RCS OF Different Fighters

[shehbazi2001]

Now that we have discussed the RCS and radar reflections, its about time to discuss a peculiarity of F-16. F-16 is the front line fighter of Pakistan AF and before using it against a high-tech adversary, it is important to investigate the radar reflections from F-16 external fuel tank of 370 gallons capacity.

F-16's external fuel tank has a flat plate at the end and it does not taper to a conical shape. This flat plate offers excellent radar reflections from the back side to a chasing fighter. These special reflections give F-16 a special radar signature and thus F-16 can even be recognized from rear just on the basis of its particular radar signature, if carrying fuel tanks.

This fuel tank configuration is obvious in following photos,

http://www.flickr.com/photos/racketrx/4085471432/

http://www.militaryfactory.com/imag...howing good detail to the engine exhaust port

This unusual problem can be solved by adding a conical after-body to the 370 gallons fuel tanks of F-16. This peculiarity is also shared by Eurofighter Typhoon's fuel tanks to a lesser extent. Apart from radar reflections, such fuel tanks are bound to generate large turbulent wakes behind the tanks.

For getting a good range or increasing its endurance, F-16 has to carry external fuel tanks but they make it possible for an radar-guided missile to get a good lock from the rear.

 

[gambit]

Now that we have discussed the RCS and radar reflections, its about time to discuss a peculiarity of F-16. F-16 is the front line fighter of Pakistan AF and before using it against a high-tech adversary, it is important to investigate the radar reflections from F-16 external fuel tank of 370 gallons capacity.

F-16's external fuel tank has a flat plate at the end and it does not taper to a conical shape. This flat plate offers excellent radar reflections from the back side to a chasing fighter. These special reflections give F-16 a special radar signature and thus F-16 can even be recognized from rear just on the basis of its particular radar signature, if carrying fuel tanks.

This fuel tank configuration is obvious in following photos,

F-16 Rear View | Flickr - Photo Sharing!

Rear left side view of the General Dynamics F-16 Fighting Falcon showing good detail to the engine exhaust port - Military Aircraft Pictures

This unusual problem can be solved by adding a conical after-body to the 370 gallons fuel tanks of F-16. This peculiarity is also shared by Eurofighter Typhoon's fuel tanks to a lesser extent. Apart from radar reflections, such fuel tanks are bound to generate large turbulent wakes behind the tanks.

For getting a good range or increasing its endurance, F-16 has to carry external fuel tanks but they make it possible for an radar-guided missile to get a good lock from the rear.

In this situation, the engine exhaust will offer a more assured radar target than the external fuel tanks. Certainly, those tanks will be contributors, but humans are naturally attracted to the stimuli (visual, auditory, or tactile) with the highest intensity. Radar detection is no different.

Look at it this way...

That largest spike produced by the vertical stab, not the rest of the aircraft, will be the focus of any radar processor. Shaping the rest of the aircraft to put it below the graph will be worthless. Likewise with the external fuel tank, eliminating that flat tail area will be financially wasted when the engine exhaust radar echo dominate the view.

 

[shehbazi2001]

In this situation, the engine exhaust will offer a more assured radar target than the external fuel tanks. Certainly, those tanks will be contributors, but humans are naturally attracted to the stimuli (visual, auditory, or tactile) with the highest intensity. Radar detection is no different.

Look at it this way...

That largest spike produced by the vertical stab, not the rest of the aircraft, will be the focus of any radar processor. Shaping the rest of the aircraft to put it below the graph will be worthless. Likewise with the external fuel tank, eliminating that flat tail area will be financially wasted when the engine exhaust radar echo dominate the view.

If by engine exhaust you mean the nozzle, then a nozzle simply can't do as much radar reflections as a flat plate of F-16 fuel tank. In your example, the vertical fin had the largest contribution because its a vertical flat plate, in fact the largest one of an aircraft but it matters only from sideways.

