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Stealth Radar


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Stealth Radar, technology, low frequency radar, phase array radar, radar absorbing materials, IR sensors, Stealth planes and ships, AESA, Radar jamming

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Stealth Radar

  1. 1. 1 Indian Institute of Space Science and Technology Thiruvananthapuram Department of Avionics AV323: Radar Systems Course Seminar Report on Stealth Radar G Ram Prabu K V R Dinesh Kumar Reddy D Pramod Reddy
  2. 2. 2 Acknowledgement First of all, we would like to thank Sanjeev Kumar Mishra sir, for his encouragement to study about Stealth Radar which is one of the major research area. We are sure that the study made in this topic will be of definite help in our future. Next we thank all the various sources which provided us with sufficient references and material to do this study.
  3. 3. 3 Abstract of Report The general idea about stealth and the ways in which stealth technology implemented in planes and ships is described. The ways to counter is illustrated and the radars used for this are depicted in detail. The concept of jamming is pictured in a systematic manner and the radar technology used to counter jamming is stated in a brief and clear manner.
  4. 4. 4 Contents 1. Introduction 2. What is Stealth Technology? 3. Reduction of the Radar Cross section 3.1. Bistatic Radar 3.2. Low Frequency Radar 3.3. Phased Array Radar operating in L-band 3.4. Countermeasures to the Radar Absorbing Materials (RAM) 3.5. Heat and IR detection using IR sensors 3.6. Acoustic countermeasures 3.7. Other Advanced Methods 4. Stealth Planes and Ships 5. Radar Technologies used to detect Stealth objects 6. What is Radar detection and jamming? 7. Active electronically scanned array 8. Summary 9. References
  5. 5. 5 1. Introduction Stealth means movement that is quiet and careful in order not to be seen or heard, or secret action. Radars are associated with stealth in two ways. Radars that detect incoming targets which are in stealth mode and radars which operate in stealth mode (radar signals which cannot be detected by enemies and jammed). 2. What is Stealth technology? The radar antenna to send out a burst of radio energy, which is then reflected back by any object it happens to encounter. The radar antenna measures the time it takes for the reflection to arrive, and with that information can tell how far away the object is. The metal body of an airplane is very good at reflecting radar signals, and this makes it easy to find and track airplanes with radar equipment. The goal of stealth technology is to make any object invisible to radar. There are two different ways to create invisibility:  The object can be shaped so that any radar signals it reflects are reflected away from the radar equipment.  The object can be covered in materials that absorb radar signals. The first two mean reducing the radar cross section  The acoustic and thermal aspects of the object are also considered. 3. Reduction of the radar cross section Radar cross-section (RCS) is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected. An object reflects a limited amount of radar energy. A number of different factors determine how much electromagnetic energy returns to the source such as:
  6. 6. 6 1) Material of which the target is made 2) Absolute size of the target 3) Relative size of the target (in relation to the wavelength of the illuminating radar) 4) The incident angle (angle at which the radar beam hits a particular portion of target which depends upon shape of target and its orientation to the radar source) 5) Reflected angle (angle at which the reflected beam leaves the part of the target hit, it depends upon incident angle) 6) The polarization of transmitted and the received radiation in respect to the orientation of the target. All these factors can be altered to make sure the RCS is very less. Changes that needs to be made on the design of objects like the plane or ship to make it invisible to radar: 1) Main design modification: The stealth aircraft and ships are designed in such a manner that the reflection from surface occurs like this 2) Vehicle shape and structure: 1) Smooth edges receive maximum radio wave reflectors. 2) A stealth aircraft on the other hand, is made up of flat surfaces. When signal hits a stealth plane the signal deflects away. 3) Mainly plain form alignment. 4) The leading edges of wing and tail surfaces set at same angles. 5) Use of re-entrant triangles behind skin. 6) Distinctive serrations used in external airframes. 7) Propulsion subsystem shaping. 8) Now in research is fluidic nozzles for thrust vectoring.
