Stealth Technology

STEALTH TECHNOLOGY
SEMINAR REPORT

Submitted By ASHISH ZACHARIAH 06 401 015 S7 MECHANICAL
In partial fulfillment of the requirement for the award of the degree Of Bachelor of Technology in Mechanical Engineering

DEPARTMENT OF MECHANICAL ENGINEERING GOVERNMENT ENGINEERING COLLEGE BARTON HILL, THIRUVANANTHAPURAM NOVEMBER 2009

DEPARTMENT OF MECHANICAL ENGINEERING GOVT. ENGINEERING COLLEGE, BARTON HILL THIRUVANANTHAPURAM-35

CERTIFICATE
This is to certify that Seminar Report entitled STEALTH TECHNOLOGY submitted by ASHISH ZACHARIAH 06 401 015 S7 MECHANICAL to the University of Kerala in partial fulfillment of the requirement for the award of the degree of Bachelor of Technology in Mechanical Engineering is a bonafide record of the seminar presented by him.

CO-ORDINATOR

HEAD OF THE DEPT .

ACKNOWLEDGEMENT

I express my sincere gratitude to Prof. G Ramachandran, Head of the Department, Department of Mechanical Engineering, Government Engineering College, Barton Hill, Thiruvananthapuram for his valuable suggestion, advice, guidance and encouragement in carrying out the seminar. I especially thank Mr. Ranjith S. Kumar and Mr. Gopakumar S., our staff advisor and seminar coordinator, for their guidance and help rendered for the successful completion of my seminar and for making available the facilities of the department for the presentation of this seminar. I also express my gratitude to all members of staff, my parents and friends who were very co-operative for the successful presentation of this seminar report. Last but not the least I thank the God Almighty for his abundant grace on preparing this seminar report.

Ashish Zachariah

ABSTRACT
Stealth refers to the act of trying to hide or evade detection. Stealth technology is ever increasingly becoming a paramount tool in battle especially “high technology wars” if one may occur in the future where invincibility means invincibility. Able to strike with impunity, stealth aircraft, missiles and warships are virtually invisible to most types of military sensors. The experience gained at the warfront emphasizes the need to incorporate stealth features at the design stage itself. According to conventional military wisdom, surprise is the best form of attack. With evermore sophisticated methods of detection, however, catching the enemy unawares has becoming increasingly difficult. Thus paving way to the development of increasingly sophisticated technologies that help in evading the enemy's ever vigilant “eyes”. Stealth Technology essentially deals with designs and materials engineered for the military purpose of avoiding detection by radar or any other electronic system. Stealth, or antidetection, technology is applied to vehicles (e.g., tanks), missiles, ships, and aircraft with the goal of making the object more difficult to detect at closer and closer ranges thus providing an element of surprise in the attacks. Attacking with surprise gives the attacker more time to perform its mission and exit before the defending force can counterattack. For example, If a surface to air missile a type of antiaircraft battery defending a target observes a bomb falling and surmises that there must be a stealth aircraft in the vicinity it is still unable to respond if it cannot get a lock on the aircraft in order to feed guidance. As stated earlier stealth technology can be looked upon as a perfect blend between the engineering skills of "designing" and "technology". And for attaining stealth various dectection techniques have to be surpassed

CONTENTS
TOPIC
1. INTRODUCTION 2. STEALTH TECHNOLOGY 1. STEALTH PRINCIPLES 2. THE TERM “SIGNATURE” OF A VEHICLE 3. WHAT’STHE NEED FOR STEALTH 4. HISTORY OF STEALTH

PAGE NO. 1 2 2 2 2 3 5 6 6 6 6 8 10 10 11 15 15 16 16

3. DECTECTION METHODS AND THE FIELDS USED 4. RADAR

4.1 RADAR (Radio Dectection AND Ranging) 4.2 WHY RADIO WAVES AND NOT SOUND WAVES 4.3 RADAR CROSS SECTION (RCS) 4.4 RCS PATTERNS
5. RADAR STEALTH

5.1 VEHICLE SHAPE 5.2 COATINGS AND ABSORBERS
6. INFRARED (IR)

6.1 INFRARED (IR) 6.2 THERMAL RADIATION
3. GENERALIZED IR SYSTEM

7. INFRARED STEALTH

17 17 17 18 20 20 22 23 23 24 25 25 26 27 27 27 28 30 31 32 32

7.1 BLACK HOLE OCARINA (BHO) IRSS SYSTEM 7.2 FILM COOLED TAIL PIPE (FCT) IRSS SYSTEM 7.3 NEAR INFRARED ABSRBING (NIR) MATERIALS
8. VISUAL STEALTH

8.1 CAMOUFLAGE 8.2 DECOYS
9. ACOUSTICS

9.1 SONAR (Sound Navigation And Ranging) 9.2 THE SONAR DECTECTORS
10. SONAR STEALTH

10.1 BATHYTHERMOGRAPH 10.2 SONAR ABSORBERS
11. LIDAR

11.1 LIDAR (LIght Dectection And Ranging) 11.2 LIDAR PROPERTIES 11.3 LASER RADAR CROSS SECTION (LRCS) 11.4 LIDAR STEALTH
12. PLASMA STEALTH 13. ADAPTOVE WATER CURTAIN TECHNOLOGY (AWCT)

13.1 FEATURES

14. ADVANTAGES AND DISADVANTAGES OF STEALTH TECHNOLOGY 33

14.1 ADVANTAGES OF STEALTH TECHNOLOGY 14.2 DISADVANTAGES OF STEALTH TECHNOLOGY
15. CONCLUSION 16. REFERENCES

33 33 34 35

LIST OF FIGURES
Fig no 2.1 2.2 2.3 3.1 3.2 3.3 4.1 4.2 4.3 4.4 5.1 5.2 5.3 5.4 5.5 5.6 6.1 6.2 6.3 7.1 7.2 TOPIC F-117A nicknamed the “NIGHTHAWK” Sea Shadow Visby-class corvettes RADAR SUBMARINE DECTECTION LIDAR EQUIPMENT CONCEPT OF RADAR CROSS SECTION RCS VS PHYSICAL GEOMETRY RCS Patterns Reduction of RCS Affects Radar Detection, Burn-through, and Jammer Power Corner Reflectors10 F117 –A the “NIGHTHAWK” F22-A called the “RAPTOR” Properties if RAMs based on angle of incidence Freeofluid used as a RAM material Iron carbonyl RAM Electromagnetic Radiation Elements of a passive IR system Elements of an active IR system Black Hole Ocarina (BHO) IRSS system Film Cooled Tailpipe PAGE NO. 3 4 4 5 5 5 7 7 8 9 11 11 12 14 14 15 16 16 17 18

7.3 7.4 7.5 8.1 8.2 8.3 8.4 8.5 8.6 10.1 10.2 10.3 10.4 11.1 11.2 13.1

Absorption of the radical anion of poly-4 on ITO electrode NIR-absorbing binuclear mixed-valence ruthenium complexes Spectroelectrochemical spectra for the film of polymer 7 on ITO electrode in chloroform with 0.1 M TBAH F117-A nicknamed the “Nighthawk” as it conducted almost all its flights during night Aircraft Camouflage of F-16 uses Digital Camouflage Patterns Visual Stealth Plane- Hawk GB Vehicle Camouflage Visby –Ship Camouflage Rubber tank used by German’s during WWII- A Decoy Properties of Bathythermograph

19 19 19 20 21 21 21 21 22 25

The submarine is submerged immediate below the layer of thermo cline 25 The submarine is submerged below the layer of thermo cline Example of Sonar Absorbing Material Relationship between propagation range and atmospheric transmission under different weather conditions Lidar Equipment The Adaptive Water Curtain Technology (AWCT) 26 26 28 30 32

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1. INTODUCTION
Stealth technology also known as LOT (Low Observability Technology) is a technologies which covers a range of techniques used with aircraft, ships and missiles, in order to make them less visible (ideally invisible) to radar, infrared and other detection methods. From the late years of World War II to today's computer enabled design changes, stealth has been a major factor in the improvement of reconnaissance and attack aircraft. The term "stealth", is thought to have been coined in 1966 by Charles E. "Chuck" Myers, combat pilot and later an exec at Lockheed. When we think of stealth today, immediately images of the B-2 bomber or the F-117A Nighthawk fighter comes to mind. In simple terms, stealth technology allows an aircraft to be partially invisible to Radar or any other means of detection. This doesn't allow the aircraft to be fully invisible on radar. Stealth technology cannot make the aircraft invisible to enemy or friendly radar. All it can do is to reduce the detection range or an aircraft. This is similar to the camouflage tactics used by soldiers in jungle warfare. Unless the soldier comes near you, you can't see him. Though this gives a clear and safe striking distance for the aircraft, there is still a threat from radar systems, which can detect stealth aircraft. Stealth technology is expanded into each of those areas which seek to detect the aircraft, ships & missiles. Thus it is essential to develop visual, infrared acoustic and radar stealth. However many countries have announced that they have developed counter-stealth techniques that allow them to negate stealth.