On the contrary, I am concerned with rear-sector RCS in relation to fuel tanks. A flat surface is almost certain death in air combat of BVR missiles and modern airborne radars. Even surface discontinuities and antennae can give enough radar reflections to show up an aircraft on radar screen. Thats why stealth aircraft like F-117 do not open their internal weapon bays when being painted by radar waves.

Financially, it is a mediocre expenditure with potentially multiple rewards. AWACS will not be able to recognize F-16 just on the basis of its special radar signature and it would improve survival against radar-guided weapons fired from six'o clock.

 

[gambit]

If by engine exhaust you mean the nozzle, then a nozzle simply can't do as much radar reflections as a flat plate of F-16 fuel tank.

Not the nozzle but the engine itself, as in straight into the exhaust.

In your example, the vertical fin had the largest contribution because its a vertical flat plate, in fact the largest one of an aircraft but it matters only from sideways.

I think you missed my point which is that when there are many contributors, we are naturally attracted to the one or the ones with the higher or highest intensity regardless of viewing angle.

On the contrary, I am concerned with rear-sector RCS in relation to fuel tanks. A flat surface is almost certain death in air combat of BVR missiles and modern airborne radars. Even surface discontinuities and antennae can give enough radar reflections to show up an aircraft on radar screen. Thats why stealth aircraft like F-117 do not open their internal weapon bays when being painted by radar waves.

Incorrect. When necessary, the F-117s did opened their weapons bays regardless of whether there were impinging radar signals or not. Your argument is based upon the automatic assumption that a radar will have an ideal viewing angle of the target every time. Nothing is further from the truth. Against dynamic targets and especially highly dynamic like a fighter versus an airliner, any radar system would require several sweep cycles in order to secure a lock because of the rarity of these ideal viewing angles. Further, an F-117 is not a WW II era bomber where the bomber must fly straight and steady with bomb bay doors opened for minutes. The F-117 will have its weapons bay exposed for at best a few seconds. Once the bombs are gone and bay doors closed, any radar detection will be lost again.

 

[Tariq Mahmood Khan]

Pakistan Aeronautical Complex (PAC) at Kamra rolled out the 26th JF-17 made by them.

A total of 250 JF-17 fighters would be made, some of them in China others in Pakistan. The main problem of this fighter is its Engine, which is the WS-10. Due to its performance and lack of reliability, WS-13 is currently under testing, our inside sources within ranks of Pakistan Air Force said.

JF-17 Thunder has been first jet made by Pakistan - and any muslim Nation of the world. It not only shows the talent of Pakistani engineers, but also their ability to perform miraculously well within tough financial limitations.

Pakistan aims to produce Stealth versions of the JF-17 aircraft, which is currently under development. It will give Pakistan Air Force air supremacy in the region.

Additional: The RCS of JF-17 is less than that of F-16, which makes it a little tough for Radars to detect within certain ranges. It is till date, the fighter with minimum RCS.

 

[shehbazi2001]

Original Post By gambit

I think you missed my point which is that when there are many contributors, we are naturally attracted to the one or the ones with the higher or highest intensity regardless of viewing angle.

The first thing that a radar can see into the exhaust is an area of the turbine equal to the opening of nozzle at that time. 50% of the turbine can't give more reflections or RCS than a flat plate of the same radius. If the exhaust of jet engine is more important on radar then why the designers try to hide the front of jet engine (ie fan and compressor) by designing S-shaped intakes like on the Super Hornet and Neuron drone? On the contrary, I dont see as much efforts to hide the turbine blades.

Incorrect. When necessary, the F-117s did opened their weapons bays regardless of whether there were impinging radar signals or not. Your argument is based upon the automatic assumption that a radar will have an ideal viewing angle of the target every time. Nothing is further from the truth. Against dynamic targets and especially highly dynamic like a fighter versus an airliner, any radar system would require several sweep cycles in order to secure a lock because of the rarity of these ideal viewing angles. Further, an F-117 is not a WW II era bomber where the bomber must fly straight and steady with bomb bay doors opened for minutes. The F-117 will have its weapons bay exposed for at best a few seconds. Once the bombs are gone and bay doors closed, any radar detection will be lost again.

