Fórum Defesa Brasil

The four basic methods of reducing RCS are shaping, use of radar energy absorbing materials, passive cancellation and active cancellation ([18]), and will be analyzed in the following paragraphs.

The most important factor affecting the RCS is the geometry or the shape of the target, not its size. In order to reduce the RCS, the surfaces and edges should be orientated in such way so as to reflect the radar energy away from an expected radar antenna and not back to it. Considering the flat surfaces (facets) and the acute angles of the F-117, it is understood that it was designed in a way that the expected radar energy would be reflected to irrelevant directions and not back to the emitting radar.

The designers tried to avoid any possible surface or edge whose normal vectors would look at a direction where a possible enemy radar might be found, especially for the frontal aspect.
Therefore, in the frame of RCS reduction, all bumps, curves etc should be avoided. In the same way, any external load (pylons, bombs, missiles, fuel tanks, pods) would considerably augment the total RCS. This is the reason why l.o. aircraft carry their armament internally, in special bays. Furthermore, armament bay and landing gear bay doors should close tightly, with no gaps in between. Generally, any irregularity of the surface could incur an RCS increase.

Propellers are strictly forbidden, while the first stage engine blades should be carefully hidden inside the intake duct. The whole air intake construction is critical, when designing a low RCS aircraft.

Low Observable Principles, Stealth Aircraft and Anti-Stealth Technologies

It is a "technology demonstrator" developed by Dassault (France), Alenia Aermacchi (Italy), SAAB
(Sweden), EADS-CASA (Spain), Hellenic Aerospace Industry (Greece) and RUAG (Switzerland). Sharp dihedral corners and parallel surfaces contribute also to the RCS.

Therefore, the twin vertical fin empennage, as in the classic F-15E (not the F-15SE), is prohibited. Stealth aircraft have either canted tail fins or no tail fin at all (as in the B-2 Spirit). Regarding existing stealth aircraft, it is also evident that the leading edges of the wings and of the horizontal tail fins (stabilizers) are parallel. This applies to the trailing edges, as well. The aim is always the same: reflect the radar energy to certain, irrelevant directions, and thus keeping the (monostatic) RCS low.

Conventional (mechanically scanning) radar antennas should also be avoided, since the antenna itself is an ideal radar energy reflector, increasing the RCS when another radar is looking at it.

For this reason, the F-117A carried no radar at all. More recent l.o. aircraft make use of electronically scanned array radars, which offer lower RCS contribution, notably AESA (Active Electronically Scanning Array) radars.

Furthermore, these radars should exhibit LPI (Low Probability of Intercept) characteristics, in an attempt to avoid detection by enemy ESM (Electronic Support Measures) systems, trying to detect and locate radar emissions. Apart from the reduction of the aerodynamic drag, which is a positive sideeffect of the absence of external loads, optimizing the aircraft design for RCS reduction is generally incompatible with the aerodynamic principles.

Furthermore, it would be impossible to control aircraft such as the F-117A and the B-2 without the
help of the electronic flight control system (Fly-By-Wire). In general, the application of low RCS principles is a trade-off of cost, aerodynamic performance, RCS and other parameters.

Stealth technology has become sine qua non: all military aircraft, tanks, ships etc, are designed or redesigned according to low observable (l.o.) principles. This technology allows a potential intruder to enter enemy area undetected and deliver a first strike before the defender realizes he is being attacked or at least before he has the time to respond effectively. Furthermore, l.o. techniques help to "break the kill chain": even if the l.o. aircraft is detected, it cannot be easily engaged by a fire control radar or a missile radar seeker.

On the other hand, stealthiness is not panacea: stealth aircraft are not invincible, they are just detected at shorter distances. Generally, the application of l.o. principles incurs a considerable cost, both in procurement as well as for maintenance. Actually, the l.o. approach is a trade-off among cost, stealth capability and operational performance, i.e., maneuverability, amount of weapons and fuel, etc. The point is that a few fighters with l.o. characteristics but with inherent operational constraints will 158 Low Observable Principles, Stealth Aircraft and Anti-Stealth Technologies not necessarily prevail against more fighters with not so l.o. characteristics but better operational capabilities.

In any case, a stealth threat is a serious threat, which should be dealt with appropriately. Traditional surveillance and engagement systems have proven to be inadequate. In fact, no radar system alone seems capable to confront effectively such threat.

