Эта версия самолета действительно интересная. Вспомогательная турбина (или как ее назвать?) смотрится странно, но это наверное логичное решение, по другому или малозаметность будет хуже или вертикальную посадку не cделать.
Интересно, сколько реально полезной нагрузки F-35b сможет взять и как далеко донести? Идея сделать из УДК Америка еще 8 авианосцев - интересная конечно, правда там от УДК одно название, это малый авианосец размером почти с наш ТАВКР. У многих стран АВ меньше. (Например - Де Голль). Еще интересно, с Уоспа его можно юзать или нет?
13:19 11.11.2016
privet75 (privet75) писал(а) в ответ на :
> Интересно, сколько реально полезной нагрузки F-35b сможет взять и как далеко донести?
Есть вот такая мурзилка/брехушка от локхида..
Но она не совсем правдивая... Радиус тупо указали 50% от дальности... Хотя всем известно, что это всегда 35%.
6800 - даже и не знаю, чем они его нагрузят так... а) только стелс: 2*GBU-31(907 кг) + 2*AIM-120(161.5) = 2137 кг б) не стелс (внешний подвес) 6*GBU-31(907 кг) + 2*AIM-120(161.5) + 2*AIM-9(85,3) = 5935,6кг Что еще массивней на него можно повесить - я не знаю...
По дальности совсем все плохо... Вот тот вентилятор забрал место под бак, на 2 с лишним тонны. По моим подсчетам, не особо нагруженный он имеет радиус меньше чем у "хариера ||". около 300 км, с дальностью (как раз) 833. Может перепутал локхид в мурзилке? 833 не радиус, а дальность.
18:32 11.11.2016
За счет чего в первую очередь F-35 будет громить 4ое поколение?
1) Stealth.
Ранее я приводил статью из мини-блога Павла Румянцева из ТМ. В данном случае я приведу статью RF-IR Stealth (Techniques/Benefits)
Skip to content Aircraft 101 RF-IR Stealth (Techniques/Benefits) MARCH 4, 2016 F-35 Brief :
The advantage of detecting, identifying and engaging a target while stay invisible is undeniable, thus for years designers has been attempting to minimize the ability of radar , RWR and Infrared system to detect aircraft . Aircraft with significant low observability characteristics embodied in are called stealth aircraft . This article will discuss some common techniques used by stealth aircraft , their benefits and clear out some common misconception .
Low Observability in Electronic Spectrum.
Radar is the main sensor systems for most aircraft and air defense systems so it is not a surprise that most of detection reduction efforts are concentrated in electronic spectrum.
Recalling the basic radar range equation discussed before:
radar-fundamentals-18-638
It easy to see that the radar detection range is proportional to σ∧¼ where σ is the radar cross-section (the RCS ,the measure of a target’s ability to reflect radar signals in the direction of the radar receiver, often measured in dBsm or m2). Reducing the cross-sectional area therefore affects radar range, although only according to the fourth root. However, by carefully designing an aircraft, the value of σ may be reduced by many hundreds times thus present an effective approach.
Contributors to high RCS:
RCS contributors
Basic RCS reduction approaches: 1) Shaping
Orientation and curvature of surfaces Alignment of edges Shielding of cavities and ducts 2) Materials selection
Composites and RAM Metamaterials and other artificial materials RCS reduction techniques:
Surface orientation:
Surface orientation
Make sure surface normals do not point in high priority threat directions Specular component is frequency independent, but scattering lobe widths decrease with increasing frequency Principal planes (i.e., cuts with the highest sidelobes) are perpendicular to edges Example above: square versus diamond with the same surface area
Retro-directive Reflectors:
conner reflector
Avoid corner reflectors (dihedral and trihedral reflectors) No vertical/horizontal tail surfaces on aircraft Surface orientation is the most important feature of a stealth aircraft , even without radar absorbing material, a stealth airframe can achieve much lower RCS compare to conventional aircraft Example picture : Computer simulation radar scattering characteristics of Mig-21 ( on the left )and F-35 ( on the right ), frontal radar cross section of F-35 fluctuated between -20 and -30 dBsm while Mig-21’s radar cross section fluctuated between 10 and 0 dBsm (both model are without radar absorbing material ) RCS graph
Edge Serration and Alignment:
edge diffraction
The maximum intensity of the diffracted lobe from an edge (in the Keller cone direction) increases with edge length.
Edge serration
Serrations break up edges to reduce lobe intensities ,serrations are applied to both edges. Example using a rectangular plate is shown. Edge agliment
Edges are generally aligned so that their lobes occur in low priority region, leading edge scattering is dominated by TE polarization. Trailing edge by TM polarization. Example shown: 5λ plate with wave incident perpendicular to edge, 70 degrees from normal incidence (green arrow is E; red arrow is ki) Current Discontinuities:
Gaps in conductivity lead to edge diffraction. A seam can look continuous, but there may not be good conduction between the two sides. Ex: wire with a break.