Lets not talk of exceptions (still you can give some examples). The SOP is not to open them or to close them before the FIRST sweep and not multiple sweeps, according to Jane's Publication "F-117A Stealth Fighter" by JON LAKE on page 78. He mentions that DOOR opening is linked to RHAWS (Radar Homing and Warning System) and this link is not for fun.

On page 76, he states that radar reflections are so sensitive that even a MINOR bird strike on F-117 can easily quadruple its RCS. (some people may be tempted to keep large bevies of birds near vital points ).

 

[Black Widow]

Pakistan Aeronautical Complex (PAC) at Kamra rolled out the 26th JF-17 made by them.

Congrats camra for producing 26th JF16.


A total of 250 JF-17 fighters would be made, some of them in China others in Pakistan. The main problem of this fighter is its Engine, which is the WS-10. Due to its performance and lack of reliability, WS-13 is currently under testing, our inside sources within ranks of Pakistan Air Force said.

The engine is not WS10, but russian RD93 (correct me if I am wrong )

JF-17 Thunder has been first jet made by Pakistan - and any muslim Nation of the world. It not only shows the talent of Pakistani engineers, but also their ability to perform miraculously well within tough financial limitations.

Its many time discussed.. Please edit this line, This is not entirely truth.

Pakistan aims to produce Stealth versions of the JF-17 aircraft, which is currently under development. It will give Pakistan Air Force air supremacy in the region.

Please mention , where the development is going on? china or Kamra?? AFAIK Pakistan is working with china on J2X, are both same project ??? Will it be stealth JF17 or LO (low obser..) JF17?? what bout payload?? internal or external??

Additional: The RCS of JF-17 is less than that of F-16, which makes it a little tough for Radars to detect within certain ranges. It is till date, the fighter with minimum RCS.

What will be RCS of loaded JF17 (4 AAMs, Targetting pods and couple of AGM/LGB) ?? Will it be lesser than 4-5m2?? if yes then its a nightmare for others... Clear your second point ( It is till date, the fighter with minimum RCS.)

comment inline...

 

[gambit]

The first thing that a radar can see into the exhaust is an area of the turbine equal to the opening of nozzle at that time. 50% of the turbine can't give more reflections or RCS than a flat plate of the same radius.

Sorry...But you are way off base here. Keyword search for you: 'radar cavity resonance'. A cavity under direct radar impact will sort of 'ring' due to multiple reflections and the interior of a jet engine, regardless of entrant, is ideal for such an effect. Further, from my 5 yrs on the F-16, the exhaust nozzle of the F100 is large enough to accommodate a person sitting inside its diameter while that flat tail end of the external wing tank is just larger than my hand span.

If the exhaust of jet engine is more important on radar then why the designers try to hide the front of jet engine (ie fan and compressor) by designing S-shaped intakes like on the Super Hornet and Neuron drone? On the contrary, I dont see as much efforts to hide the turbine blades.

Inlet tunnels are not part of the jet engine so it is easier to conceal the inlet side of a jet engine via serpentine inlet tunnels. The exhaust end is a different matter because the distance between the exhaust nozzle and the engine core is much shorter than between the inlet lip and the first turbine stage. Lockheed removed the engine's afterburner stage so they were able to create the non-circular nozzles and that was to reduce the nozzle themselves as contributors, not the engine itself.

The above is a typical 'pod' layout of an airliner's engine. We can see that it is possible to lengthen the inlet side and direct inlet airflow as we need. But unlike exhaust gases in other combustion type engine where they are not used, a jet engine's exhaust is necessary for the aircraft's ability to fly: Thrust. Any attempt to manipulate that force reduces the possibility for flight. So we want to keep that flow as short and as direct as possible. Hence, there is no attempt to 'serpentine' the exhaust end of the F-22. For the F-35 VTOL version, the afterburner stage will not be available when the nozzle is turned to allow vertical operation of the aircraft, so we have not violated anything regarding exhaust direction.