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2.STEALTH TECHNOLOGY
“Oh divine art of subtlety and secrecy! Through you we learn to be invisible, through you inaudible and hence we can hold the enemy’s fate in our hands.” Sun Tzu – Chinese General, The Art of War, c.490 B.C. Stealth technology also known as LO technology (low observable technology) is a subdiscipline of military electronic countermeasures which covers a range of techniques used with aircraft, ships, submarines, and missiles, in order to make them less visible (ideally invisible) to radar, infrared, sonar and other detection methods. 2.1 Stealth principles Stealth technology (or LO for "Low Observability") is not a single technology. It is a combination of technologies that attempt to greatly reduce the distances at which a vehicle can be detected; in particular radar cross section reductions, but also acoustic, thermal, and other aspects. Stealth technology aims at minimizing signatures and signals, and prevent/delay detection and identification, thus increasing the efficiency of the vehicles own countermeasures and sensors. Ben Rich, the leader of the Lockheed team that designed the F117, pretty much sums up stealth technology when he say: “A stealth aircraft has to be stealthy in six disciplines: radar, infrared, visual, acoustic, smoke and contrail. If you don’t do that, you flunk the course.” However, not all disciplines are equally important when discussing any given platform category. Underwater warfare will naturally hand dominance to the acoustic spectrum. However, land combat will emphasize visual, infrared and acoustic signatures. Radar and infrared bands dominate the scene of airspace surveillance. 2.2 The term “Signature” of a Vehicle Signature - Any unique indicator of the presence of certain materiel or troops; especially the characteristic electronic emissions given off by a certain type of vehicle, radar, radio, or unit. Thus Signature can be concluded as any activity or radiation or the characteristic of the body that helps to revile its presence at a particular point. All the dectection methods used that be in military and civil systems are by dectecting the signature of the body. This signature is called by different names in different contexts. Radar Signature is called Radar Cross Section or RCS and so on. Thus signature can be rightly called as observability of an object and stealth vehicles can be called as low-observable vehicles or low-signature vehicles. 2.3 What’s the need for Stealth? It’s a matter of fact that the rapid development of stealth technology occurred due to the pronounced improvement of the dectection techniques like radar’s as they were the most commonly used dectection methods in the 1930’s & 40’s. There are some key strategies that triggered the development of the Stealth technology like the use of Radar Aided-Anti aircraft systems and the use of Sonar’s for detecting the Submarines by the Ships etc. Thus the rapid development was the need of time to reduce causalities. And that still remains so. As Stealth technologies touching new heights day by day in the other side AntiStealth technologies are also in full momentum to outdate the Stealth technologies. Thus stating the need of STEALTH TECHNOLOGY. 2

2.4 History of Stealth In the late 1930’s and 1940’s Radar technology was commonly used for dectecting aircrafts. Since radar technology was developed during the Second World War, it should not be surprising to learn that the first attempts at stealth technology occurred during this period also. It might be surprising to learn, however, that it was the Germans, not the Allies, who worked on the project. The Germans were responding to the success the Allies were having with the early radar sets. Not only was their radar very effective at spotting incoming enemy bombers, but it was also very important in the battle for the Atlantic. The Germans developed a radar absorbing paint. While this ferrite-based paint was much too heavy for aircraft, it could be used on submarines The United States' first stealth development was totally accidental and quickly forgotten. Shortly after the war, Northrop Aircraft developed an experimental bomber called the YB-49 Flying Wing. As the name implies, the aircraft had no body or tail; it was simply a large flying wing. The aircraft was assigned to perform a normal test flight over the Pacific. When the test was completed, they turned and headed for home, pointing the slim wing edge directly at the base radar station. The radar crew was shocked to see the aircraft suddenly appear almost overhead because they had seen no evidence of it on the radar screen. Interest in the project quickly faded after the bomber crashed in the Mojave Desert in 1948. The plane was very unstable in flight and this stability problem was listed as the cause of the crash. With the “cold war” and the Soviet Union well under way in the early 1950s, it became imperative that the U.S. should learn about military developments deep inside the country. Old bombers were converted to spy planes, but they soon proved to be very vulnerable to attack. In order to plug this intelligence gap, a new plane was designed. The idea was to create a plane that could cruise safely at very high altitudes, well out of the reach of any existing fighter. The design specification required that “consideration is given…to minimize the delectability by enemy radar.” The task of making this plane a reality fell upon the Advanced Development Projects team at Lockheed in California. This was a small team of highly qualified and highly motivated engineers and pilots. This highly secret facility became known as the “Skunk Works” and has been on the leading edge of stealth technology since the early 1950s. The aircraft they developed became known as the U-2, and it was highly successful. After much effort they were successful in building an aircraft that could evade the enemy RADAR’s called the F-117A nicknamed as the “Nighthawk”, developed by Lockheed Martin in 1983.

Fig 2.1:- F-117A nicknamed the “NIGHTHAWK”

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There is a boat the Skunk Works developed shortly after the F-117A. It is called the "Sea Shadow" and was built in 27 months and operated secretly in the late 1980 for $200 million dollars. The Sea Shadow was first unveiled on April 9, 1993. The barge used for the program was the Hughes Mining Barge (HMB-1), a vessel was originally built for a secret CIA project in the early '70s, and had been in mothballs for years. The CIA project, it has since come out, was an attempt to recover a Soviet nuclear sub that sank off the coast of Hawaii in 1968. The project included two ships, the Gosimir Explorer which was basically a ship capable of deep Sea mining, and the HMB-1 which actually submerged under the Gosimir Explorer. The HMB-1 had a claw to retrieve the USSR submarine, which was operated by the drill on the Gossimir Explorer. (The operation was partially successful with half of the ill-fated Soviet sub and crew being brought up from the ocean bottom.) The Sea Shadow's stats are: Length: 160 ft. Width: 68 ft. Draft: 14.5 ft. Displacement: 560 tons (full load). In May 1999, the Sea Shadow was reactivated by the Navy for a 5 year program in order to "research future ship engineering concepts and to serve as a host vessel for companies to demonstrate advanced naval technologies." The Sea Shadow is currently operation out of San Francisco Bay.

Fig 2.2:- Sea Shadow

Fig 2.3:- Visby-class corvettes

Sweden that gave us Volvos, Saabs and ABBA has developed what it claims is the world’s first fully operational stealth warship that is essentially invisible to radar. The two Visby-class corvettes will enter service by the end of the year. They are made from composite materials and use Rolls-Royce water jets to make them electronically undetectable at more than eight miles in rough seas and more than 14 in calm waters. The ship’s acoustic and optical signatures are lowered by its non-magnetic hull that, like the F-117 Nighthawk, features large, flat surfaces and sharp angles. The water jets are 10 to 15 decibels quieter than propellers. "It’s very hard for a submarine to detect a water jet vessel," Patric Hjorth, technical manager of the Swedish Defense Materiel Administration "It has a very different signature from a propeller-driven craft as it fades into the background."

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3. DETECTION METHODS AND THE FIELDS USED
 RCS: - Aircrafts, Missiles, Ships, Land Vehicles...

 Infrared signature:-Aircrafts, Missiles, Ships, Land Vehicles, Submarines.