Surface Discontinuities
Passive Cancellation:
Approach: add a secondary scatterer and adjust it so its scattered field cancels that of the bare target Only effective over a narrow range of angles and frequency bandwidth Only practical for canceling low RCS levels Examples: parasitic elements and lumped loads Passive Cancellation
Passive cancellation structure ( or RAS ) Passive Cancellation Structure
Intake RCS reduction:
Intake cavity and engine fan blade are a great source of radar reflection . Stealth aircraft intake are often more sophisticated, being a serpentine duct rather than a direct, more conventional intake , they use complex techniques to reduce reflection over a range of frequencies. The intake is designed to counter radar threats at three wavelengths loosely termed long ( 30 cm), medium ( 10–20 cm) and short ( 3 cm),equating to 1 GHz (long-range surveillance radar), 1.5–3 GHz (AWACS radar) and 10 GHz(fighter radar) respectively. At long wavelengths (30 cm ) the stealth fighter inlet ducts behaves as follow :
F-35 inlet again L band
At medium wavelengths (10-20 cm ) the stealth fighter inlet ducts behaves as follow :
F-35 inlet again medium wavelength
At medium-short wavelength , stealth fighter inlet ducts behave as follow : F-35 inlet again X band
Randome RCS reduction:
Antenna mode reflections. The antenna mode reflections mimic the antenna main beam and sidelobes.
Radar RCS reduction
Random scattering. This is caused if the antenna characteristics are not uniform across the antenna. Radar RCS reduction 2
Radar antenna edge diffraction. Mismatches of impedances at the perimeter of the antenna can cause reflections called edge diffraction. In effect the outer perimeter of the antenna acts as a loop and reflections tend to be abeam of the antenna rather than fore and aft. Radar RCS reduction 3
Canopy RCS reduction :
Radar wave can go through canopy and reflected off object inside the cockpit , thus increase RCS significantly. Solution : coat the inner of canopy with a thin layer of gold to prevent radar wave from entering the cockpit , the outter cockpit is coated with transparent radar absorbing material. Example : Canapy RCS reduction
Weapons RCS reduction :
Missiles , bombs are all great contributors to radar reflection due to the perpendicular angle of their wings, fins , Pylons is another great contributors because of the corner they make with aircraft wing. As a result, stealth aircraft often carry weapons internally, the added benefits is the reduction in drag .However due to limitation in size when using internal configuration , a stealth fighter cannot carry as much weapons as a normal fighter.
Example: F-35 weapons bay
Skin RCS reduction :
No matter what shape they have , airframe will always reflect radarwave , so along with unique shaping to redirect radarwave from the original source, stealth aircraft often have radar absorbing paint or use radar absorbing material (RAM ) , most RAM on fighters works better at high frequency than low frequency . For example:
Results:
Оценки ЭПР ниже чем у F-22
With careful design stealth aircraft can have RCS equal a fraction of conventional aircraft.
rcs-of-aircraft
f-35-production-12-728
Benefit of low RCS
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.
Radar detection range
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 ) Jamming- burn throgh
Burn-through Range is the radar to target range where the target return signal can first be distinguished from the Jamming signal ( rendering jamming ineffective).
Burn through effect
Low Observability In Infrared Spectrum.
All objects with a temperature above absolute zero emit heat energy in the form of radiation. Usually this radiation is invisible to the human eye because it radiates at infrared wavelengths, but it can be detected by electronic devices designed for such a purpose , these devices are called infrared (IR )sensor. With the improvement in optics and processing power of CPU , nowadays infrared sensors can see much further than human eyes .As a result, all stealth aircraft use one way or another to suppress their infrared signature.
Infrared signature suppression has two objectives:
Reduce the range at which an IR missile or sensor can detect and track the aircraft. Increase the effectiveness of countermeasure systems and devices. It is important to note that infrared sensor detecting assets by comparing the contrast of such assets infrared signature with background radiation , thus the effectiveness of infrared suppression is affected significantly by the temperature of background. In general clear sky is the worst background due to their low temperature while cloud and/or hot land surface make the best backgrounds for stealth aircraft to hide from adversary infrared sensor.( for the same reason, aircraft fly higher are much easier to detect by IRST )
Example: infrared photo of C-130 in 2 different background
Like all electromagnetic radiation, IR interacts with matter in a variety of ways:
Reflects—A wave is reflected from a surface. The angle of reflection equals the angle of incidence. Refracts—The direction of a wave bends when passing between two transparent media with different propagation speeds (Snell’s law). Scatters—Scattering occurs upon interaction with particles whose size approaches the length of the wave (why the sky is blue). Diffracts—This interaction occurs around the edges of an obstruction. Interferes–This interaction occurs in both a constructive and destructive manner. Absorbs—When absorbed by matter, radiation is converted into another form of energy. The most common conversion is to heat. Emits—Radiation is emitted from matter by conversion from another form of energy. Transmits—IR propagates through a transparent medium (or vacuum). Polarizes—An electric field is partially polarized by reflection from dielectric Infrared wavelength range from 0.7-14 µm , divided to short ( 0.7-1.5 µm ) medium (1.5-6 µm ) and long infrared wave (7-14 µm ), with different characteristics they all have different military application.