Lets not talk of exceptions (still you can give some examples). The SOP is not to open them or to close them before the FIRST sweep and not multiple sweeps, according to Jane's Publication "F-117A Stealth Fighter" by JON LAKE on page 78. He mentions that DOOR opening is linked to RHAWS (Radar Homing and Warning System) and this link is not for fun.

Lake did not definitively state that there is such a link. As far as I know, such a link is only 'reported', which is a loaded word.

Before you take in too much of Lake's words, take a brief study on how a radar warning receiver set works...

RADAR WARNING RECEIVERS AND DEFENSIVE ELECTRONIC COUNTERMEASURES

For instance a SAM fire control illuminator locked on to the aircraft is a far greater threat than a surveillance radar. Modern RWRs employ microprocessors to perform this task often in conjunction with the signal processing function. The prioritised threat data is then fed to a cockpit display usually as synthetic symbols. This provides the pilot or weapon system operator with plan position indication of threats to facilitate defensive manoeuvring.

If the mere presence of a 'Search' mode is enough to lock out the ordnance delivery of the F-117 for fear of exposure, then we might as well scrap the entire 'stealth' aircraft program.

This alone is of limited use as in practice it is desirable to know in which direction the radar is. This is accomplished by using a set of four identical matched crystal video receivers each fed by an antenna which covers a quadrant of space about the carrying aircraft. By comparing the strength of the output signals from the receivers, the direction of the radar can be estimated with reasonable accuracy.

If the pilot knows that the threat direction is away from his position, why should he abort the mission just because his aircraft is impacted by the more general and coarse resolution 'Search' frequency? It make no operational sense. You read too much into Lake's writing.

On page 76, he states that radar reflections are so sensitive that even a MINOR bird strike on F-117 can easily quadruple its RCS. (some people may be tempted to keep large bevies of birds near vital points ).

Birds are unpredictable. They may not go where you want them to go.

 

[shehbazi2001]

Original Post By gambit

Sorry...But you are way off base here. Keyword search for you: 'radar cavity resonance'. A cavity under direct radar impact will sort of 'ring' due to multiple reflections and the interior of a jet engine, regardless of entrant, is ideal for such an effect. Further, from my 5 yrs on the F-16, the exhaust nozzle of the F100 is large enough to accommodate a person sitting inside its diameter while that flat tail end of the external wing tank is just larger than my hand span.

The exhaust nozzle of F-100 or F-110 is variable-area depending on the thrust and flight regime. I do not think that a man-sized object can sit there when the engine is not using afterburner.

Original Post By gambit

If the mere presence of a 'Search' mode is enough to lock out the ordnance delivery of the F-117 for fear of exposure, then we might as well scrap the entire 'stealth' aircraft program. If the pilot knows that the threat direction is away from his position, why should he abort the mission just because his aircraft is impacted by the more general and coarse resolution 'Search' frequency? It make no operational sense. You read too much into Lake's writing.

It is amazing that even after 5 years on F-16, you are still not clear about radar theory but there is no shame in learning from each other through a healthy discussion. The presence of surveillance or search radar is not a problem for the door opening of F-117. Interval between two successive radar beams gives enough time for the door opening and weapon release. What are the antenna rotation rates of surveillance radars? 3rpm, 6rpm, 10rpm or maximum around 40rpm. At 3rpm, the aircraft has 20 seconds before the next radar beam hits him. At 5rpm, he has 12 seconds of interval and even at 40rpm, there is an interval of 1.5 seconds to release the bombs.

Returning back to radar reflections of flat surfaces, these days even the aircraft such as NH-90 have diamond-shape fuselages to reduce RCS. The fuselage of Commanche helicopter was also diamond-shape. The use of flat-sided fuselages or round-fuselages is on rapid decline.

 

[gambit]

The exhaust nozzle of F-100 or F-110 is variable-area depending on the thrust and flight regime.

Really? I did not know that...

I do not think that a man-sized object can sit there when the engine is not using afterburner.