 Acoustic Signature: - Predominantly for Submarines (SONAR), Ships, Aircrafts etc...  Visible Signature: - Predominantly for Land Vehicles, Aircrafts, and Ships.
 Laser Cross Section:-Aircrafts, Missiles, Ships, Land Vehicles.  Magnetic Signature: - Submarines, Ships.

Fig 3.1:- RADAR

Fig 3.2:- SUBMARINE DECTECTION

FIG 3.3:- LIDAR EQUIPMENT

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4.RADAR
In the early 1930's and 1940's radar technology was increasingly used to dectect aircrafts. During the Second World War all counters Germany, Great Britain, France and The United States of America used this technology for navigating ships and to dectect approaching enemy aircrafts. This technology didn't pose much of a threat then as this was not incorporated into Antiaircraft defenses then. This whole story changed during The Vietnam and Yom Kipper wars. to make the feet more secure for use and more effective the Americans who were the sheet anchor needed to develop an effective way to evade radar. 4.1 RADAR (RAdio Dectection And Ranging) Thus RADAR as it is abbreviated so uses radio waves for dectection of the target. Radar basically works on two major principles. 4.1.a Echo Echo can be considered as a wave bouncing off a surface and coming back to the source. This Principle can be applied for all types of waves starting from sound waves to light waves. the time for the reception of the transmitted signal to reach the transmitter cum receiver can be effectively used to calculate the distance of the target from the transmitter. 4.1.b The Doppler Shift This being the second principle of the radar. This effect is more commonly felt for sound. The sound that you hear as a vehicle is approaching you is at a higher pitch or a higher frequency than the sound you hear when the vehicle is moving away from you. This property when applied to radar can be used to determine the speed of the object. The frequency of the reflected wave can be the same, greater or lower than the transmitted radio wave. if the reflected wave frequency is less then this means that the target is moving away from the transmitter and if higher then moving close to the transmitter and if constant then the target is not moving like a helicopter hovering at a point. This can be used to predict the speeds of the target too. 4.2 Why Radio waves and not sounds waves? Although the above said principles are applicable to sound waves radio waves are used for dectection and ranging due to the following reasons. The speeds of the radio waves are comparable with that of light and are much higher than that of sound. Sound waves cannot travel as far as light in the atmosphere without significant attenuation. And finally, electromagnetic echo is much easier to dectect than a sound echo.

4.3 Radar Cross Section (RCS)
Radar cross section is the measure of a target's ability to reflect radar signals in the direction of the radar receiver, i.e. 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.

6

The RCS of a target can be viewed as a comparison of the strength of the reflected signal from a target to the reflected signal from a perfectly smooth sphere of cross sectional area of 1

m2 as shown in Figure. The conceptual definition of RCS includes the fact that not all of the radiated energy falls
Fig 4.1:on the target. A target’s RCS (σ) is most easily visualized as the product of three factors:

σ = Projected cross section x Reflectivity x Directivity.
Reflectivity: The percent of intercepted power reradiated (scattered) by the target. Directivity: The ratio of the power scattered back in the radar's direction to the power that would have been backscattered had the scattering been uniform in all directions (i.e. isotropically). For a sphere, the RCS, σ = πr2 , where r is the radius of the sphere.

Fig 4.2:-

The RCS of a sphere is independent of frequency if operating at sufficiently high frequencies where λ<<Range, and λ<< radius (r). Experimentally, radar return
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reflected from a target is compared to the radar return reflected from a sphere which has a frontal or projected area of one square meter (i.e. diameter of about 44 inches). Using the spherical shape aids in field or laboratory measurements since orientation or positioning of the sphere will not affect radar reflection intensity measurements as a flat plate would. The sphere is essentially the same in all directions. The flat plate has almost no RCS except when aligned directly toward the radar. The corner reflector has an RCS almost as high as the flat plate but over a wider angle, i.e., over ±60 ο. The return from a corner reflector is analogous to that of a flat plate always being perpendicular to your collocated transmitter and receiver. Targets such as ships and aircraft often have many effective corners. Corners are sometimes used as calibration targets or as decoys, i.e. corner reflectors. An aircraft target is very complex. It has a great many reflecting elements and shapes. The RCS of real aircraft must be measured. It varies significantly depending upon the direction of the illuminating radar.

Fig 4.3:- RCS Patterns

Significance of the Reduction of RCS If each of the range or power equations that have an RCS (σ) term is evaluated for the significance of decreasing RCS, Therefore, an RCS reduction can increase aircraft survivability. The equations used are as follows: Range (radar detection):2-way range equation: Pr = Pt Gt Gr λ2 σ :Thus, R4 α σ or σ 1/4 α R

(4π)3 R4
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Range (radar burn-through):Crossover equation: RBT2 = Pt Gt σ : Thus,RBT2 α σ or σ 1/2α RBT

Pj Gj 4π Power (jammer): Equating the received signal return (Pr) in the two way range equation to the received jammer signal (Pr) in the one way range equation, the following relationship results: Pr = Pt Gt Gr λ2 σ = Pj Gj Gr λ2 (4π)3 R4 (4πR)2 Therefore, Pj α σ or σ α Pj Note: jammer transmission line loss is combined with the jammer antenna gain to obtain Gt .
Thus the Deductions can be made from the figure given below. This shows an example of the effects of RCS reduction. Thus if the RCS of an aircraft is reduced to 0.75 (75%) of its original value, then the jammer power required to achieve the same effectiveness would be 0.75 (75%) of the original value (or -1.25 dB). Likewise, If Jammer power is held constant, then burn-through range is 0.87 (87%) of its original value (-1.25 dB), and the detection range of the radar for the smaller RCS target (jamming not considered) is 0.93 (93%) of its original value (-1.25 dB)

Fig 4.4:- Reduction of RCS Affects Radar Detection, Burn-through, and Jammer Power

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5. RADAR STEALTH
There are two broad aspects of RCS minimization techniques. One falls under the effort to restructure the frame, and covers the geometric design considerations that are taken into account when aiming for a low RCS. The other principle is referred to as “radar absorbent materials” and is concerned with the materials that help to reduce the reflectivity of the airframe, as well as the structures that will support these materials and integrate them into the airframe often referred to as “Radar-absorbent structures”. These two axes are of course not taken in isolation during the design; trade-offs often have to be made between them.

5.1 Vehicle Shape
The stealth designer's mission starts with the same words as the physician's Hippocratic Oath: "First, does no harm." The prime most concern being that of the aircraft's The possibility of designing aircraft in such a manner as to reduce their radar cross-section was recognized in the late 1930s, when the first radar tracking systems were employed, and it has been known since at least the 1960s that aircraft shape makes a significant difference in detectability. The Avro Vulcan, a British bomber of the 1960s, had a remarkably small appearance on radar despite its large size, and occasionally disappeared from radar screens entirely. It is now known that it had a fortuitously stealthy shape apart from the vertical element of the tail. On the other hand, the Tupolev 95Russian long range bomber (NATO reporting name 'Bear') appeared especially well on radar. It is now known that propellers and jet turbine blades produce a bright radar image; the Bear had four pairs of large (5.6 meter diameter) contra-rotating propellers. Another important factor is the internal construction. Behind the skin of some aircraft are structures known as re-entrant triangles. Radar waves penetrating the skin of the aircraft get trapped in these structures, bouncing off the internal faces and losing energy. This approach was first used on the F-117. The most efficient way to reflect radar waves back to the transmitting radar is with orthogonal metal plates, forming a corner reflector consisting of either a dihedral (two plates) or a trihedral (three orthogonal plates). This configuration occurs in the tail of a conventional aircraft, where the vertical and horizontal components of the tail are set at right angles. Stealth aircraft such as the F-117 use a different arrangement, tilting the tail surfaces to reduce corner reflections formed between them. A more radical approach is to eliminate the tail completely, as in the B-2 Spirit.

