...If one fighter is physically larger than the other, does that mean it has a larger RCS as well? Not necessarily. Even a B-52 can have an RCS of a bird, but at a very far distance. The problem is that if both fighters detect each other at the same time despite being physically dissimilar, it mean both have the same RCS -- FROM THEIR RADARS' PERSPECTIVES. An RCS value depends on the transmitting radar's signal and data processing sophistication. It mean a physically smaller body can have the same RCS as the larger body at the same distance because radar sophistication varies widely between manufacturers.

If two fighters of physically dissimilar sizes detect each other at the same time, then it mean the larger fighter have the superior radar system to compensate for its larger physical dimensions. If we assume that both fighters have the same radar sophistication -- no matter what -- then your scenario is impossible. The larger fighter will be detected first and will die first REGARDLESS OF MISSILE SOPHISTICATION.

Let us use 1m2 at 100km distance for example. If both fighters that are physically dissimilar detect each other at the same time, it mean both fighters have the same RCS of 1m2 according to their respective radars' sophistication at 100 km distance. Get it?

It mean the smaller fighter have an inferior radar because if it have the same level of sophistication, it should have detected the physically larger fighter at 120 or even 150 km distance without itself being detected. In other words, assuming if both fighters have the same radar sophistication, the larger fighter would be 1m2 at 150 km distance while the smaller fighter would be 1m2 at 100 km distance. Who would die first? The larger fighter.

For your scenario that have an engagement between physically dissimilar fighters where both detect each other AT THE SAME TIME the smaller fighter must have an inferior radar, and if both shoot at roughly the same time, then it depends entirely upon missile sophistication for the kill.

The reason why I often say '150-200' km distance for 1m2 RCS is precisely because of variations in radar sophistication. That 50 km distance variable is a terrible figure but it is the truth about the industry. That figure is about the distance for several missiles so you can see how important it is to gain even just 5 km of further out detection distance.


Is it possible to have even a rough RCS value guesstimate for any fighter? No.

But...You can place it in the same class -- base upon 'eyeballing' -- as long as you have a reasonably accurate RCS value from one or several aircrafts that set the standard for that class. The clean F-16 pretty much set the bar for 'stealth', meaning you must get below 1m2 at 150-200 km distance in order to be a credible 'stealthy' threat. So can you say 2.85m2 at 121.8 km distance based upon pure eyeballing? No.

Personally, I would place the JF into the F-16 class based upon what I personally know about the F-16's RCS and based upon my 'eyeballing' the JF.

I do not know how your friend had this 2.6 figure 'confirmed' to him. Absent assurance on how this figure came to be, such as if it was measured in isolation as in enclosed anechoic EM chamber, that decimal level of precision is dubious.

Same for the RCS reduction value of the DSI structure. Each DSI structure must be carefully custom tailored for the aircraft out of aerodynamic demands and because of that, its purported RCS reduction or RCS contributorship compare to the diverter plate is difficult to assess in regard to that percentage you cited. I mean...Were there measurements on the design that have the diverter plate assembly? If the design never intended to have the diverter plate in the first place, then how credible is that 30-35% figure?

That does not mean the DSI structure is not beneficial in trying to effect RCS contributorships from diverse structures on as complex a body like an aircraft. It is beneficial because you want to have as low a contributorship FOR EACH structure as possible. On the other hand, if there is one or if there are several large contributorships from several different structures that utterly dominate measurements then it is pointless to debate on whether to install the diverter plate or the DSI structure.

To sum it up...It is reasonable to presume a 'class' but not reasonable to declare a value, and if said declaration involve a decimal point, time to be suspicious.


just thought to share some info -- credits to gambit


 
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A low frontal RCS is important but how can it be achieved with a big radar antenna in the nose radome which is transparent to radar waves? While special techniques have been adopted for stealth fighters, others still lack a low frontal RCS. Sometimes it is shown that AESA radars antennae are mounted at an angle in the nose to reduce frontal RCS but it still not sufficient to achieve low-observability in front.

A different approach to reduce frontal RCS seems to have been adopted in YF-23 and Boeing X-32 with very slender noses which apparently can't house a medium-sized airborne radar. It is apparent in the below shots,

To reduce frontal RCS, it is logical that adversary's radar waves should not enter into the nose radome and get reflected back from a flat plate like the antenna of an airborne radar.

FSS - frequency selective surfaces is the answer. But still some compromises will be made. Some links shared by Gambit in past on FSS

Antenna System Utilizing A Frequency Selective Surface
ScienceDirect.com - Composite Structures - Nanocomposite stealth radomes with frequency selective surfaces
 
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The frontal RCS of steakth aircraft doesnt contain any of the 5 long sharp peaks.

rcs.jpg
 
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radar waves impacting an aircraft. a part of the signal is lost, some of it comes back.
radar_return_mechs.jpg
 
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No matter what and how the RCS is removed, it will some how stay. Whether one makes nose as antenna itself or the edges are lover than 90 or greater than 90 degrees. The radar technology has become very advance. They can even detect a small photon from miles RCS is a cake walk.
 
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