Infrared band
Infrared signature of aircraft:
An aircraft’s infrared signature is a complex mixture of emissions and reflections from different materials with different emissivity and different areas. Signature is complex in its spectral distribution, in its contrast against background, and in its dependence on conditions. Aspect angle, altitude, airspeed, ambient air temperature, power setting, and sun angle are only a partial list of conditions affecting signature values.
F-4 IR signature
F-14A IR image
High infrared signature component of fighter aircraft:
Engine “hot parts,” which usually consist of the aft turbine face, engine center body, and interior nozzle sidewalls. Engine exhaust plumes, which are emissions from the combustion constituents of CO2 and water vapor. Airframe, which includes all of the external surfaces of the wings, fuselage, canopy, etc. Airframe signature includes solar and terrestrial reflections, mach shock wave in addition to direct emissions. Similar to radar cross section, IR signature of an aircraft is very aspect angle dependence thus lead to very different detection range, For example : OLS-35 ( IRST system on Su-35 ) can easily detect an aircraft from 90 km aways from tail aspect, however in head on aspect the detection range reduce significantly down to 30 km
Infrared band
infrared percentages
Airframe Aerodynamic Heating:
The temperature of the airframe is warmer than ambient by the amount of aerodynamic heating. A good estimate of airframe temperature is given by the formula for the recovery temperature given below. Note that the temperature units are Kelvin.The temperature of the skin of an aircraft stabilizes at the ambient air temperature plus aerodynamic heating. Aero heating increases as the square of Mach number. The formula below gives a good approximation:
Aircraft moving at supersonic speed also produces compressed air ( Mach cone ) which not only increase the airframe temperature significantly but also increase frontal area present to the infrared sensor.
sOTlm
Aircraft moving at Mach 1 can be detected by IR sensor at twice the distance compare to aircraft moving at Mach 0.8
supersonic and IR detection
As shown in table above flying supersonic can increase aircraft infrared signature significantly, so the most simple solution is to stay subsonic, the trade off of such decision is smaller weapon engagement envelope, longer reaction time for adversary , this solution works well in design that high speed is not a requirement such as F-117 , B-2 . Another solution is to use fuel as heat sink , most modern stealth aircraft have internal fuel tanks distributed evenly through out the airframe, the fuel being use all the time thus they can transfer the built up heat aways from the aircraft.Fuel can also be used to reduce heat generated by electronic equipments,avionics systems like radar and jammer can generate very high amount of heat.Example : fuel tank contribution of conventional aircraft ( on the left) vs stealth aircraft ( on the right)
Modern stealth assets also use carbon composite material in the leading edge of the wing, such material has good IR dissipation ability, some RAM paints also have modest infrared reduction ability, For example the Top coat on F-22 , F-35 reported to reduce their skin infrared signature in long infrared wavelength (8–12 microns) by more than half.
Internal Equipment Heating:
Electronics equipment such as radars and jammers generate significant amount of heat, the more sophisticated and powerful the equipment is the more heat they will create , cooling are required not only to reduce enemy’s infrared sensor detection range but also to prevent the equipment from being overheat and shutdown
aircraft that lack significant cooling features for electronics often have higher body temperature , thus easier to detect.
For example:
picture of F-16 in mid-infrared wavelength.
Solution : As mentioned earlier , avionics can be cooled using fuels , furthermore aircraft can use open vents, thus atmosphere air can act as heat exchanger with the fuel which got heated by avionics
For example : F-35 has a scoop located on the top of the right wing-glove to provide air to the fuel/air heat exchanger. A deployable scoop is located on the left-aft fuselage to provide air to the IPP and to the avionics
The aircraft also use engines bypass air as heat exchanger
F-135 heat exchanger
Airframe heating due to engines:
With the core temperature of several hundreds degrees of modern jet engines , without appropriate measuares they can increase aircraft body temperature dramatically
For example :picture of Typhoon in mid infrared wavelength
Solution :
Airframe heating due to jet engines can be reduced by extensive use of cooling vents, the cold air at high altitude provide an isolation layer between the engine and the airframe
For example : the F-35 has two scoops located in the wing/fuselage to provide nacelle bay ventilation ,
Temperature from the exhaust pumes:
The biggest contributors of signature in mid-infrared wavelength on a jet aircraft is their exhaust pumes , reduction of exhaust temperature as little as 100 degrees can reduce aircraft infrared emission by more than haft
One very common misconception about jet engine and infrared signature is : an engine with higher thrust will always have higher infrared signature ,however that is an inaccurate assumption. To understand why, let look at the design of a turbojet engine
Below is a diagram of a normal turbofan engine commonly used in all aircraft flying nowadays: The 2 main components that responsible for thrust are the Fan and turbine.