  7. 7. 7 3) Acoustic precautions: Acoustic stealth plays a primary role in submarine stealth as well as for ground vehicles. Submarines use extensive rubber mountings to isolate and avoid mechanical noises that could reveal locations to underwater passive SONAR arrays. The noise from engines in air and that of the rotor blades in helicopter have to be reduced to prevent detection. In supersonic speeds sonic boom can be used for detection also. 4) Visibility and Infrared (heat): The vehicles are painted black or white to enable visual camouflage. The heat radiation from the engines and other parts must be minimum to hide from IR or Heat detectors. The trail heat is also to be reduced to hide from the incoming missiles. 5) Radar Absorbing Materials (RAM): Radar-absorbent material (RAM), often as paints, are used especially on the edges of metal surfaces. While the material and thickness of RAM coatings can vary, the way they work is the same: absorb radiated energy from a ground or air based radar station into the coating and convert it to heat rather than reflect it back. Current technologies include dielectric composites and metal fibers containing ferrite isotopes. Paint comprises of depositing pyramid like colonies on the reflecting superficies with the gaps filled with ferrite-based RAM. The pyramidal structure deflects the incident radar energy in the maze of RAM. A commonly used material is known as Iron Ball Paint. Iron ball paint contains microscopic iron spheres that resonate in tune with incoming radio waves and dissipate the majority of their energy as heat, leaving little to bounce back to detectors. FSS are planar periodic structures that behave like filters to electromagnetic energy. The considered frequency selective surfaces are composed of conducting patch elements pasted on the ferrite layer. FSS are used for filtration and microwave absorption.
  9. 9. 9 4. Famous Stealth Planes and Ships Sukhoi_T-50_Maksimov- Stealth fighter Indian air force The various generation fighters A design of Stealth ship (Picture1-US Navy Sea shallow) Stealth Helicopters US stealth copter RAH -66
  10. 10. 10 5. Radar Technologies used to detect Stealth objects 1. Bistatic Radar: It has potential advantages in detection of stealthy targets which are shaped to scatter energy in directions away from the monostatic radars. 2. Low Frequency Radar: Shaping offers far fewer stealth advantages against low-frequency radar. If the radar wavelength is roughly twice the size of the target, a half-wave resonance effect can still generate a significant return. Low-frequency radar is radar which uses frequencies lower than 1 GHz, as opposed to the usual radar bands, which range from 2 GHz and up, and the maximum is 40 GHz. The radar cross section of any target depends on the radar transmitted frequency. Below 900 MHz the target radar cross section increases exponentially, however the increased radar cross section means that there is much more radar return from undesirable sources, such as cloud cover and rain (cf. weather radar). It is because of this that radars are traditionally at much higher frequency, with an exception being the radars operated in the 3-30 MHz band which are used as over-the-horizon radar stations because signals in that range are able to reflect off the ionosphere. Radars do not absorb this frequency.
  11. 11. 11 This technology was uncovered by the Serbs in Kosovo during the Kosovo War in the 1990s. This technology was used to shoot down an F-117 nighthawk via a specially modified anti-aircraft missile to use this radar to guide it towards the target. The Serbs say that this radar's drawback is that of the huge amount of clutter it creates because of the sensitivity of this radar. They also say that it is highly effective against stealth aircraft, stealth ships, etc. pending clutter can be reduced. Recent interest has accumulated in developing radars which operate in these low frequencies to help counter the advancement in stealth technology by applying advanced digital signal processing to these bands in order to reduce radar clutter. If the radar wavelength is roughly twice the size of the target, a half-wave resonance effect can still generate a significant return. However, low-frequency radar is limited by shortage of unused frequencies, lack of accuracy given the long wavelength, and by the radar’s size, making it difficult to transport and making for an easy target. A long-wave radar may detect a target and roughly locate it, but not identify it, and the location information lacks sufficient weapon targeting accuracy. The disadvantage is the size is large as seen in the diagram and has severe Doppler ambiguities, range is unambiguous. A low frequency radar used to shoot down F-117 nighthawk stealth flight
  12. 12. 12 3. Phased Array Radar operating in the L-band: The use of geometry to deflect radar return (bounce) energy away from the radar receiver represents the corner stone of modern stealth technology. The technique is based on the fact that only those surfaces parallel to the electromagnetic wave front will reflect the wave back to the receiver. In optical terms this is stated as: "The angle of reflection is equal and opposite of the angle of incidence". In other words, only a wave front incident at 90 degrees to a surface will be reflected back to the source (radar transmit/receive antenna). By angling all surfaces with respect to the probable direction of incoming radar emissions (in most cases, the horizon), the radar wave is reflected away from the receiver. The scientific paradigm that underlies this technique is known as ray trace optics, and is based on Pierre Fermat's principle of least time. Fermat's principle or the principle of least time is the principle that the path taken between two points by a ray of light is the path that can be traversed in the least time. This principle is sometimes taken as the definition of a ray of light. However, this version of the principle is not general; a more modern statement of the principle is that rays of light traverse the path of stationary optical length with respect to variations of the path. In other words, a ray of light prefers the path such that there are other paths, arbitrarily nearby on either side, along which the ray would take almost exactly the same time to traverse. Fermat's principle can be used to describe the properties of light rays reflected off mirrors, refracted through different media, or undergoing total internal reflection. It follows mathematically from Huygens Principle (at the limit of small wavelength). A wave front is created in this manner where all the waves having the same phase is obtained.
