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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

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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.

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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.

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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

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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:

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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.

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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.

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THE TOTAL STRUCTURE OF THE LOCKHEED F-19 FIGHTER AND NORTHROP ANDVANCED TECHNOLOGY BOMBER

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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

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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.

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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

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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.

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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.

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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.

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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

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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.

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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

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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.

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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

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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

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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.

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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)

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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

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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.

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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.

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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

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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

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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

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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

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

References:

www.wikipedia.org

www.defensemedianetwork.com

www.foia.cia.gov

Books-advantages of bistatic radar

Introduction to radar systems text book

www.whale.to

www.dailymail.co.uk

www.researchinventy.com

www.defense-update.com