?w=1200
The compressor , turbine and combustor ( also know as the engine core ) move air at very high speed hence, they are less dependence on air density and aircraft velocity. On the other hand, the fan stage moves air at much slower rate ,which is much more fuel efficient and also mix the exhaust plume with cold air, thus reduce the temperature of the plume.Not all air ##### in by the first stage fan will go through the engine’s core (compressor , turbine and combustor ) some will pass through the outer duct. The air that passed through the outer duct is called bypass air.To get to a certain thrust level, an engine can either have very big fan and small core ( good for combat radius and thermal signature ) or very big core and relatively small fan (good for speed and high altitude performance )
Due to reasons stated above one of the solution for exhaust temperature reduction is to use engines with higher bypass ratio , the trade off of such design choice is the aircraft will not be able to fly very high or very fast. ( F-135 have much higher bypass ratio compared to F-119 , EJ200 , Snecma M88 , R-15 ,F404 ) A common misconception is that engine turbine inlet temperature is also proportional to exhaust temperature, that is wrong however, in reality turbine inlet temperature does not reflect the engine case temperature or even the exhaust plume temperature. It simply means that the gases entering the turbine is at a higher energy state and the engine will yield more gross energy per drop of fuel or air entering the combustor(s). That energy however, is extracted to do work first by the high-pressure turbine, then by the low-pressure turbine before going out the tail pipe at a given velocity. The final temperature depends on how much energy is extracted to drive the compressors and the fan, and how much bypass air is mixed into the exhaust. The F-35 has twice as many low pressure turbine stages which in theory will extract more energy. It also has a bigger and higher pressure ratio fan which adds energy to the exhaust as well as introduce relatively cold air into the mix. The exhaust plume temperature and engine case temperature can never be derived from the turbine inlet temperature alone. It is also important to remember that unlike rocket, jet engines are air-breathing engine, which mean their performance depending alot on air density, the thinner the air the less thrust they will be able to generate, so aircraft thrust will reduce as they go higher. For example : a jet engine that can generate more than 190kN at sea level can be struggled to push out 10kN at 40-50K feet. On the other hand, a rocket engine can generate the same amount of thrust regardless of altitude Modern stealth aircraft also use exotic engines nozzles that either long and flat or with various spike so that exhaust pumes become unstable and mixed quicker with cool ambient air .As a result, the heat will be dissipated rapidly Example :
www.richard-seaman.com
2) F-35 with spikes nozzles :
LOAN nozzles
Stealth aircraft are also designed so that from front , the view of their engines nozzles will be blocked by their vertical and horizontal stabilizer Example: conventional aircrafts ( on the left ) have exposed engine nozzles while stealth aircraft ( on the right ) have masked nozzles
References : Barr, E. S. “Historical Survey of the Early Development of the Infrared Spectral Region,” American Journal of Physics, Vol. 28, No. 1 (January 1960), p. 49. Herschel, W. “Experiments on the Refrangibility of the Invisible Rays of the Sun,”Philosophical Transactions of the Royal Society of London, Vol. 90 (1800), pp. 284–292. Herschel, W. “Investigation of the Powers of the Prismatic Colours to Heat andIlluminate Objects; With Remarks, That Prove the Different Refrangibility ofRadiant Heat. To Which Is Added, an Inquiry Into the Method of Viewing the SunAdvantageously, With Telescopes of Large Apertures and High Magnifying Powers,” Philosophical Transactions of the Royal Society of London, Vol. 90 (1800),pp. 255–283. Hudson, R. Infrared System Engineering. Hoboken, New Jersey, John Wiley & Sons,1969. Huygens, C. Treatise on Light. London, Macmillan and Co., Limited, 1690. Naval Surface Warfare Center. Genesis of Infrared Decoy Flares. The Early Years from 1950 into the 1970s, by B. Douda. Crane, Indiana, NSWC, 26 January 2009. (ADA495417, UNCLASSIFIED.) Newton, I. Opticks: or a Treatise of the Reflections, Refractions, Inflections & Colours of Light. The Second Edition, With Additions. Printed for W. and J. Innys, printers to the Royal Society, at the Prince’s- Arms in St. Paul’s Church-Yard, London, 1718. Planck, M., and Willis, A. P. Eight Lectures on Theoretical Physics. Mineola, NewYork, Dover Publications Inc., 1998. P. 92. Sylvania Electronic Defense Laboratories. Radiometry, by F. Nicodemus. Mountain View, California, Sylvania Electronic Defense Laboratories, 15 March 1965.