  13. 13. 13 The wave front appears to converge on the focal point because this point is the only location where all in phase waves originating at the incoming wave front plane, are in phase after reflection by the mirror. The microwave transmitter array is phased such that the wave front plane (shown in yellow) is off axis from the antenna array. Now consider the target plane (shown in red). Because the transmitted microwave energy exists at all points (not just the in phase plane), AND the target plane angle is complimentary to the wave front plane angle, the resulting reflected wave plane (from the target plane) will be parallel to the antenna array plane. In other words, by pre-distorting the transmitted radar pulse so the wave front plane is complimentary to the target plane, the reflected wave plane will be in phase at the antenna array, and therefore detectable by the radar receiver array. In short if all the waves generated are in phase then the waves that are reflected will also remain in phase and can be detected by any antenna array which detects the phase. The disadvantage of this type is the radar system must have some prior knowledge of the expected range of target angles in order to pre-distort the transmitted microwave pulse. However once a set of actual angles are obtained by painting the target, this represents additional signature information that can be used to identify the type of target.
  14. 14. 14 The advantage is that the target does not recognize or identify that it is being traced. Most phased array radar systems use hundreds, and in some cases, thousands of transmit/receive channels in the array, and are therefore large and very costly. However, these systems were designed and built in an era when computer technology was still relatively crude by the standards of today. With the advent of small, fast, inexpensive computers, the number of microwave transmit/receive channels required for an effective stealth countermeasure phased array radar would be less than sixty, and with fine tuning of the computer hardware, software and antenna geometry, might be as few as ten. Obviously these systems would be both very portable, and inexpensive to build in mass production. shows the phased array radar used on the F/A 22 Raptor. This radar employs approximately 2000 microwave transmitter/receiver pairs, each the size of a pack of chewing gum.
  15. 15. 15 The radar used in the Russian ships used to shoot down the US Stealth planes, this operates at the L band Basic block diagram of a phased array radar The transmit chain (Tx) consists of a phase shifter, attenuator and several gain stages to achieve the desired output power. The receive chain (Rx) similarly consists of a phase shifter, attenuator, and low noise amplifiers. In addition, there is often a limiter added to the receive chain to protect the low noise amplifier. Isolation of the transmit channel from the receive channel is accomplished using either a ferrite-based circulator or a high power switch. A number of control signals must be supplied to the module to set
  16. 16. 16 the states of the main control components. In addition, DC power and monitoring functions are often implemented. Increased capability and complexity can be added to the system through the implementation of multiple polarization states, digital waveform generation and other features. Making the appropriate semiconductor technology choices and applying commercial practices at the component and module level enables opportunities for significant cost reduction, while maintaining the required high performance to be realized. With the proper choice of semiconductor based technology, cost reduction is routinely achieved through functional integration at the Integrated Circuit (IC) level. Aggressive integration and size reduction at the die level leads to increased functionality per square millimeter of semiconductor and ultimately, lower cost per function. 4. The countermeasures to the Radar Absorbing Materials (RAM): Radar Absorbent Material. In this case the object to be protected is given a coating of successive layers of magnetic composition material such as Ni-Mn-Zn sandwiched with dielectrics that convert 95% of incident RF energy to heat. This material can be made as thin as 1.75 cm, which is practical for aircraft use; however, the weight penalty of 24.9 kg per m2 is excessive. This would not eliminate their use aboard ship or at ground-based facilities. Another approach, involving continuing research, consists of a phenolic-fiberglass sandwich material. This structure again converts 95% of incident RF energy to heat by using a resistive material consisting of carbon black and silver powder. This material is effective over the range of 2.5 to 13 GHz, which encompasses many fire control and weapon-guidance radars. The disadvantage of this approach is that while it is lightweight and relatively thin, it is not able to handle the high temperature and erosion processes at supersonic speeds. These methods, though promising, still cannot deal with some of the lower radar frequencies. As this material is effective over the range of 2.5 to 13 GHz, therefore low frequency radars of operating frequency less than 2.5 GHz are used. 5. Heat and IR detection using IR sensors: Any engine liberates some amount of heat and chemical effluents. Infrared stealth is accomplished by mixing hot exhaust gases with air at ambient temperature, prior to release into the atmosphere. A related technique involves spreading the hot exhaust gas plume over a large area as it's released into the atmosphere. Both methods are designed to lower the effective temperature of the exhaust plume, thereby making infrared detection more difficult. However, the exhaust plume has other characteristics that are detectable, and when coupled with absence of heat it is certainly a stealth aircraft.
