Simulated radar cross section of a stealth aircraft as a function of aspect angle.
An unique characteristic of a sphere is that it’s radar cross section is not affected by it’s orientation to the radar beam unlike others shapes such as cylinder or plate. As a result aircraft’s RCS are often be compared to a sphere of a certain size.
Another unique characteristic of sphere is that if 2πa/λ >10 (with a is the sphere radius, λ is radar operating wavelength) then the RCS(σ) of the sphere approaches its geometric projected area πa², this is called theoptical region where the sphere RCS does not change along with the radar’s operating frequency as long as initial condition is met.
When 1<2πa/λ<10, it is called Mie region, in this region a creeping wave travels around the sphere and back towards the receiver where it interferes constructively or destructively with the specular backscatter to produce a variation in the sphere’s RCS , the sphere RCS could varied between 0.26 to 4 times the value calculated in optical region.
2πa/λ < 1
When 2πa/λ < 1, it is called Rayleigh Region, in this region the sphere’s RCS is inversely proportional to the 4th power of the wavelength (exponentially smaller than values calculated in optical and Mie region).
Fluctuation of RCS in Mie region and Rayleigh region happened not only to simple body like a sphere but also to complex bodies like aircraft, however it happened in a much more complex manner that will be explained below:
To start with, the total radar reflection of a complex body such as aircraft made from several different kinds of reflections:
Specular return: this is the most significant form of reflection in optical region (when structure size >10 times wavelength), surface acts like a mirror for the incident radar pulse.
Most of the incident radar energy is reflected according to the law of specular reflection (the angle of reflection is equal to the angle of incidence). This kind of reflection can be reduced significantly by shaping.
Traveling/Surface wave return: an incident radar wave strike on the aircraft body can generate a traveling current on surface that propagates along a path to surface boundaries such as leading edge, surface discontinuous …such surface boundaries can either cause a backward traveling wave or make the wave scattered in many directions .
This kind of reflection can be reduced by radar absorbing material, radar absorbing structure, reduce surface gap or edges alignment (so that their lobes occur in low priority region )
Diffraction: wave striking a very sharp surface or edge are scattered instead of following law of spectacular reflection.
Creeping wave return: this is a form of traveling wave that doesn’t face surface discontinuous and not reflected by obstacle when traveling along object surface , thus it is able to travel around object and come back at the radar.
Unlike normal traveling wave, creeping wave traveled along surface shadowed from incidence wave (because it has to go around the object). As a result, the amplitude of creeping wave will reduce the further it has to travel because it can’t feed energy from the incident wave in shadow region.
Creeping wave mostly traveled around a curved or circular object. Hence, stealth fighters and stealth cruise missiles do not use tube fuselage. Nevertheless, creeping wave return are often much weaker than spectacular return.
A high-frequency regime (or optical region) applies when the structure is at least 10 times longer than the incident radar wave.
In this regime, specular mechanisms dominate the radar, (the angle of reflection equals the angle of incidence), like billiard balls colliding.
Reflection towards the emitting radar is reduced by angling surfaces so that they are rarely perpendicular to radars and suppressing the reflections from re-entrant structures such as engine intakes and antenna cavities with combinations of internal shaping, radar absorbent material (RAM) or frequency selective surfaces.
In this regime, “surface wave” mechanisms are small contributors to RCS, but are still present. If the wavelength is small relative to the surface, these waves are weak and their overlap will generate maximum backscatter when the radar signal is at grazing angles.
When these currents encounter discontinuities, such as the end of a surface, they abruptly change and emit “edge waves.”
The waves from different edges interact constructively or destructively due to their phases. The result is they strengthen the reflection in the specular direction and create “sidelobes” – a fan of returns around the specular reflection which undulate rapidly and weaken as the angle deviates from the specular direction.
The currents can also swing around to a structure’s back side, becoming “creeping waves” that shed energy incrementally and contribute to backscatter when they swing back toward the radar. Surface wave reflections are generally very small in the optical region.
So why is stealth less effective at low frequency?