(Report Number EDL-G324, publication UNCLASSIFIED.) Westrum, R. Sidewinder: Creative Missile Development at China Lake. Annapolis,Maryland, Naval Institute Press, 1999. Schleher, C.D. (1978) MTI Radar, Artech House. Schleher, C.D. (1999) Electronic Warfare in the Information Age, Artech House. Skolnik, M.I. (1980) Introduction to Radar Systems, McGraw-Hill. Stimson, G.W. (1998). Introduction to Airborne Radar, 2nd edn, SciTech Publishing Inc. Thornborough, A.M. and Mormillo, F.B. (2002) Iron Hand – Smashing the Enemy’s Air Defences,Patrick Stephens.Joint Advanced Strike Technology Program, Avionics architecture definition – issues/decisions/rationale document, version 1, 8 August 1994. A. Doucet, N. Gordon, and V. Krishnamurthy. Particle lters for state Joint Advanced Strike Technology Program, Avionics architecture definition – issues/decisions/rationale document, version 1, 8 August 1994. A. Doucet, N. Gordon, and V. Krishnamurthy. Particle lters for state estimation of jump Markov linear systems. IEEE Transactions on Signal Processing, 49(3):613-624, 2001.N.J. Gordon, D.J. Salmond, and A.F.M. Smith. A novel approach to nonlinear/non-Gaussian Bayesian state estimation.In IEE Proceedings on Radar and Signal Processing, volume 140, pages 107-113, 1993.F. Gustafsson. Adaptive Filtering and Change Detection. John Wiley & Sons Ltd, 2000. Share this: TwitterFacebook13Google
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22:30 11.11.2016
shizl_badizl (shizl_badizl) писал(а) в ответ на :
> За счет чего в первую очередь F-35 будет громить 4ое поколение? > > 1) Stealth.
Шизл, не читали бы вы глупостей на ночь. Если действительно хотите разобратся в теме, а не просто копи-пастить чужие не совсем корректные статьи... Рекомендую
07:16 12.11.2016
_developer (_developer) писал(а) в ответ на :
> Шизл, не читали бы вы глупостей на ночь. > Если действительно хотите разобратся в теме, а не просто копи-пастить чужие не совсем корректные статьи... > Рекомендую
Я вам рекомендую не писать глупости. Radar cross section это отдельная тема которая прекрасно изложена в статье Radar Fundametals из 2 частей....позволю себе сделать копипаст 1 части.
Radar Fundamentals (Part I)
Introduction
The word “RADAR” is an acronym for RAdio Detection And Ranging. As it was originally conceived, radio waves were used to detect the presence of a target and to determine its distance or range.The reflection of radio waves by objects was first noted more than a century ago. In 1903, the reflection of radio waves was employed in Germany to demonstrate detection of ships at sea.In 1922, Marconi presented the same idea in Britain but received little official interest. These early experiments used continuous wave, or CW, transmissions and relied on the reflection of a transmitted wave from a target to indicate the presence of a target. CW transmissions can detect the presence of an object and, if the radio wave is formed into a narrow beam, can also provide azimuth information. CW transmissions cannot provide range.
The lack of range information was a serious limitation but was finally overcome by modulating the radio wave transmissions to send out a train of short pulses . The time between pulse transmission and an echo return to the receiver provides a direct measurement of range
Target angle discrimination is another critical capability of radar systems. In order for a radar system to detect a target, the antenna must be pointed at the target during the transmission and reception of RF energy. The ability of a radar system to accurately determine angle is a function of the horizontal beamwidth of the antenna. If the radar sweep is referenced to true North, the angle of a radar return can be measured relative to true North.
Determining targets velocity is an important capability of radar systems, to achieve that radars take advantage of Doppler effect. Doppler effect is the phenomenon that frequency of radio waves will be changed or shifted when reflected from a target moving relative to the radar. To measure exact speed the central processor of radar calculate the different between the frequency of transmitted wave and frequency of reflected wave . In pictures below fo is the transmitted frequency of the radar, and ft is the frequency of the reflected wave.
For a stationary target, the frequency of the reflected signal will equal the frequency of the transmitted signal
For a target moving toward the radar, the frequency of the reflected signal will be higher than the transmitted signal
The reflected frequency for a target moving away from the radar will be lower than the transmitted frequency
It important to remember that factors affect Doppler shift is not target absolute velocity but target’s radial velocity. As a result aspect angle between target and radar is very important.
Пропустил в статью часть глоссария как то длина волны итд....