  17. 17. 17 The detectable signatures of the exhaust gas plume fall into two broad categories. A) Chemical signatures B) Physical signatures The chemical signatures of exhaust gas plumes result from the combustion process itself, and include elevated levels of oxides of carbon and nitrogen (along with water vapor), relative to the surrounding atmosphere. These chemical signatures are detectable with properly designed radar systems. For instance nitric oxide (NO) has a resonance at 1.665 GHz, and carbon monoxide has a resonance at 9.361 GHz. A dual band backscatter search radar operating at these frequencies, in conjunction with a coaxial mounted focal plane infrared detector would make an ideal detector for stealth platforms. The use of multi-wavelength backscatter Lidar offers nearly unlimited flexibility in chemical signature analysis of exhaust gas plumes. The physical signatures of the exhaust gas plume result from the large velocity differentials relative to the surrounding atmosphere. This is especially true for jet aircraft. Currently, backscatter Doppler radar in the 500MHz to 1500MHz region is used to directly measure the motion of the atmosphere in the study of weather related phenomena. Since these systems can accurately measure atmospheric motion in the 10 kilometers per hour range, the measurement of jet exhaust plumes at 100 to 600+ kilometers per hour range will prove very easy to accomplish. As with chemical signature analysis, the use of a coaxial mounted focal plane infrared detector will confirm the stealth nature of the platform. The Chilbolton ACROBAT (Advanced Clear-air Radar for Observing the Boundary layer And Troposphere) is an example of backscatter clear air Doppler radar technology. An artistic view of a IR sensor to detect stealth radar
  18. 18. 18 6. Acoustic countermeasures: Acoustic stealth plays a primary role in submarine stealth as well as for ground vehicles. Submarines use extensive rubber mountings to isolate and avoid mechanical noises that could reveal locations to underwater passive SONAR arrays. Early stealth observation aircraft used slow-turning propellers to avoid being heard by enemy troops below. Stealth aircraft that stay subsonic can avoid being tracked by sonic boom. The presence of supersonic and jet-powered stealth aircraft such as the SR-71 Blackbird indicates that acoustic signature is not always a major driver in aircraft design, although the Blackbird relied more on its extremely high speed and altitude. One possible technique for reducing helicopter rotor noise is 'modulated blade spacing'. Standard rotor blades are evenly spaced, and produce greater noise at a particular frequency and its harmonics. Using varying degrees of spacing between the blades spreads the noise or acoustic signature of the rotor over a greater range of frequencies. Another method of physical detection is worthy of mention. Although widely used in WWII, it seems acoustic signature analysis has fallen out of favor in recent decades. While most stealth aircraft are very quiet during approach, the authors firsthand experience with an over flight by a B2 bomber indicates this is certainly NOT the case as the aircraft was departing. This observation may not appear to be useful, until you consider the situation depicted.