Fig 5.1:- Corner Reflectors

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In addition to altering the tail, stealth design must bury the engines within the wing or fuselage, or in some cases where stealth is applied to an existing aircraft, install baffles in the air intakes, so that the turbine blades are not visible to radar. A stealthy shape must be devoid of complex bumps or protrusions of any kind; meaning those weapons, fuel tanks, and other stores must not be carried externally. Any stealthy vehicle becomes non stealthy when a door or hatch is opened. Stealth airframes sometimes display distinctive serrations on some exposed edges, such as the engine ports. The YF-23 has such serrations on the exhaust ports. This is another example in the use of re-entrant triangles and planform alignment, this time on the external airframe. Ships have also adopted similar techniques. The Visby corvette was the first stealth ship to enter service, though the earlier Arleigh Burke class destroyer incorporated some signaturereduction features. In designing a ship with reduced radar signature, the main concerns are radar beams originating near or slightly above the horizon (as seen from the ship) coming from distant patrol aircraft, other ships or sea-skimming anti ship missiles with active radar seekers. Therefore, the shape of the ship avoids vertical surfaces, which would perfectly reflect any such beams directly back to the emitter. Retro-reflective right angles are eliminated to avoid causing the cat's eye effect. A stealthy ship shape can be achieved by constructing the hull and superstructure with a series of slightly protruding and retruding surfaces. This design was developed by several German shipyards, and is thus extensively applied on ships of the German Navy.

RADAR scattering due to vehicle shape Fig 5.2:- F117 –A the “NIGHTHAWK” Fig5.3:- F22-A called the “RAPTOR”

5.2 Coatings and Absorbers
5.2. a RAMs (Radar Absorbing Materials) Radar-absorbing materials (RAMs) are used to dissipate the energy of the radar wave so to prevent the reception of a reflected signal by an antenna. Usually, the dissipation process converts the radio frequency (RF) energy to a negligible quantity of heat.

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RAMs are one of four ways of reducing the radar cross-section of an object, which is a measure of the reflection of radar waves by an object. A larger radar cross-section (RCS) of an object corresponds to a longer detection range and a higher signal-to-noise ratio for the observing radar operator. A 747 would have a huge RCS, whereas a bumblebee would have an insignificant RCS. Other ways of reducing RCS include passive cancellation, incorporating an echo source which by design cancels another echo source for a certain frequency and angle, active cancellation, incorporating a sensor and emitter which cooperate to radiate waves which interfere with incident radar waves, and by geometric shaping and design modifications. Only the last will be discussed, as the former two are rather impractical and are less dependent on material or process properties. Dielectric and magnetic RAMs are the two main types (along with various combinations of these) of RAMs in current operational use; these will be explored in further detail as we go along.

Fig 5.4:- Properties if RAMs based on angle of incidence

5.2. b Types of RAMs
(i) Iron ball paint One of the most commonly known types of RAM is iron ball paint. It contains tiny spheres coated with carbonyl iron or ferrite. Radar waves induce molecular oscillations from the alternating magnetic field in this paint, which leads to conversion of the radar energy into heat. The heat is then transferred to the aircraft and dissipated. The iron particles in the paint are obtained by decomposition of iron pentacarbonyl and may contain traces of carbon, oxygen and nitrogen.A related type of RAM consists of neoprene polymer sheets with ferrite grains or carbon black particles (containing about 30% of crystalline graphite) embedded in the polymer matrix. The tiles were used on early versions of the F-117A Nighthawk, although more recent models use painted RAM. The painting of the F117 is done by industrial robots with the plane covered in tiles glued to the fuselage and the remaining gaps filled with iron ball paint. The United States Air Force introduced a radar 12

absorbent paint made from both ferrofluidic and non-magnetic substances. By reducing the reflection of electromagnetic waves, this material helps to reduce the visibility of RAM painted aircraft on radar. (ii) Foam absorber It is used as lining of anechoic chambers for electromagnetic radiation measurements. This material typically consists of fireproofed urethane foam loaded with carbon black, and cut into long pyramids. The length from base to tip of the pyramid structure is chosen based on the lowest expected frequency and the amount of absorption required. For low frequency damping, this distance is often 24 inches, while high frequency panels are as short as 3-4 inches. Panels of RAM are installed with the tips pointing inward to the chamber. Pyramidal RAM attenuates signal by two effects: scattering and absorption. Scattering can occur both coherently, when reflected waves are in-phase but directed away from the receiver, and incoherently where waves are picked up by the receiver but are out of phase and thus have lower signal strength. This incoherent scattering also occurs within the foam structure, with the suspended carbon particles promoting destructive interference. Internal scattering can result in as much as 10dB of attenuation. Meanwhile, the pyramid shapes are cut at angles that maximize the number of bounces a wave makes within the structure. With each bounce, the wave loses energy to the foam material and thus exits with lower signal strength.[4] Other foam absorbers are available in flat sheets, using an increasing gradient of carbon loadings in different layers. (iii) Jaumann absorber A Jaumann absorber or Jaumann layer is a radar absorbent device. When first introduced in 1943, the Jaumann layer consisted of two equally-spaced reflective surfaces and a conductive ground plane. One can think of it as a generalized, multi-layered Salisbury screen as the principles are similar. Being a resonant absorber (i.e. it uses wave interfering to cancel the reflected wave), the Jaumann layer is dependent upon the λ/4 spacing between the first reflective surface and the ground plane and between the two reflective surfaces (a total of λ/4 + λ/4). Because the wave can resonate at two frequencies, the Jaumann layer produces two absorption maxima across a band of wavelengths (if using the two layers configuration). These absorbers must have all of the layers parallel to each other and the ground plane that they conceal. More elaborate Jaumann absorbers use series of dielectric surfaces that separate conductive sheets. The conductivity of those sheets increases with proximity to the ground plane.

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Fig 5.5:- Freeofluid used as a RAM material

Iron ball paint has been used in coating the SR-71 Blackbird and F-117 Nighthawk, its active molecule is made up by an iron atom surrounded by five carbon monoxide molecules. Iron ball paint (paint based on iron carbonyl) a type of paint used for stealth surface coating. The paint absorbs RF energy in the particular wavelength used by primary RADAR. Chemical formula: C5FeO5 / Fe (CO)5 Molecular mass: 195.9 g/mol Apparent density: 76.87 g/cmc Molecular structure: An Iron atom surrounded by 5 carbon monoxide structures (it takes a balllike shape, hence the name) Melting point: 1536° C Hardness: 82-100 HB It is obtained by carbonyl decomposition process and may have traces of carbon, oxygen and nitrogen. The substance (iron carbonyl) is also used as a catalyst and in medicine as an iron supplement however it is toxic. The painting of the F-117 is done by industrial robots however the F-117 is covered in tiles glued to the fuselage and the remaining gaps filled with iron ball paint. This type of coating converts the radar wave energy into heat (by molecular oscillations) the heat is then transferred to the aircraft and dissipated. It is the exact same principle by which water is heated in the microwave oven (radar uses microwaves).

Fig5.6:- Iron carbonyl RAM material

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6. INFRARED (IR)
6.1 Infrared (IR) Passive IR detection techniques rely on the fact that every atom of matter continuously sends electromagnetic radiation at an IR wavelength which corresponds to its temperature. IR detectors identify an aircraft by discriminating its IR radiation with that of the background; hence it is desirable to have an IR emission from the aircraft close to the background radiation. Since controlling an IR emission during a Military operation is not always feasible; IR emission control has to be incorporated at the design stage of the aircraft itself. The major IR signature contributors are the airframe, engine casing and the plume. The amount of incident IR radiation in the detector’s band depends upon the amount of radiation emitted by the source, its position with respect to the detector, and the amount of radiation that is attenuated (absorbed and scattered) by the atmosphere on its way to the detector. It is not possible to always operate in a position that results in minimum amount of incident IR on the detector in its band. Also it is not possible to control the amount of atmospheric attenuation of the IR emitted by the source in the direction of the detector. Hence the only operation that remains is to control the IR intensity emitted by the source. Infrared Signatures Suppression Systems (IRSS) like Black Hole Ocarina, Film cooled tail pipe and Centre Body tail pipe; are some of the popular IR countermeasures adopted. Further, in order to avoid IR seeking missiles, countermeasures such as infrared jamming systems, infrared flares or decoys are frequently employed.