Radar Detection Range
When discussing about aircraft and modern avionics, especially stealth aircraft like F-22, modern jammers like ALQ-99 or modern surface to air missiles systems such as S-300/400, many enthusiasts have inaccurate assumption that radar detection, tracking is a binary quality ( either they assume that radar will always detect the aircraft or the aircraft is completely invisible ).That assumption is inaccurate, however.Regardless of the systems involves, radar detection is a quantity characteristic that affected by factors such as radar peak power, pulse repetition frequency, target’s radar cross section , radar gain and so on.Radar detection range can be estimated by the equation above.
Radar cross section
Radar cross section is not the same as the area of the target even though square meters unit is often used to measure radar cross section (another unit that can be used to measure RCS is dBsm ).Radar cross section is the measure of a target’s ability to reflect radar signals in the direction of the radar receiver. In others words, it is a measure of the ratio of backscatter power per steradian (unit solid angle) in the direction of the radar (from the target) to the power density that is intercepted by the target .The conceptual definition of RCS includes the fact that not all of the radiated energy falls on the target.
Projected cross section: The geometric cross section refers to the area the target presents to the radar, or its projected area. This area will vary depending on the angle, or aspect, the target presents to the radar. In other words, the target will probably present the smallest projected area to a radar if it is flying directly toward the radar and is viewed head-on. A view from the side, top, or underneath will present a much larger projected area. Reflectivity: .Radar power does not necessarily reflect equally from all parts of an aircraft, and some components produce stronger radar reflections than others. In addition, some radar power is usually absorbed by the target. This absorption is especially true of aircraft coated with special substances called Radar Absorbent Materials (RAM) or those using internal reflectors called Radar Absorbent Structures (RAS) that trap incoming radar waves.Reflectivity refers to the fraction of the intercepted power that is reflected by the target, regardless of direction Directivity: Radar energy is not reflected evenly,depending on exact target’s shape radar wave will reflect toward some direction more than the others.The power that is reflected toward the radar is called the backscattered power . Directivity is defined as the ratio of the power that is backscattered in the direction of the radar to the power that would have been scattered in that direction if the scattering were in fact uniform in all directions Radar cross section of simple metal shape can be estimated by equation in table below (The variable λ represents the wavelength of the radar, which is assumed to be smaller than the dimensions of the shape.)
Unlike a sphere or a cylinder , an aircraft is a very complex target. It has a great many reflecting elements and shapes. For such targets there are no firm relationship between a target’s surface and RCS. Hence, aircraft radar cross section must be measured because it varies significantly depending upon the direction of the illuminating radar.
Example: simulated radar cross section of a Mig-29 as a function of aspect angle.
Example 2 : radar cross section of a C-29 cargo plane as a function of aspect angle
The RCS of military ground and sea-based vehicles are often larger than that of military aircraft because the latter are generally more rounded for aerodynamic reasons while ground , sea vehicles are often made up of flat armor plates and lots of brackets, antennas etc.
Aircraft can be designed to scatter over 99% of signal energy away from radar direction and absorb 99% of the rest , giving them much lower RCS.
Example :Simulated radar cross section of a stealth aircraft as a function of aspect angle.
An unquie characteristic of a sphere is that it’s radar cross section is not affected by it’s orientation to the radar beam unlike others shape 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 the optical 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.
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 , vertex …etc , 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 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 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 lobes 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 increase 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 effect of surfacewave 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 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 emissions. 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 azimuthal 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: First generation stealth aircraft on the left (F-117) , has much higher number of sharp edge than the one on the right (F-35)
Regarding the issue of stealth and low frequency, there are 3 common misconceptions .The first common misconception is 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 despite operating within low-frequency range from UHF to F band , AWACs radars 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 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
И внимание....3 misconception
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 )
Radar detection range and RCS
Radar Gain (directivity)
Most radiators emit stronger radiation in one direction than in another As a transmitting antenna, gain describes how well the antenna converts input power into radio waves headed in a specified direction . In layman term: Gain describe how narrow the radar beam is, high gain radars create narrow beamwidth, low gain radars create wide beamwidth.Narrow radar ‘s beamwidth benefit radar resolution and detection range while wide radar beam benefit sector scanning time. The relationship between radar gain and operating frequency , radar aperture is illustrated in the table below.
Radar Beamwidth and Elevation-Azimuth Resolution
As stated earlier , radar beam width play a vital role in their angular accuracy characteristics because as long as targets stay within the radar beam , there will be reflection , the problem is if several targets fly close enough that their angular separation is smaller than the radar beamwidth , all the return echoes will be blended into one return , and radar will only display a single target on screen . To display two distinct radar returns of 2 target close to each other, radar beam needs to be able to pass between them without causing a return. Elevation-azimuth resolution is the ability of a radar to display two targets flying at approximately the same range with a certain angular separation, such as two fighters flying line-abreast tactical formation. The elevation-azimuth resolution capability is usually expressed in nautical miles and corresponds to the minimum angular separation required between two targets for separate display.