  19. 19. 19 Two acoustic sensors (1 & 2) are sequentially triggered by over flight of the stealth aircraft. Since the distance between acoustic sensors 1 and 2 is known, the time interval between triggers of sensors 1 and 2 yields the velocity of the stealth aircraft. Knowing the aircraft velocity, and the distance between sensor 2 and the countermeasure weapon, allows the weapon to be triggered in advance of stealth aircraft over flight. When employed at a natural choke point such as a long narrow valley, or an artificial choke point such as the midpoint between two conventional search radars, the utility of the tactic becomes self-evident. A typical countermeasure weapon would consist of multiple mortar launched shells, containing small metal fragments dispersed by a high explosive charge, directly in the flight path of the oncoming stealth aircraft. This countermeasure system has the added advantage of being completely passive, and therefore undetectable by the stealth aircraft. The later generations of stealth aircraft have tried to strike a balance between stealth capabilities and conventional aerodynamic capabilities. This was necessary because the ideal geometry (shape) for maximum stealth is NOT the ideal shape required to achieve maximum aerodynamic performance. Sonic boom created when a stealth plane flies
  20. 20. 20 The different types of radars and the stealth helicopters detected by them 7. Other Advanced Methods: The communication satellites, surveillance and navigational satellites, cell towers and other sources act as transmitters and there are only receivers to take them and analyze the waves The waves coming to the mobile in the car can be used to detect a stealth plane
  21. 21. 21 The quantum radar is a new type of technology that is presently popular and used in some case to create an image of the incoming object which is developed with new type of stealth technology like the plasma stealth. Image of the US stealth bomber in a quantum radar Quantum radar is a theoretical remote-sensing method based on quantum entanglement. The most convincing model has been proposed by an international team of researchers. This team designed a model of quantum radar for remote sensing of a low-reflectivity target that is embedded within a bright microwave background, with detection performance well beyond the capability of a classical microwave radar. By using a suitable wavelength converter, this scheme generates excellent quantum correlations (quantum entanglement) between a microwave signal beam, sent to probe the target region, and an optical idler beam, retained for detection. The microwave return collected from the target region is subsequently converted into an optical beam and then measured jointly with the idler beam. Such a technique extends the powerful protocol of quantum illumination to its more natural spectral domain, namely microwave wavelengths. A prototype quantum radar can be realized with current technology, and is suited to various potential applications, from standoff sensing of stealth objects to environmental scanning of electrical circuits. Thanks to its quantum-enhanced sensitivity, this device could also lead to low-flux non-invasive techniques for protein spectroscopy and biomedical imaging.
  22. 22. 22 Some common radars to detect the stealth objects The low frequency russian radar ByeloRussian KB Radar (Agat) Vostok E is an entirely new 2D VHF radar design, using a unique wideband square ring radiating element design, in a diamond lattice pattern. Electron Multiplying Charge Coupled Device (EMCCD)
  23. 23. 23 A Chinese anti stealth radar to fight the Pakistani fighters A phased array radar to counter stealth The Russian low frequency and multiple phased array used to detect all the stealth US bombers like F11 and F12
  24. 24. 24 Radars that are not detected by other Radars i.e., radars that cannot be jammed are called as radars operating in stealth mode. 6. What is Radar detection and jamming? Radar jamming and deception (Electronic countermeasure) is the intentional emission of radio frequency signals to interfere with the operation of a radar by saturating its receiver with noise or false information. There are two types of radar jamming: Mechanical and Electronic jamming. Mechanical jamming is caused by devices which reflect or re-reflect radar energy back to the radar to produce false target returns on the operator's scope. Mechanical jamming devices include chaff, corner reflectors, and decoys. 1) Chaff is made of different length metallic strips, which reflect different frequencies, so as to create a large area of false returns in which a real contact would be difficult to detect. Modern chaff is usually aluminum coated glass fibers of various lengths. Their extremely low weight and small size allows them to form a dense, long lasting cloud of interference. 2) Corner reflectors have the same effect as chaff but are physically very different. Corner reflectors are multiple-sided objects that re-radiate radar energy mostly back toward its source. An aircraft cannot carry as many corner reflectors as it can chaff. 3) Decoys are maneuverable flying objects that are intended to deceive a radar operator into believing that they are actually aircraft. They are especially dangerous because they can clutter up a radar with false targets making it easier for an attacker to get within weapons range and neutralize the radar. Corner reflectors can be fitted on decoys to make them appear larger than they are, thus furthering the illusion that a decoy is an actual aircraft. Some decoys have the capability to perform electronic jamming or drop chaff. Decoys also have a deliberately sacrificial purpose i.e. defenders may fire guided missiles at the decoys, thereby depleting limited STOCKS of expensive weaponry which might otherwise have been used against genuine targets. Electronic jamming is a form of electronic warfare where jammers radiate interfering signals toward an enemy's radar, blocking the receiver with highly concentrated energy signals. The two main technique styles are noise techniques and repeater techniques. The three types of noise jamming are spot, sweep, and barrage.