As the radar wavelength of radar grows, the intensity of specular reflections is reduced but its lobes width are widen (the same phenomenon also happened to radar, if aperture size remained the same, the reduction in frequency will increase radar beamwidth).
Because the specular reflection lobes are widen, shaping become less effective because it will be harder to deflect radar wave aways from the source (it is important to note that, while this lobe widening phenomenon making shaping less effective, it also reduce the intensity of the reflection because the energy will be distributed over a wider volume)
Specular reflections from flat surfaces decrease with the square of the wavelength, but widen proportionally: at 1/10th the surface length(approaching Mie region) they are around 6 deg wide.
At lower frequency, the effect of traveling wave and diffraction is also more pronoun. For flat surfaces, traveling waves grow with the square of wavelength and their angle of peak backscatter rises with the square root of wavelength: (at 1/10th the surface length, it is over 15 deg).
Tip diffractions and edge waves from facets viewed diagonally also grow with the square of wavelength.
The end result is that the net value of stealth aircraft’s RCS often increases when wavelength approaching Mie region related to aircraft size. Thus, low-frequency radars are often regarded as a counter to stealth technology
Example: Simulated radar cross section of a B-2 aircraft and AGM-86 missiles as a function of aspect angle (at 10 Ghz and 1 Ghz respectively )
The negative effect of traveling wave and diffraction can be reduced by: aligning discontinuities to direct traveling waves towards angles of unavoidable specular return, such as the wing leading edge, thus limit their impact at other angles.
Example: serrated edges are used in place where there is current discontinuity such as weapon bay door so that traveling wave return reflected toward less important angle
Another common method to reduce the effect of surface wave is designing airframe facets with non-perpendicular corners and so radars view them along their diagonals, at low angles and across from the facets’ smallest angles, limits the area of edge-wave emission.
At high relative frequencies, surface waves can also be suppressed with RAM. Surface wave diffraction can also be reduced by blending facets.
The first stealth aircraft, the F-117, was designed with a computer program that could only predict reflections from flat surfaces, necessitating a fully faceted shape, but all later stealth aircraft such as B-2 , F-35 , F-22, X-47 use blended facets.
Shapes composed of blended facets are not only more aerodynamic but also allow currents to smoothly transition at their edges, reducing surface-wave scattering. Therefore, blended bodies have the potential for a lower RCS than fully faceted structures, especially at low-frequency regime.
And blending the curves around an aircraft in a precise mathematical manner can reduce RCS around the azimuth plane by an order of magnitude.
The penalty is often a slight widening of the specular return at the curves, but in directions at which threat radars are less likely to be positioned. This was one of the great discoveries of the second generation of stealth technology.
Example: Airframe of first generation stealth aircraft such as F-117 are fully facet. Whereas second and third generation of stealth aircraft such as F-35, B-2, X-47 uses blended facet design where needed.
It is, however, important to remember that, even though a blended body shape can benefit stealth characteristics because they reduce surface scattering compared to sharp facet design.
A full circular (tube) body is extremely bad for stealth application, the reason is that the surface wave doesn’t get scatter but will travel a full circle around the object and come back to the source (also known as creeping wave return).
Regarding the issue of stealth and low frequency, there are 3 common misconceptions.
The first common misconceptionis that any low-frequency radar can render stealth useless regardless of their transmitting power or aperture size (Ex: Tikhomirov NIIP L-band transmitter on the leading edge of Flanker series are often cited by enthusiasts as a counter stealth system ) , that is wrong however.
While it is true that stealth aircraft will often have higher RCS in Mie region. It is important to remember that given equal radar aperture area, lower frequency radars will have much wider beam compared to high-frequency radars, thus, the concentration of energy is much lower making them more vulnerable to jamming, lower gain also result in lower accuracy.
Moreover, as mentioned earlier lower frequency also resulted in wider reflection beamwidth, hence weaker reflection. As a result, most low-frequency radars have much bigger transmitting antenna compared high-mid frequency radar (to get narrow beamwidth)
it is also the reason that fighters fire control radar still work in X-band, because a L-band, VHF band radars of similar size would be too inaccurate for any purpose others than early warning (their accuracy can be estimated by radar gain equation that will be provided later).