Angular resolution in nautical miles ( 1.852 km ) can be estimated by equation below
It important to remember that a radar vertical beamwidth is not necessarythe same as it’s horizontal beamwidth . Hence, the azimuth and elevation resolution may be different.
Range Resolution Range resolution is the ability of a radar to separate two targets that are close together in range and are at approximately the same azimuth . The range resolution capability is determined by pulse width.A radar pulse in free space occupies a physical distance equal to the pulse width multiplied by the speed of light, which is about 984 feet per microsecond. If two targets are closer together than one-half of this physical distance, the radar cannot resolve the returns in range, and only one target will be displayed.
The range resolution of the radar is usually expressed in feet and can be computed using equation below. It is the minimum separation required between two targets in order for the radar to display them separately on the radar scope
Radar Resolution Cell
A radar’s pulse width, horizontal beamwidth, and vertical beamwidth form a three dimensional resolution cell . A resolution cell is the smallest volume of airspace in which a radar cannot determine the presence of more than one target. The resolution cell of a radar is a measure of how well the radar can resolve targets in range, azimuth, and altitude. The horizontal and vertical dimensions of a resolution cell vary with range. The closer to the radar, the smaller the resolution cell will be .The physical dimention of resolution cell can be easily calculated using equation given for range and angular resolution given above.
Radar gain and radar detection range
Duty Cycle
Duty cycle is the fraction of time that a system is in an “active” state. Duty cycle is the proportion of time during which a component, device, or system is operated. If a transmitter operates for 1 microsecond, and is shut off for 99 microseconds, then is run for 1 microsecond again, and so on. The transmitter runs for one out of 100 microseconds, or 1/100 of the time, and its duty cycle is therefore 1/100, or 1 percent. The duty cycle is used to calculate both the peak power and average power of a radar system.
Peak Power
The energy content of a continuous-wave radar transmission may be easily figured because the transmitter operates continuously. However, pulsed radar transmitters are switched on and off to provide range timing information with each pulse. The amount of energy in this waveform is important because maximum range is directly related to transmitter output power. The more energy the radar system transmits, the greater the target detection range will be. Peak power is the amplitude, or power, of an individual radar pulse.
Average Power
Average power is the power distributed over the pulse recurrence time. . The energy transmitted by average power can be computed by multiplying average power by PRT. Since the energy in a set of pulses determines detection range, average power or energy provides a better measure of the detection range of a radar than does peak power. Average power can be increased by increasing the PRF, by increasing the pulse width, or by increasing peak power.
Minimum Detectable Signal ( P-min )
Radar send out pulses and analyze reflection to detect and track targets. The reflection signal power is competing with some interfering signal in order to be detected or recognized. Interfering signal sources may be ground or sea returns, meteorological clutter returns, atmospheric reflections, jamming, or more likely, random noise generated within the receiving circuitry. The latter source is always present to some degrees, while the other sources are variable and can be zero. Therefore, the internal noise power is normally used for determining the maximum range of the radar system.The maximum range performance is determined by a very small signal to noise ratio when the signal begins to fade and become indistinguishable from the noise.
Rest Time
Rest time is the time between the end of one transmitted pulse and the beginning of the next. It represents the total time that the radar is not transmitting. Rest time is measured in microseconds
Recovery Time (RT)
A radar is not only a transmitter but also a receiver .Recovery time is the time immediately following transmission time during which the receiver is unable to process returning radar energy. RT is determined by the amount of isolation between the transmitter and receiver and the efficiency of the duplexer. A part of the high power transmitter output spills over into the receiver and saturates this system. The time required for the receiver to recover from this condition is RT
Listening time (LT)
Listening time is the time the receiver can process target returns often express in micro seconds . Listening time is measured from the end of the recovery time to the beginning of the next pulse, or PRT- (PW + RT).