  25. 25. 25 1) Spot jamming occurs when a jammer focuses all of its power on a single frequency. While this would severely degrade the ability to track on the jammed frequency, a frequency agile radar would hardly be affected because the jammer can only jam one frequency. While multiple jammers could possibly jam a range of frequencies, this would consume a great deal of resources to have any effect on a frequency-agile radar, and would probably still be ineffective. 2) Sweep jamming is when a jammer's full power is shifted from one frequency to another. While this has the advantage of being able to jam multiple frequencies in quick succession, it does not affect them all at the same time, and thus limits the effectiveness of this type of jamming. Although, depending on the error checking in the device(s) this can render a wide range of devices effectively useless. 3) Barrage jamming is the jamming of multiple frequencies at once by a single jammer. The advantage is that multiple frequencies can be jammed simultaneously; however, the jamming effect can be limited because this requires the jammer to spread its full power between these frequencies, as the number of frequencies covered increases the less effectively each is jammed. 4) Base jamming is a new type of Barrage Jamming where one radar is jammed effectively at its source at all frequencies. However, all other radars continue working normally. 5) Pulse jamming produces noise pulses with period depending on radar mast rotation speed thus creating blocked sectors from directions other than the jammer making it harder to discover the jammer location. 6) Cover pulse jamming creates a short noise pulse when radar signal is received thus concealing any aircraft flying behind the EW craft with a block of noise. Digital radio frequency memory, or DRFM jamming, or Repeater jamming is a repeater technique that manipulates received radar energy and retransmits it to change the return the radar sees. This technique can change the range the radar detects by changing the delay in transmission of pulses, the velocity the radar detects by changing the Doppler shift of the transmitted signal, or the angle to the plane by using AM techniques to transmit into the side lobes of the radar. Electronics, radio equipment, and antenna can cause DRFM jamming causing false targets, the signal must be timed after the received radar signal. By analyzing received signal strength from side and back lobes and thus getting radar antennae radiation pattern false targets can be created to directions other than one where the jammer is coming from. If each radar pulse is uniquely coded it is not possible to create targets in directions other than the direction of the jammer. Deceptive jamming uses techniques like "range gate pull-off" to break a radar lock.
  26. 26. 26 The methods to counter radar jamming (Electronic counter measures used in stealth radar): 1) Blip enhancement is an electronic warfare technique used to fool radar. When the radar transmits a burst of energy some of that energy is reflected off a target and is received back at the radar and processed to determine range and angle. The reflected target energy is called skin return, and the amount of energy returning to the originating radar is directly proportional to the radar cross-section of the target. Basic radars present the target information on a display and displayed targets are referred to as blips. Based on the relative size of the blips on the display, a radar operator could determine large targets from small targets. When a blip
  27. 27. 27 enhancing technique is used, small targets returns are augmented to look like large targets. One early maritime application of this technique was used with an aircraft carrier and its escort ships. Because the aircraft carrier physically dwarfed the other vessels its radar return was much larger making it relatively easy for a radar operator to pick it out as a target. Escort ships were fitted with blip enhance transmitters that received and amplified the radar signal so that all of the escort ships looked like they were aircraft carrier-sized targets. When all the escort ships activated their blip enhance transmitters, all the ships blips grew on the radar display masking the true aircraft carrier blip, and confusing any attempt to target the aircraft carrier for a missile attack. 2) Constantly alternating the frequency that the radar operates on (frequency hopping) over a spread-spectrum will limit the effectiveness of most jamming, making it easier to read through it. Modern jammers can track a predictable frequency change, so the more random the frequency change, the more likely it is to counter the jammer. 3) Cloaking the outgoing signal with random noise makes it more difficult for a jammer to figure out the frequency that a radar is operating on. 4) Limiting unsecure radio communication concerning the jamming and its effectiveness is also important. The jammer could be listening, and if they know that a certain technique is effective, they could direct more jamming assets to employ this method. 5) The most important method to counter radar jammers is operator training. Any system can be fooled with a jamming signal but a properly trained operator pays attention to the raw video signal and can detect abnormal patterns on the radar screen. 6) The best indicator of jamming effectiveness to the jammer is countermeasures taken by the operator. The jammer does not know if their jamming is effective before operator starts changing radar transmission settings. 7) Using EW countermeasures will give away radar capabilities thus on peacetime operations most military radars are used on fixed frequencies, at minimal power levels and with blocked Tx sectors toward possible listeners (country borders) 8) Mobile fire control radars are usually kept passive when military operations are