Fig6.1:- Electromagnetic Radiation Spectrum

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6.2 Thermal Radiation The total amount of radiation emitted is dependent on emissivity and the fourth power of absolute temperature as given by Stefan Boltzmann Law,

e = ε. σ. T4
, where, Stefan Boltzmann constant, σ = 5.67 x 10-8 W/m2K4 From electromagnetic considerations, Planck’s Law gives the monochromatic emissive power of a black surface as

ebλ= λ [e
5

1 (C /λT) 2

2πC

-1]

, where C1 and C2 are constants whose values are 0.596 x 10-16 W/m2 and 0.014387mK respectively. For a non black surface, monochromatic emissive power is given by,

eλ= ελ . ebλ
The emissive power within a specified band of wavelengths is obtained by integrating the Planck’s law within that wavelength interval. The total radiant emittance increases rapidly with temperature. The wavelength of maximum spectral radial emittance shifts towards shorter wavelengths with the increase in temperature. Individual curves never cross one another and hence higher the temperature, higher will be the radial emittance at all wavelengths. 6.3 Generalized IR System Every typical IR system’s components are designed to optimize the system performance for a specific wavelength region, for maximum detectivity, for high resolution, and so on, depending upon the type of source to be detected and the kind of information the system is required to furnish. Consider a model of a generalized IR system, in order to help us understand the principles underlying the many IR systems. Every IR system model is composed of basic building blocks and every IR system whether active or passive, is composed of most if not all of these building blocks. For example, all IR systems include a source or target, a background, an atmosphere or environment, optics and a detector. With the aid of this generalized model, the path of IR radiation from its sources can be analyzed, step – by – step through the various modifications necessary for its final presentation in some form of display

Fig6.2:-Elements of a passive IR system

Fig6.3:-Elements of an active IR system

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7. INFRARED STEALTH
The passive IR sensors detect energy emitted by the aircraft. Since radiation from the aircraft cannot be avoided, the signatures have to be reduced or modified to match with the background in order to increase the probability of the success of the mission. Infrared Signature Suppression system 7.1 Black Hole Ocarina (BHO) IRSS system The Black Hole Ocarina IRSS has the advantage of optical blocking hot engine parts, and cooling exhaust duct and plume. The Black Hole System is a finned nozzle with internal bends to prevent the direct view of hot internal exhaust surfaces as shown. The bending of the nozzle avoids the direct line of sight of hot engine parts. Ocarina is a system of multiple exhausts (Fig 4.3) is devised to dissipate the exhaust plume and reduce the plume radiation. Both, Black Hole system and Ocarina system have been merged to make the Black Hole Ocarina (Fig 4.4), which has the advantages of a bent nozzle of a black hole system and the multiple outlets of an ocarina system. As shown in Fig 4.2, the nozzle acts as an optical block for the direct view of the hot engine parts, and the multiple exhausts with the ejector system cools the plume as well as dissipates the plume so that the plume radiation is reduced. The hot exhaust air sucks in the cold air from the engine compartment and reduces plume temperature. The external air passing over the finned nozzle cools it.

Fig7.1:- Black Hole Ocarina (BHO) IRSS system 7.2 Film Cooled Tailpipe (FCT) IRSS system Film cooled tailpipe (FCT) entrains secondary air by ejector action for cooling the hot tailpipe and plume. This is a passive system depending on the static pressure distribution along the length of the device to draw ambient air. The FCT was designed to be a "mission kit", and as such is easily retrofit able with the aircraft's factory exhaust without any modifications to the aircraft. As shown in the Figure. FCT consists of a nozzle, a flow wedge down the stream of the nozzle and film cooling slots for cold air entry. Due to ejector action, cold air enters through the film cooling slots and cools the exhaust air. FCT provides passive cooling of both metal and plume. The FCT needs minimum modifications on helicopter for installation and it is moderately effective in all flight conditions. The reported engine power loss due to FCT installation is of the order of 2% of maximum engine power.

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Fig7.2:- Film Cooled Tailpipe

7.3 Near Infrared Absorbing (NIR) Materials
Organic solids and polymers that absorb in the near-infrared (NIR) region (1000–2000 nm) represent a class of emerging materials and show a great potential for use in photonics, telecommunications. The radical anions of stacked aromatic imides, fused phorphyrin arrays, polythiophenes, sandwich-type lanthanide bisphthalocyanines, semiquinones, and mixed-valence binuclear metal complexes are a few known examples of NIR- absorbing organic materials. Most of these NIR-absorbing materials are also electrochemically active or electrochromic (EC). NIR-absorbing organic materials are low bandgap materials (e.g., 0.75 eV or 1550 nm) and thus must contain an extended conjugation or mixed valence system. Organic materials that are electrically, optically, or thermally active in the NIR region, specifically at the telecommunication wavelengths (e.g., 1310 and 1550 nm) can in principle be used in a device for optical attenuation and absorption or antireflection, owing to their unique electrical and optical properties, low-cost fabrication, and feasibility for use in a monolithically integrated optical device.

7.3. a NIR-Absorbing Inorganic Materials
Metal oxides such as tungsten oxide (WO3), IrOx, and Ta2O5 have been known for many years to be EC in the visible region and have been extensively investigated as an EC thin film. The application has been realized in smart windows and antireflection rear mirrors in cars. The film of tungsten oxide deposited on an electrode can be electrochemically reduced in the presence of an electrolyte according to the following electrochemical reaction: Where M+ is H+ or Li+, x is the so-called ion insertion coefficient. Reduction of tungsten oxide involves entry of electrons into the tungsten oxide film from the electrode and of proton or lithium ion from other (electrolyte-facing) side. The reduced tungsten oxide is typically deep blue in color and also absorbs broadly in the NIR region. The absorbance in the vis–NIR region depends on the degree of reduction or ion insertion coefficient and as well WO3 film morphology.

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Fig7.3:- Absorption of the radical anion of poly-4 on ITO electrode.

7.3. b NIR-Absorbing Organic Materials
Several types of NIR-absorbing organic materials are reported in literature, including stacked naphthalimide anion radicals, fused phorphyrin arrays, doped polythiophenes and other related conducting polymers, sandwich-type lanthanide bisphthalocyanines, radical anions of conjugated diquinones (also called semiquinones), and mixed-valence binuclear metal complexes. NIR-absorbing semiquinones For a long time, quinones have been known as disperse dyes and also as electron acceptors. Although monoquinones and their corresponding radical anions have been thoroughly investigated, the radical anions of aromatic diquinones or semiquinones, have only recently received some attention, due to their unique NIR absorbing and semiconducting properties. E.g. pentacenediquinone is known for its NIR electrochromic.

Fig7.4:- NIR-absorbing binuclear mixed-valence

ruthenium complexes
.

Fig7.5:- Spectroelectrochemical spectra for the film of polymer 7 on ITO electrode in chloroform with 0.1 M TBAH.

The study of symmetric d5/d6 mixed-valence binuclear ruthenium (II/III) species has contributed significantly to the understanding of bonding and electron transfer in and between metal complexes such as a classical example of the molecule-bridged Creutz–Taube ion. The binuclear mixed-valence ruthenium complexes are known to be EC and NIR absorbing 19

8. Visual Stealth
Historically, stealth aircrafts like the F-117 and the B-2 Spirit were painted black and were supposed to fly only during the night time for effective camouflaging. However, the concept of day-time stealth has been researched by Lockheed Martin, such a plane would need to blend into the background sky and also carry antiradar and infrared stealth technology. Researchers at the University of Florida are in the process of developing an ‘electro chromic polymer’. These thin sheets cover the aircraft’s white skin and sense the hue, color and brightness of the surrounding sky and ground. The image received is then projected onto the aircraft’s opposite side. When charged to a certain voltage, these panels undergo color change. At the Tonopah test range airstrip in Nevada, another system was tested; as claimed by a technician working at the base, an F-15 equipped with this technology took off from the runway only to disappear from sight 3 Km away. Yet another similar “skin” is being tested at the topsecret Groom Lake facility at Area 51 in Nevada. It is composed of an electro-magnetically conductive polyaniline-based radar absorbent composite material. The system also disposes photo-sensitive receptors all over the plane that scans the surrounding area; subsequently the data is interpreted by an onboard computer which outputs it much like a computer screen. Perhaps one day, in the very near future, one may fly in a completely invisible aircraft. B-2 Spirit bomber, Boeing’s Bird of Prey and the F-35 Joint Strike Fighter represent the pinnacle of modern day advancements in this particular field of human endeavor Fig8.1:- F117-A nicknamed the “Nighthawk” as it conducted almost all its flights during night.