A promising approach relies on a combination of the following:

- Very low frequency band radars, for medium to high altitude surveillance: as the frequency decreases, the wavelength increases and becomes comparable with major parts of the aircraft. Thus, scattering enters the resonance region, exhibiting a higher Radar Cross Section (RCS), at least momentarily. Also, the Radar-Absorbent Materials (RAM) are not very effective at lower frequencies. For these reasons, radars operating, e.g., in the VHF band, are expected to see a l.o. target
at a longer distance with respect to “conventional”, higher frequency radars, transmitting in the L or S-band.

- Passive radars can complement their active counterparts, covering low to medium altitudes: such radars detect and track targets passively, measuring distortions and disturbances on existing signals from radio, TV, mobile telephony, Wi-Fi etc. They feature low frequency and bistatic operation (different locations of transmitter and receiver), offering increased probability of revealing stealth aircraft, which are optimized for monostatic radars.

- All information from every radar system or sensor should be fused in a central command and control entity (data fusion). If possible, low level, raw data should be taken into account. A few “hits” from a radar may not result to a “plot”, however a few “hits” from different radars corresponding to the same location certainly indicate a possible target.

- Any unidentified track should be transferred to combat aircraft via tactical data link, in order to be intercepted.

- The interceptors should be equipped with an IRST (InfraRed Search and Track) system, allowing detection and tracking of l.o. targets at longer distances, with respect to their radar sets. Ideally, they should be also capable of engaging a K. C. Zikidis et al. designated target sent to them via data link, even if they cannot “see” the target by their on-board sensors.

- Following the same way of thinking, IR air-to-air missiles should be preferred to radar seeker missiles, even for BVR (Beyond Visual Range) distances. As a final conclusion, it should be noted that the development of l.o. technology and the proliferation of stealth aircraft have changed the modern warfare, rendering most legacy systems almost useless. In order to cope with this new type of
threats, older systems should be upgraded, modified accordingly and interconnected, following the principles of the net-centric warfare doctrine, while new, suitable systems should also be employed, as described in this paper. If not, the danger of realizing you are being attacked, after having received several bombs on your airforce bases and facilities, is looming.

However, as Dr. W. Edwards Deming has put it:
“It is not necessary to change. Survival is not mandatory.”

If HF-band (3 – 30 MHz) waves are transmitted towards the ionosphere, under certain conditions, they may be reflected back to earth. The ionospheric reflection of the HF waves is the principle of operation of the Over-The-Horizon Radars (OTHR).

Such radars offer extremely long detection ranges (from 700 to 4000 km) but also very low resolution (from some hundreds of meters up to 20 km). Apart from using very low frequencies, they “see” their targets from above, offering considerable capabilities of detecting l.o. targets. It is noted that stealth aircraft are optimized for radars in front and from below, while from above they may present higher RCS, due to the cockpit and the engine intake and exhaust.

An OTHR system requires large facilities of antenna arrays, offering a strategic advantage. Obviously, their cost and limitations, especially their very high minimum range, constrain their use. Some of the few existing OTHR systems are the following:

– AN/FPS-118 Over-The-Horizon-Backscatter (OTH-B) Radar, covering
distances from 900 to 4800 km (L.M., U.S.A.).
– AN/TPS71 Relocatable Over The Horizon Radar (ROTHR), covering
distances from 925 to 3000 km (Raytheon, U.S.A.).
– Jindalee Operational Radar Network (JORN), covering distances up to
4000 km (Australia) [88][89]. It has been reported that this system has detected F-117
aircraft.
– NOSTRADAMUS, from 700 to 2000 km (ONERA, France)

Conventional (mechanically scanning) radar antennas should also be avoided, since the antenna itself is an ideal radar energy reflector, increasing the RCS when another radar is looking at it.

Mathias escreveu:Porém o fabricante pode considerar o RCS com a antena rebatida e fixa o que minimizaria a reflexão. Porém, estando assim o radar do avião é inoperante, inútil.
Em termos de marqueting faz muito efeito e quase ninguém vai se preocupar em saber desses detalhes, simplesmente se assume que o RCS do avião é X e pronto, aquilo fica valendo ara tudo.

Isso só seria uma limitação com esse radar em modo passivo, se ativo as emissões dele serão detectadas a uma distância muito maior do que o eco do radar inimigo.

Mathias escreveu:Acho que não entendi...

Marechal-do-ar escreveu:A propósito, tem fácil esses estudos?

Mathias escreveu:Não se vê radares com o desempenho do IRBIS-E simplesmente porque eles não existem, fora o APG-77 de F-22.
Que radar tem uma potência de pico de 20Kw e uma antena de 1 m de diâmetro?