You completely missed the point. This is about area not about if anything can sit in the nozzle of a running engine. Area wise, regardless of whether the nozzle is wide open or constricted, the cavity hole created by the exhaust nozzle is much greater than the flat tail end of the external wing fuel tanks.

It is amazing that even after 5 years on F-16, you are still not clear about radar theory but there is no shame in learning from each other through a healthy discussion.

Here is what is truly amazing...

First -- No one can be an expert on an aircraft on everything.

Second -- I would think that given my previous posts in this discussion, you could gleam a little bit that perhaps I have equal knowledge about radar theory as you, or even greater.

The presence of surveillance or search radar is not a problem for the door opening of F-117. Interval between two successive radar beams gives enough time for the door opening and weapon release. What are the antenna rotation rates of surveillance radars? 3rpm, 6rpm, 10rpm or maximum around 40rpm. At 3rpm, the aircraft has 20 seconds before the next radar beam hits him. At 5rpm, he has 12 seconds of interval and even at 40rpm, there is an interval of 1.5 seconds to release the bombs.

Returning back to radar reflections of flat surfaces, these days even the aircraft such as NH-90 have diamond-shape fuselages to reduce RCS. The fuselage of Commanche helicopter was also diamond-shape. The use of flat-sided fuselages or round-fuselages is on rapid decline.

Good. But I see nothing here to suggest that there is a either a 'hard' or 'soft' link between the RHAWS and the bomb bay doors on the F-117. Keep in mind that the F-117 was flying in the Nevada desert and elsewhere in the US for over a decade before it was officially acknowledged. We had plenty of experience in exploring the various scenarios where such a link would be necessary. So unless you can bring to the discussion credible evidences where such a link exist...

 

[Tempest II]

Apparently CCTV “leaked” (hinted) that the RCS of the J-11 to be 4 sqm ... ... Yes I too you take that to be a clean configuration. Will post TV screen shots later.

 

[gambit]

A question was asked about this comment from somewhere else...

LPI means low not no probability of interception. It's rather optimistic to believe that LPI signals won't be detected by advanced RWR/ESM. Every transmission can be detected and if a radar wants to detect something it must receive strong enough returns to make sense of them. The waves have to travel to and from the target and will be much stronger when hitting the target than at the time they are received by the source of origin.

It is true that this is NOT zero interception and that when operating in LPI mode, quite often the seeking radar's signals will be detected by a radar warning receiver set.

But here is the problem...

Imagine the above illustration to be a very small slice of a transmission in a very small slice of time. The signals that rose above the 'clutter rejection threshold', aka 'noise', and above the higher line labeled 'Threshold' will be detected by an RWR set, however, the BANDWIDTH SPREAD and irregularity of these signals will make it extremely problematic for an RWR set to statistically calculate and determine from a pattern that there is a seeking radar signal. In background noise there are always sporadic spikes that are higher than a set threshold level. These spikes contributed to what is called Constant False Alarm Rate (CFAR) processing...

Constant false alarm rate - Wikipedia, the free encyclopedia

Constant false alarm rate (CFAR) detection refers to a common form of adaptive algorithm used in radar systems to detect target returns against a background of noise, clutter and interference. Other detection algorithms are not adaptive. Non-adaptive detectors are sometimes referred to as clairvoyant detectors.

What LPI mode does is to create a pattern of pulses that mixes amplifications, freqs, and other pulse characteristics over a period of time and repeat those patterns (plural) again. The greater the sophistication of the threat radar which naturally will have the greater memory capability, the greater the variability of these patterns.

The greater the complexity of KNOWN threat signals, the greater the complexity of those CFAR algorithms must be in the ATTEMPT to distinguish a pattern within a time slice. All CFAR algorithms are programmed to make a determination after a certain amount of time elapsed, so the greater the bandwidth spread, the greater the odds of defeating a CFAR algorithm because that pattern can be spread out over a longer period of time. The simpler the CFAR algorithm, the shorter the time slices and the sooner the RWR set will dismiss these spikes, leaving the aircraft vulnerable.