8.1 Camouflage 8.1.a Aircraft Camouflage The design of camouflage for aircraft is complicated by the fact that the appearance of the aircraft's background varies widely, depending on the location of the observer (above or below) and the nature of the background. Many aircraft camouflage schemes of the past used counter shading, where a light color was used underneath and darker colors above. Other camouflage schemes acknowledge that the aircraft will be twisting and turning while in combat, and the camouflage pattern is applied to the entire aircraft. Neutral and dull colors are preferred, and two or three shades selected, depending on the size of the aircraft. Though air-to-air combat is often initialized outside of visual range, at medium distances camouflage can make an enemy pilot hesitate until certain of the attitude, distance and maneuver of the camouflaged aircraft. The higher speeds of modern aircraft and the reliance on radar and missiles in air combat have reduced the value of visual camouflage, while increasing the value of electronic "stealth" measures. Modern paint is designed to absorb electromagnetic radiation used by radar, reducing the signature of the aircraft, and to limit the emission of infrared light used by heat seeking missiles to detect their target. Further advances in aircraft camouflage are being investigated in the field of active camouflage 20

Fig8.2:- Aircraft Camouflage of F-16 uses Digital Camouflage Patterns.

Fig8.3:- Visual Stealth PlaneHawk GB

8.1.b Vehicle Camouflage The purpose of vehicle and equipment camouflage differs from personal camouflage in that the primary threat is aerial reconnaissance. The goal is to disrupt the characteristic shape of the vehicle, reduce shine, and make the vehicle difficult to identify even if it is spotted. Methods to accomplish this include paint, nets, ghillie-type synthetic attachments, and natural materials. Paint is the least effective measure, but forms a basis for other techniques. Military vehicles often become so dirty that pattern-painted camouflage is not visible. Patterns are designed to make it more difficult to interpret shadows and shapes; matte colors are used to reduce shine, but a wet vehicle can still be very shiny, especially when viewed from above. Nets can be highly effective at defeating visual observation, but are useful mostly for stationary vehicles. They also take a lot of time to set up and take down. Nets are occasionally fixed in place around gun tubes or turrets, and if adequately attached can remain in place while the tank is moving. Nets are far less effective in defeating radar and thermal sensors. Synthetic attachments, analogous to ghillie-suit attachments, are sometimes used to break up shape. These are prone to loss as AFVs move across terrain, but can be effective. Natural materials, such as tree branches, bundles of leaves, piles of hay or small bits of urban wreckage can be highly effective when the vehicle is in a defensive position.

Fig8.4:- Vehicle Camouflage 21

Fig8.5:- Visby –Ship Camouflage

8.1.c Ship Camouflage Until the 20th century, naval weapons had a very short range, so camouflage was unimportant for ships or the men on board them. Paint schemes were selected on the basis of ease of maintenance or aesthetics, typically buff upperworks (with polished brass fittings) and white or black hulls. At the turn of the century the increasing range of naval engagements, as demonstrated by the Battle of Tsushima, prompted the introduction of the first camouflage, in the form of some solid shade of gray overall, in the hope that ships would fade into the mist. 8.2 Decoys Decoys were extensively used during the Second World War. Rubber tanks were used to distract the enemy and know their position during that time. Nowadays decoy’s are said to be used during missile launches like the ICBM’s (Inter Continental Ballistic Missiles) a number of missiles will be launched to their orbits in which say only one or two will have the payload the others would be dummies to confuse the enemy any to increase the probability of counter missiles like the scud missiles.

Fig8.6:- Rubber tank used by German’s during WWII- A Decoy

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9. ACOUSTICS
Acoustics means Sound and Acoustic signature is used to describe a combination of acoustic emissions of ships and submarines. Although Acoustic Signature are found in for land and ariel units acoustic signature turns out to be the key method of dectection for Naval field rather than the other two. 9.1 SONAR (SOund Navigation And Ranging) Sonar is very important part of anti submarine warfare. The sonar is a device for detecting and locating objects submerged in water by means of the sound waves they reflect or produce. It means that the active sonar wasn’t used in fight against submarines. The first active sonar was constructed in 1918, in the Admiralty Experimental Station (UK). On the first testing the sonar found merged submarine on distance of a few hundred meters. 9.1.a Hydro Acoustics Sound is mechanical oscillating. Spreading of the sound is possible because of elastic connection between molecules. Molecules in liquids are closer one to another than in the air. Because of that the sound spreads faster in the water than in the air. Speed of the sound in the water is 4.4 times faster than in the air. Exact speed of the sound in the water is 1438 m/s, when temperature of the water is 8 degrees Celsius. Speed and direction of the sound wave spreading depend about temperature, salinity and depth of the water. The speed with which sound is transmitted is a characteristic of the material in question, proportional to the modulus or stiffness of the material and inversely proportional to its density. For example, the speed of sound in sea water can be calculated as follows: Where, C = Speed of sound in sea water, approx. 57,735 in/sec, or 4,800 ft/sec, disregarding effects of temperature, salinity, and pressure.~ 1450 m/s K = Bulk modulus of sea water = 300,000 psi. ρ = Density of sea water, based on a specific weight of 64 lbs./cu.ft. = 9 x 10-5 slugs/cu.in. 9.1.b Sonar Properties The first practical sonar units have been constructed between WW1 and WW2. The best working frequency was 20 kHz, pulse power was 50 W. Range was 1000 to 1500 meters (good working conditions) or 500 to 700 meters (bad working conditions). In WW2 there are two types of sonar, projection type and panoramic type.

(i) Projection sonar: beam 5 to 15 degrees, frequency 10 to 50 kHz, output pulse power 50 to
200 W, duration of signal 30 to 200 ms. Range was 800 to 4500 meters. In winter range was better than in summer. In WW2 average range of submarine detecting was 1350 meters (from a destroyer). Range of sonar depended about:
• •

power of output signal; working frequency – if frequency is lower range is bigger, but there is problem of direction; 23

•

•

Shape and size of ultra sound beam – narrow beam makes longer range and better directing than broad beam but with narrow bean it is harder keep contact with the submerged submarine and it is need more time to survey sector around the ship. Time of duration of output signal – for longer range survey the sonar needs longer duration of output signal.

(ii) Panoramic sonar: The sonar transmits its sound beam in all directions immediately. The
sonar’s receiver receives echo of the sound from all directions and shows possible contact on its screen, with direction and range of the contact.

9.1.c Working frequency: 20 and 25.5 kHz, output power 200 to 800 W, duration of signal 6,
30 or 80 ms. Range was up to 3000 metros. The first successful panoramic sonar in the United States was QHB-1, in 1943. There was also sonar for detecting depth of submerged submarine. It was additional unit and it works together with standard sonar unit. Working frequency of that additional sonar was from 15 to 100 kHz. Depended about construction, there are: hull mounted sonar and towed sonar. 9.2 The Sonar Dectectors Sonar dectectors are simply devices that dectect the presence of Sonar beams. They can be classified as Passive: - Passive sonar’s listen without transmitting. They are usually military (although a few are scientific). Sonar in freshwater lakes is different in operation from sonar at sea. In salt water sonar operation is affected by temperature. Ocean temperature varies with depth, but at between 30 and 100 meters there is often an marked change, called the thermocline, dividing the warmer surface water from the cold, still waters that make up the rest of the ocean. Regarding sonar, a sound originating from one side of the thermocline tends to be reflected off the thermocline, unless it is very noisy. The thermocline is not present in shallower coastal waters. Pressure also effect sound propagation as convergence zones (CZ). Sound waves that are radiated down into the ocean bend back up to the surface in great arcs due to the effect of pressure on sound. Under the right conditions these waves will then reflect off the surface and repeat another arc. Each arc is called a CZ annulus. CZs are found every 33 nm, forming a annular pattern of concentric circles around the sound source. Sounds that can be detected for only a few miles in a direct line can therefore also be detected hundreds of miles away. The signal is naturally attenuated but modern sonar suites are very sensitive. Active:-.Active sonar creates a pulse of sound, often called a "ping", and then listens for reflections of the pulse. To measure the distance to an object, one measures the time from emission of a pulse to reception. To measure the bearing, one uses several hydrophones, and measures the relative arrival time to each in a process called beam-forming. The first active sonar technology was originally called ASDIC after the "Allied Submarine Detection Investigation Committee". .