The relationship between recovery time , rest time and listening time is illustrated in the diagram below
Список литератруры
References
Adamy, D., “Seduction Decoys”, Journal of Electronic Defense, Vol. 20, No 7,pp. 58-59, July, 1997. Adamy, D., “EW 101”, Journal of Electronic Defense, Vol. 21, No 1, pp. 14-19,January, 1998. Chrzanowshi, E. J., “Active Radar Electronic Countermeasures”, Artec House, Inc., Norwood, MA, 1990. Neri, F.,” Introduction to Electronic Defense Systems”, Artec House, Inc., Norwood, MA, 1990. Skolnik, M. L., “Introduction to Radar Systems”, McGraw-Hill, Inc., New York, NY, 1980. 4513 ATTG/INW, Advanced Radar Principles for Electronic Combat, 15 April 1991. “Electronic Warfare Fundamentals”NOVEMBER 2000,Nellis AFB NV J.M. Headric and M. I. Skolnik. “Over-the-horizon radar in the HF band.” in Proceedings of IEEE, Vol. 6, pp. 664-672, 1974. J. M. Headrick. “HF over-the-horizon radar.” in Radar Handbook, 2nd ed., M. I. Skolnik, New York: McGraw-Hill,1990. L. Sevgi. “Stochastic modeling of target detection and tracking in surface wave high frequency radars.” Int. J. of Numerical Modeling, Vol. 11, No 3, pp.167-181, May 1998. L. Sevgi, A. M. Ponsford. “Multi-sensor, multi-resolution integrated maritime surveillance systems.” inIEEE Signal Processing and Applications Symp. (SIU’99), June 16-19, 1999, Ankara, Turkey. L. Sevgi, A. M. Ponsford. “Multi-sensor integrated maritime surveillance systems.” in IEEE International Multi-Conference on Circuits, Systems, Communications and Computers (CSCC’99), July 4-8, 1999, Athens, Greece. L. Sevgi, A. M. Ponsford, H.C. Chan. “An integrated maritime surveillance system based on surface wave HF radars, Part I – Theoretical background and numerical simulations.” IEEE Antennas and Propagation Magazine, V.43, N.4, pp.28-43, Aug. 2001. A. M. Ponsford, L. Sevgi, H.C. Chan. “An integrated maritime surveillance system based on surface wave HF radars, Part II – Operational status and system performance.” IEEE Antennas and Propagation Magazine, V.43., N.5, pp.52-63, Oct. 2001 M. C¸akır, G. C¸ akır, L. Sevgi. “Parallel FDTD-based radar cross section (RCS) simulations.” in Fifth International Conference on Electrical and Electronics Engineering (ELECO 2007), Dec 5-9, 2007, Bursa, Turkey. G. C¸akır, M. C¸ akır, L. Sevgi. “Radar cross section (RCS) modeling and simulation: Part II – A novel FDTD-based RCS prediction virtual tool.” IEEE Antennas and Propagation Magazine, Vol. 50, No.2, pp.81-94, Apr 2008. Gonca C¸AKIR1, Levent SEVG˙I2″Radar cross-section (RCS) analysis of high frequency surface wave radar targets”Turk J Elec Eng & Comp Sci, Vol.18, No.3, 2010 G.T. Ruck, D.E. Barrick, W.D. Stuart and C.K. Krichbaum, Radar Cross-Section Handbook, Vol. 1, New York: Plenum Press, 1970. C.W. Trueman, S.J. Kubina, S.R. Mishra and C. Larose, “RCS of four fuselage-like scatterers at HF frequencies,” IEEE Trans. Antennas and Propagation, vol. AP-40, no. 2, Feb. 1992, pp. 236-240. Dr. Robert M. O’Donnell IEEE New Hampshire Section Guest Lecturer”Radar Systems Engineering” Professor Oleg I. Sukharevsky “Electromagnetic Wave Scattering by Aerial and Ground Radar Objects”2015 Williams/Cramp/Curtis: Experimental study of the radar cross-section of maritime targets,Electronic Circuits and Systems, Vol. 2, No. 4,July 1978 P.Bhartia, I.Bahl, Millimeter-Wave Engineering and Applications, John Wiley & Sons, 1984 JIANG Hao, ANG Hai-song”The Analysis of Aerodynamic and Stealth Characteristic of F-35 Fighter”College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China (STEPHEN TRIMBLE Flight Global 2010 ) accessed 20 March 2016 < > Physics And Progress Of Low-Frequency Counterstealth Technology (accessed 25 August 2016 ) Aviation Week & Space Technology ,Daniel Katz
12:45 17.11.2016
красота
09:26 18.11.2016
ну а пока троли тужатся в невнятных потугах...схема двигателя F-35B
УДК типа "Wasp" подтвердили возможность замены движка F-35B силами маринеров.
08:18 30.11.2016
ну а пока троли тужатся в попытках доказать несостоятельность программы JSF, Пентагон выделили 1, 28 млрд в виде аванса на LRIP 10. Партию из 90 самолетов.
И таки да....новый контракт предусматривает цену за F-35A в 80 млн.
09:40 12.12.2016
F-35 начал испытания на авиабазе luke совместно с F-16
ну и годнота на последок.....
09:45 12.12.2016
ну ...и первые F-35 на пути в Израиль.
на этой неделе первые F-35 прибудут на ближний восток.
09:55 12.12.2016
F-35 на УДК. Прогресс как он есть
14:32 12.12.2016
Последние новости CNN по поводу перелета первых F-35 в Израиль.