Throw in AESA where the system can create sub-arrays each with its own LPI patterns transmitting simultaneously and the currently deployed RWR/ESM can be quite overwhelmed. The seeking AESA threat radar will be able to collect vital target resolutions over a wide bandwidth in different transmit duration that the defender simply cannot distinguish from background clutter. Or one adversary can transmit 'loud' enough to spoof the defender into focus of a region while another adversary physically attack from a different direction. Given the speed and lethality of 'first world' class air-air missiles, a few seconds of distraction will suffice to either destroy a defender or severely disrupt a formation of defenders, thereby throwing off a mission.

Note: The above illustration can be applied to a 'stealth' aircraft as well. Just imagine that the spikes are reflected signals that came off the 'stealth' aircraft. As long as these spikes are irregular over time, the greater the difficulty of any system to determine that there is a valid 'target'.

 

[gambit]

Recently, it was asserted that a 'round nose' is undesirable for a 'stealth' aircraft. The assertion was nothing more than from observation and comparison between the latest 'stealth' aircrafts and the casually labeled '4th gen' aircrafts. It was a correct assertion but truly is more like throwing a pair of dice and received a favorable result. In short, it was a lucky guess.

Beyond observations and guesses...Here are the reasons why a 'round nose' or more specifically a circumference is undesirable in many cases when trying to design a radar low observable aircraft...

The flat plate perpendicular to the seeking radar will produce the highest echo intensity. An angled flat plate will produce less. Basic geometry tells us that angle of deflection is the same as angle of incidence (or approach). A joint of structures, even by lines, that ends in a 90 deg corner will produce that dreaded 'corner reflector' that is a great contributor to RCS.

On a circumference, or a sphere for that matter, regardless of radar-target aspect angles to each other, the seeking radar will ALWAYS receive that tiny amount of signal called 'specular reflection'. The rest of the signal will become assorted surface wave behaviors. In other words, no matter how the radar is positioned around the circumference there will always be a tiny amount of flat plate surface that is perpendicular to the seeking radar.

That behavior is quite evident for the F-15 and all aircrafts with similar radome or fuselage structures.

The same cannot be said for the SR-71, which is quite close to the extreme end of the shaping spectrum: The diamond shape. Any impinging radar signal will deflect away from source direction and this angled facetings technique was evident in the retired F-117.

The addition of curvatures into the diamond shape result in the (complete) ogive...

Ogive - Wikipedia, the free encyclopedia

...is the roundly tapered end of a two-dimensional or three-dimensional object.

The F-15's radome is an ogive when view from the side but have the circumference when view from the front. The SR-71's nose is an ogive in both perspectives. The curvatures induces surface wave behaviors, leaving an even smaller quantity of energy for specular reflection. While it is true that a curve is a series of very small flat surfaces, the ogive's curvatures discourages those direct views by the seeking radar, in other words, because the aircraft is a dynamic target, those small flat plates will not be seen by the radar for long. The ogive denies the seeking radar statistical certainty of this part of the aircraft. If the other regions of the aircraft, such as main fuselage, empennage, and flight control elements, receive similar treatments, then the increased statistical uncertainty will produce a radar low observable aircraft.

The F-22's and F-35's noses, which includes the radomes, may not go to the extreme diamond shape as the SR-71 but they are the results of careful balancing between EM and aerodynamic demands. The ridges on the sides are where the edge diffraction effect is deliberately introduced to scatter off the surface waves into free space, precluding the possibility of creeping wave behavior by meters length frequencies. For the F-22, its overall shape is approximate to the ogive, giving it the ability to deal with meters length frequencies, aka 'long wavelengths' of the HV/VHF/UHF long range search radars.

Note: The creeping wave behavior is usually eliminated by the 10-lambda rule. This rule states that if the circumference of the sphere or cylinder is greater than 10 wavelengths (lambda), the creeping wave behavior will not exist. All guidance radars are centimetric or millimetric, so for the F-15's radome circumference, the creeping wave behavior will not exist, only specular reflection will exist. However, if a meters length long range search radar signal is used, the creeping wave behavior will exist for the F-15 but very difficult to sustain on the F-22 and F-35.