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10. Sonar Stealth
As in case of all Stealth features is Sonar stealth the aim I to reduce sound from a Submarine or Ship thus remain undetected. There are many ways of reducing Acoustic Signature like reduction of vibration of the Submarine, reduce sounds due to cavitations etc. 10.1 Bathythermograph A bathythermograph is an instrument for recording the temperature at various depths in the ocean. Area of detecting is from –2.2 to 32.2 degrees of Celsius. The bathythermograph may be fitted on a surface ship or on a submarine. During the measurements the ship’s (or the submarines) speed can be up to 22 knots, normally up to 12 knots. A bathythermograph consists of a thermal and a depth part. Results of the measurements are shown on a bathythermogram. When a submarine is submerged, she cannot use a bathythermograph. Instead of that, she has an apparatus for continuously measuring speed of a sound at all depths of the submarine.

Fig10.1:- Properties of Bathythermograph

How a bathythermograph can be used to make sonar ineffective when we use a bathythermograph, we actually look for a thermo cline. A thermo cline is a layer of water where the temperature gradient is greater than that of the warmer layer above and the colder layer below. When the temperature gradient is greater, a sound wave rapidly bending towards the sea bottom. The sound wave goes to the sea bottom and “stay there”. The sound wave is useless. If a submarine is submerged at the layer of thermo cline or immediate below the layer, the submarine will not be “captured” from the wave, and she will stay undetected. Figure below shows situation when the submarine is submerged immediate below the layer of thermo cline, and the surface ship is fitted with the hull mounted sonar.

Fig10.2:- Hull mounted Sonar

There are usually two layers of a thermo cline in summer. One layer is on about 15 to 20 meters of depth, and another one is about 150 meters of depth. Depth of 15 to 20 meters is 25

important. During the summer, at afternoon, if weather conditions are good, a submarine could not be detected from standard (hull mounted) ship’s sonar. In the same time, the depth is good for observing and torpedo launching. If the surface ship wishes to detect a submarine, the ship has to be fitted with towed sonar. In that case, the sonar must be submerged below the thermo cline. Picture shows situation when the submarine is submerged below the layer of thermo cline and the surface ship is fitted with towed sonar. Fig10.3:Tower type Sonar

10.2 Sonar absorbers
Making an efficient, broadbanded sonar absorber presents a number of technical challenges. Most absorptive materials do not have the requisite impedance, and rigid materials are not lousy enough. In some cases, scattering can be used to enhance absorption, but this is not always practicable. Even more difficult are the effects of wavelength: absorbers designed for high frequencies are ineffective at low frequencies. Finally, whatever system is used, it must have good hydrostatic strength so that it may be used deep in the sea. The most promising development in this area is a new family of composite materials employing rigid syntactic foam in combination with a variety of fillers and ingredients. The addition of suitable additives to the syntactic system can provide a controlled amount of scattering or absorption. Reducing the elastic modulus of the resin binder to create more “rubbery” foam will introduce acoustic loss. A truly broadbanded sonar absorber with good hydrostatic strength can be made by dispersing suitably-sized elastomeric particles in a syntactic foam matrix. Using these principles, successful underwater sound absorbers have been made for a variety of military and civilian applications.

Fig10.4:- Example of Sonar Absorbing Material Syntactic foam is a lightweight, high strength composite material frequently used in the sea for floats and buoys, to support instruments, as submarine void filler, for encapsulating hydrophones, and so on.

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11. LIDAR
11.1 LIDAR (LIght Dectection And Ranging) Lidars can be used in detecting stealth targets for its higher angular resolution, strong ability of anti-jamming, good concealment, and small size and light weight. Traditional radars use microwave and centimeter wave as carriers, while the lidar uses laser, which has much shorter wavelengths. The lidar uses amplitude, phase, frequency and polarization carries information and does not have essential difference with traditional radars. Several key technologies need to be taken into consideration in detecting stealth targets by lidars.

11.2 Lidar Properties
The target designation radar needs not only discovering stealth targets but also tracking and aiming so as to antagonize them. Extending radar wavelength is necessary. Laser radar can detect stealth targets effectively because it has short wavelength, high beam quality, strong directionality, high measuring accuracy and it has functions of target identifying, posture displaying and orbit recording. The normal operational wavelengths of laser radar include 0.532¹m, 1.064 ¹m, 10.6 ¹m, etc. Target and background optical properties on different wavelengths and atmospheric effects of different wavelength need to be considered in lidar detection. 11.2.a Target and Background Optical Properties Targets act as a series of combined reflecting surfaces to lidars, and these reflection surfaces decide the electric levels of echo signals. Both relative movement effects caused by targets movement and vector speed of targets can lead to the variation in reflected signals of lidars. Observed echo signals are called lidar characteristic signals which used to obtain target information. Reflection of several typical targets on 1.064 ¹m laser is shown in Table

Table11.1:- Reflections of several typical targets on 1.064 ¹m laser.

The main background noise sources are sun light, moon light, atmospheric dispersion and its own radiation, which cause background illegibility in the FOV (Field of View) of receiver. This can be widely used in aircraft photoelectric stealth.

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11.2.b Atmospheric Effects There are three main atmospheric effects on lidar signal transmission. The first one is attenuation caused by atmosphere molecular absorption. H2O, CO2 and O3 are the primary absorbing sources. Another kind of attenuation arises from Mie scattering by floating particles. Atmospheric turbulence leads to the random changes of refractive index of atmosphere and causes wave-front aberration. Assuming that the original power of a lidar signal is P (λ), the power after transmission of x can be calculated by

P (λ¸ x) = P (λ¸ 0) exp [-k (λ) x] , where k (λ) denotes the attenuation coefficient which contains absorption and diffraction. It can
be seen from formula that atmospheric attenuation depends strongly on operational wavelengths of lidars. So it is important to choose lasers with low atmospheric attenuation as the operational wavelength, such as 10.6 ¹m and 1.064 ¹m. Figure shows the relationship between propagation range and atmospheric transmission on the operational wavelength of 1.064 ¹m. It shows atmospheric transmission under conditions of fine (with visibility of 25 km), clear (with visibility of 15 km), haze (with visibility of 5 km), mist ( with visibility of 1 km), light fog (with visibility of 0.7 km) respectively.

Fig11.1:- Relationship between propagation range and atmospheric transmission under different weather conditions.

11.3 Laser Radar Cross Section (LRCS) The LRCS of target is the symbol of laser scattering ability of target. It refers to the ratio of incident power in unit area to total scattering power when targets are isotropic scattering. This ratio has a dimension of area, and it denotes how much power stealth targets have got from the 28

incident power. The LRCS is a complex function of targets' dynamic and static features, propagation media features and incident wave features. The LRCS can be calculated approximately as RCS in radar.

σ=4πρAR ΩR , where ρ denotes the reflectivity of target surface, AR denotes the projection area of target, ΩR
denotes the solid angle of scattering beam. Reflecting signal of diffusive reflection targets will be scattered in a wide area, and the distribution of reflecting signals submit to the rule of Bidirectional Reflecting Distribution Function (BRDF). The detected power of lidars can be derived from lidar operating range formula.

,where PR denotes the receiving power, PT denotes the transmitting power, R denotes the operating range, ΩT denotes the solid angle of transmitting beam, AC denotes the effective receiving area, and т denotes the transmission of unidirectional transmission. The relationship between the LRCS and operating range can be derived from formula and

When R is the maximum operating range, σM is called the Critical LRCS, and the target is stealthy if inequality σ < σM is tenable. At this point, it is necessary to build a complex geometrical model and take account of the surface optical characters or material scattering characters to calculate LRCS of a stealth target with complicated shape. The graphic EM calculating model of a RF system can be used for reference of calculating the LRCS.