Modern stealth aircraft also use various methods to reduce their return even in Mie region , simulation done by MBDA shown that even though their radar operating within low-frequency range from UHF to F band, their AWACs still struggle to detect stealth aircraft from their frontal aspect.
The second common misconception is that the lower the operating frequency of the radar (longer wavelength), the better it would perform again stealth assets.
That is wrong, however. It is important to remember that aircraft RCS does not necessarily grow linearly with increasing in frequency.
As surface-wave effects grow, their phases can interfere constructively or destructively with specular reflections. This phenomenon is illustrated in the simple form with a sphere(as mentioned earlier).
As wavelength grows relative to the circumference, the creeping wave circling the sphere grows continuously, but its phase interference with the specular return varies.
This causes the sphere’s RCS to undulate, with successively higher peaks corresponding to phase matches between the specular return and the strengthening creeping wave.
This phenomenon is known as Mie scattering (also known as the resonance region) and this regime where the wavelength is between one and 1/10th the size of the structure. Maximum RCS is often reached when the wavelength reaches the approximate size of the structure.
Once the wavelength grows past this point (when wavelength get bigger than target size), the specifics of target geometry cease to be important and only its general shape affects reflection.
The radar wave is longer than the structure and pushes current from one side of it to the other as the field alternates, causing it to act like a dipole and emit electromagnetic waves in almost all directions. This phenomenon is known as Rayleigh scattering. At this point, the RCS for aircraft will then decrease with the fourth power of the wavelength
Example : Frontal su-27 RCS as a function of frequencies
The third common misconception is about the quarter wavelength rule, it is popular among enthusiasts to think that the RAM (radar absorbing material ) must be at least as thick as 1/4 the operating frequency of the radar to have any absorbing characteristic. That is wrong however.
While RAM absorbing capabilities often reduce at low frequency, they do not disappear completely. For example: MnZn ferrite RAM with thickness of merely 3 mm can have absorbing rating of -5dB (or absorbing by 68%) at 2 GHz (wavelength of 2 Ghz wave is about 15 cm long)
Reduce radar detection range
One easy to see benefit of RCS reduction is the deduce in enemy detection range, thus giving pilots more times to react to the threat or getting into weapon engagement zone.
Example: radar detection range between conventional and stealth aircraft.
Improve jamming effectiveness
It is a common misconception that stealth technology is short live and as radar get more powerful, soon, they will be able to out range weapon engagement envelop, thus renders all money spend on RCS reduction a waste.
This impression is inaccurate because any technology that can increase a radar peak power or gain will also benefit a jammers in the same ways. And stealth have a synergy relationship with jamming.
Another common opinion is that the gap in RCS can easily be close by using a more powerful jammer. This is also inaccurate because RCS directly proportional to the power required to jam a radar at a certain distance.
Which mean when RCS is reduced to 1/100th the original value, the required jamming power is also reduced to 1/100th the original value to achieve the same effect.
In others words, if a stealth aircraft need a 10 kW jammer, a conventional asset will need jammer with power of 10Mw or more.
If the jamming power is keeping the same then burn-through range is reduced by 10 times, which mean stealth assets (RCS=0.001m2) can get 10 times closer the threat compared to conventional aircraft (RCS=0.1m2).
In other words, even if adversary radar can see through jamming of conventional assets from 400 km aways, a stealth asset can still get within 40 km of such radar using exactly same jamming system.
Example : burn-through distance of F-35, F-18E with same jamming assets, same threat radar (image not to scale)
如果干扰能力保持不变，烧穿距离将减少10倍，也就是说隐形飞机(RCS = 0.001m2)与传统飞机(RCS = 0.1m2)相比，可以获得10倍的威胁。意味着使对方雷达可以从400公里的距离看到传统飞机的干扰，可让雷达要到40公里范围内才可看到隐身飞机。