 

[gambit]

A brief but helpful explanation of the three main RCS shapes as measured and graphed...

Keep in mind that these are to illustrate general principles and their results. They should not be taken to be %100 indicative of classified designs such as the F-22, F-35, and the B-2. Their RCS shapes will someday be known, but not today or probably for the next 20 yrs.

As to the question of why should frontal RCS matter, see this explanation => http://www.defence.pk/forums/military-aviation/20908-rcs-different-fighters-6.html#post2006035

 

[gambit]

To understand some basics of RCS control there must be some basic understanding of radar detection and target resolutions.

First...The highly desirable target resolutions are:

- Altitude
- Speed
- Aspect angle
- Heading

Second...To calculate them we must have pulses...

The illustration above is applicable to ALL wavelengths, from the meters length HF/VHF/UHF bands to the ghz centimetric and millimetric bands.

Third...Each pulse has:

- Duration
- Leading edge
- Trailing edge

The whole thing is called 'finite pulse length'.

Without pulses, we cannot calculate the lists of target resolutions. We depend on knowing from each time indexes (or slices) of those pulses, from when a pulse impact a target to when the echo received, to know how fast, how high, and which heading is the target with respect to us.

Fourth...It is self evident that the higher the freq the closer the pulses to each other that we can create, the shorter those time slices will be, the higher the target resolutions. For example, the standard light flickers at 60 cycles (hz) but in high speed camera photography the shutter speed create light pulses much greater than 60 cycles to give us those spectacular 'slo-mo' sports actions. If we have strobe lights lower than 60 cycles like those so popular in nightclubs, those time slices are too far apart so we see movements that are 'jerky' and 'disjointed'.

All of this means that the closer the attacker is to us, the higher his target resolutions we want so we can defend ourselves, and this mean the world over is restricted to the centimetric and some millimetric bands to create those target resolutions. The downside to using higher freqs with shorter pulses is that since each pulse has a finite amount of energy (finite pulse length) the higher the freqs we use to calculate high target resolutions, the closer we must allow the attacker to come to us before we use those limited energy to find his target resolutions. It is the typical Catch-22 dilemma in using high freqs.

This is why air defense radar systems have multiple antennas for multiple stages of target detection:

- Meters length freqs for long range search purposes. Very poor target resolutions but at least we know the general direction of a potential threat. For a busy civilian airport, we only need to know the coarse information of what is 200 km out. No need for fine grain information.

- Low end of the centimetric freqs for increased target resolutions for threat assessments and assignments. For a busy civilian airport, we need to know a bit more so we can negotiate landing permissions and priority.

- High end of the centimetric and millimetric freqs to missile guidance. This is where all threats are no longer potential but are genuine danger. We need the maximum fine grain possible of all threats' altitudes, speeds, headings, and aspect angles.

Because active cancellation is not yet possible, the first law of RCS control is:

1- Design for specific threat freqs. This mean shaping the aircraft against the highly useful and popular X-band.

Next are:

2- Use angle facetings to control exposure of large expanse of surfaces.

3- Use 'lossy' material or absorber whenever possible to control surface wave behaviors.

4- Enforce tolerances across large expanse of surfaces.

5- Treat edges to control diffractions, this includes plan forming of all flight control elements.

6- Avoid corner reflectors of any degree when possible. If not possible, then avoid the 90 deg kind.

7- Avoid straight line cavities. Or heavily diffuse entrant signals before the cavities. Straight line cavities like inlet tunnels can induce 'resonance' or a 'ringing' in the EM spectrum.

8- Avoid surface discontinuities whenever possible. If not possible, see law 5.

9- Shield high-gain antennas from out-of-band freqs. This mean use law 2 to prevent non-threat freqs from exposing the aircraft via the aircraft's own radar antenna.

If the F-35 is more visible in the EM spectrum than the F-22 it will be because of considerations that compelled the designer to focus less on some of the above laws than others. This is applicable to all future 'stealth' designs: Mission requirements can trump some measures of RCS control.