  28. 28. 28 not ongoing to keep radar locations secret 9) Active electronically scanned array (AESA) radars are innately harder to jam and can operate in Low Probability of Intercept (LPI) modes to reduce the chance that the radar is detected. 10)A quantum radar system would automatically detect attempts at deceptive jamming, which might otherwise go unnoticed. 7. Active electronically scanned array An active electronically scanned array (AESA), also known as active phased array radar (APAR), is a type of phased array radar whose transmitter and receiver (transceiver) functions are composed of numerous small solid-state transmit/receive modules (TRMs). AESA radars aim their "beam" by emitting separate radio waves from each module that interfere constructively at certain angles in front of the antenna. Advanced AESA radars can improve on the older passive electronically scanned array (PESA) radars by spreading their signal emissions out across a band of frequencies, which makes it very difficult to detect over background noise, allowing ships and aircraft to broadcast powerful radar signals while still remaining stealthy. Radar systems generally work by connecting an antenna to a powerful radio transmitter to emit a short pulse of signal. The transmitter is then disconnected and the antenna is connected to a sensitive receiver which amplifies any echoes from target objects. By measuring the time it takes for the signal to return, the radar receiver can determine the distance to the object. The receiver then sends the resulting output to a display of some sort. The transmitter elements were typically klystron tubes or magnetrons, which are suitable for amplifying or generating a narrow range of frequencies to high power levels. To scan a portion of the sky, the radar antenna must be physically moved to point in different directions. Starting in the 1960s new solid-state devices capable of delaying the transmitter signal in a controlled way were introduced. That led to the first practical large-scale passive electronically scanned array, or simply phased array radar. PESAs took a signal from a single source, split it into hundreds of paths, selectively delayed some of them, and sent them to individual antennas. The radio signals from the separate antennas overlapped in space, and the interference patterns between the individual signals was controlled to reinforce the signal in certain directions, and mute it in all others. The delays could be easily controlled electronically, allowing the beam to be steered very quickly without moving the antenna. A PESA can scan a volume of space much quicker than a traditional mechanical system. Additionally, thanks to progress in electronics, PESAs added the ability to produce several active beams, allowing them to continue scanning the sky while
  29. 29. 29 at the same time focusing smaller beams on certain targets for tracking or guiding semi-active radar homing missiles. PESAs quickly became widespread on ships and large fixed emplacements in the 1960s, followed by airborne sensors as the electronics shrank. AESAs are the result of further developments in solid-state electronics. In earlier systems the transmitted signal was originally created in a klystron or traveling wave tube or similar device, which are relatively large. Receiver electronics were also large due to the high frequencies that they worked with. The introduction of gallium arsenide microelectronics through the 1980s served to greatly reduce the size of the receiver elements, until effective ones could be built at sizes similar to those of handheld radios, only a few cubic centimeters in volume. The introduction of JFETs and MESFETs did the same to the transmitter side of the systems as well. It gave rise to Amplifier-Transmitters with a low-power solid state waveform generator feeding an amplifier, allowing any radar so equipped to transmit on a much wider range of frequencies, to the point of changing operating frequency with every pulse sent out. Shrinking the entire assembly (the transmitter, receiver and antenna) into a single "transmitter-receiver module" (TRM) about the size of a carton of milk and arraying these elements produces an AESA. The primary advantage of an AESA over a PESA is capability of the different modules to operate on different frequencies. Unlike the PESA, where the signal is generated at single frequencies by a small number of transmitters, in the AESA each module generates and radiates its own independent signal. This allows the AESA to produce numerous simultaneous "sub-beams" that it can recognize due to different frequencies, and actively track a much larger number of targets. AESAs can also produce beams that consist of many different frequencies at once, using post-processing of the combined signal from a number of TRMs to re-create a display as if there was a single powerful beam being sent. However, this means that the noise present in each frequency is also received and added. Block diagram
  30. 30. 30 Advantages: 1)Low probability of intercept. 2)High jamming resistance. Disadvantages: 1)The highest Field of View (FOV) for a flat phased array antenna is currently 120°. 8. Summary Stealth in air crafts, planes and ships are no more a concealed secret. The technology of stealth is understood by many and radars to counter stealth are present. High signal processing knowledge and use of multiple anti stealth radars (ex: low frequency radar, phased array radar) give better result. Radar jamming is an electronic warfare method. Radars which operate in stealth mode (ex: AESA) prevents jamming and is an electronic counter warfare method. Most technologies in this field are not revealed and are kept veiled (secret).
  31. 31. 31 References:     Books-advantages of bistatic radar  Introduction to radar systems text book    