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Fig11.2:- Lidar Equipment 11.4 Lidar Stealth As said early LIDAR can be considered as a special case of RADAR and hence the almost all stealth methods adopted for radars stated above are applicable for Lidars too like the Lidar jammer’s etc.

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12.Plasma Stealth
Plasma Stealth can be considered as a specific Stealth method employed for Ariel stealth. Couple of things to keep in mind: plasma is ionized gas particles. Therefore, plasma flow is a flow of ionized gas particles. Ion is an electrically charged particle or group of atoms. Plasma cloud is a quasineutral (total electrical charge is zero) collection of free charged particles. The vast majority of matter in the universe exists in plasma state. Near the Earth plasma can be found in the form of solar wind, magnetosphere and ionosphere. The main property of plasma (for our purposes) is its frequency, which is equal to a square root of a ratio of 4 * Pi * square of ion charge * concentration of ions to the mass of ion: SQRT ((4 * Pi * n * e^2) / m), , where e is electron or ion charge, n is concentration of ions per volume of plasma and m is mass of ion. There are several types of oscillations in plasma: low frequency (ion-sound waves), high frequency (oscillations of electrons relative to ions), spiral waves (in the presence of a magnetic field - "magnetosound"), and cross waves propagating along a magnetic field. A device for generating plasma is called plasmatron. This device generates the so-called low-temperature plasma. There are several types of oscillations in plasma: low frequency (ion-sound waves), high frequency (oscillations of electrons relative to ions), spiral waves (in the presence of a magnetic field - "magnetosound"), and cross waves propagating along a magnetic field. A device for generating plasma is called plasmatron. This device generates the so-called low-temperature plasma. This is truly unbelievable, but even this theoretically and technologically is perfectly possible. It is not known whether the plasma stealth system developed by the Russians employs a plasma laser or some other method for creating a plasma field. My personal opinion is that it has nothing to do with a plasma laser (which is a very large and very power-hungry device.) Plasma physics was given priority in Russia many years ago, which resulted in a number of breakthroughs in theory as well as practical applications of plasma. Perhaps one of the most interesting and promising applications of plasma is the so-called ion thruster, used to propel spacecraft. This technology was first developed in Russia (mainly by Keldysh Research Center) and recently successfully used on an American satellite.

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13. Adaptive Water Curtain Technology (AWCT)
The Adaptive Water Curtain Technology (AWCT) is intended to deflect and scatter enemy radar waves thus reducing the ship’s radar cross section (RCS). It consists of (highly conductive) sea water sprayed in a fashion that effectively creates an angled radar reflective curtain around the ship. To reduce the ship’s remaining RCS, the water curtain can be "modulated" such that the returns appear as "Sea Clutter." This could be done by determining the surrounding Sea State-either locally, or from satellite Sea State data, i.e., deriving the Sea Clutter Spectrum; and applying the appropriate coefficients to the modulating process for optimum mimicry. This approach is suggested as an "Add-On" to existing surface ships, an interim measure until the next generation DD(X) of stealthy surface ships has replaced this class. The Arleigh Burke class Destroyer--which has rudimentary stealth technology, is used as an example of a recipient ship for this technology. Although this class of ship has a reduced RCS over its predecessor, it can still benefit significantly from the proposed technology. This technology can reduce a surface ship's vulnerability to Radar cross-section (RCS), Infrared signature (IR), and Visual signature reduction. 13.1 FEATURES: Reduced RCS. IR Signature Mitigation of Ship Stacks by the use of "Pre-Cooled" Water Curtain. Reduced Visual Signature (Camouflage). Possible EMP Protection System able to mimic Sea Clutter. Water streams can be "Modulated" for enhanced concealment. Uses Fire Fighting Technology. Compensation for finite Water Stream boundaries (gaps), using Spray and Misting. System uses Feedback for accurate positioning of Water Stream "Landing zone." Able to (actively) Resist Wind Loading on Water Curtain. Can Selectively open Gaps in Water Curtain for Radar, IR, Communications, etc. Satellite (or RPV) for Interactive Sensing and Alignment for Stealth Optimization. System Cleaning by Periodic Flushing with brackish or clean water.

Fig13.1:- The Adaptive Water Curtain Technology (AWCT) is intended to deflect and scatter enemy radar waves thus reducing the ship’s radar cross section (RCS). It consists of highly conductive sea water sprayed in a fashion that effectively creates an angled radar reflective curtain around the ship

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14. ADVANTAGES AND DISADVANTAGES OF STEALTH TECHNOLOGY
14.1 Advantages of Stealth Technology 1. A smaller number of stealth vehicles may replace fleet of conventional attacks vehicles with the same or increased combat efficiency. Possibly resulting in longer term savings in the military budget. 2. A Stealth vehicles strike capability may deter potential enemies from taking action and keep them in constant fear of strikes, since they can never know if the attack vehicles are already underway. 3. The production of a stealth combat vehicles design may force an opponent to pursue the same aim, possibly resulting in significant weakening of the economically inferior party. 4. Stationing stealth vehicles in a friendly country is a powerful diplomatic gesture as stealth vehicles incorporate high technology and military secrets. 5. Decreasing causality rates of the pilots and crew members. 14.2 Disadvantages of Stealth Technology 1. Stealth technology has its own disadvantages like other technologies. Stealth aircraft cannot fly as fast or is not maneuverable like conventional aircraft. The F-22 and the aircraft of its category proved this wrong up to an extent. Though the F-22 may be fast or maneuverable or fast, it can't go beyond Mach 2 and cannot make turns like the Su-37. 2. Another serious disadvantage with the stealth aircraft is the reduced amount of payload it can carry. As most of the payload is carried internally in a stealth aircraft to reduce the radar signature, weapons can only occupy a less amount of space internally. On the other hand a conventional aircraft can carry much more payload than any stealth aircraft of its class. 3. Whatever may be the disadvantage a stealth vehicles can have, the biggest of all disadvantages that it faces is its sheer cost. Stealth aircraft literally costs its weight in gold. Fighters in service and in development for the USAF like the B-2 ($2 billion), F-117 ($70 million) and the F-22 ($100 million) are the costliest planes in the world. After the cold war, the number of B-2 bombers was reduced sharply because of its staggering price tag and maintenance charges. 4. The B-2 Spirit carries a large bomb load, but it has relatively slow speed, resulting in 18 to 24 hour long missions when it flies half way around the globe to attack overseas targets. Therefore advance planning and receiving intelligence in a timely manner is of paramount importance. 5. Stealth aircraft are vulnerable to detection immediately before, during and after using their weaponry. since reduced RCS bombs and cruise Missiles are yet not available; all armament must be carried internally to avoid increasing the radar cross section. As soon as the bomb bay doors opened, the planes RCS will be multiplied. 6. Another problem with incorporating "stealth" technology into an aircraft is a wing shape that does not provide the optimum amount of lift. The resulting increase in drag reduces flight performance. "Stealth" shapes, such as the "faceting" found on Lockheed's F-117 "stealth" fighter, also tend to be aerodynamically destabilizing. This is brought under control only through the use of highly sophisticated computers that serve to electronically balance the aircraft in flight through its autopilot and control system. All of these modifications, however, hurt the plane's performance, adding weight, affecting aerodynamics, and altering the structure of the aircraft. The advantages of stealth technology must always be weighed against its disadvantages impossible. 33

15. CONCLUSION
The Detection and Stealth Technology has improved significantly more advanced in the last fifty years or so. This trend is likely to continue as these two oppose each other. Till date stealth aircraft have been used in several low and moderate intensity conflicts, including operation Desert Storm. Operation Allied Force and the 2003 invasion of Iraq .In each Case they were employed to strike high value targets which were either out of range of conventional aircraft or which were too heavily defended for conventional aircraft to strike without a high risk of loss. In addition, because The stealth aircraft aren’t going to be dodging surface to air missiles and anti-aircraft artillery over the target they can aim more carefully and thus are more likely to hit the high value targets early in the campaign (or even for it) ,Before other aircraft had the opportunity to degrade the opposing air defense. However, given the increasing prevalence of excellent Russion-bilt Surface –to-air missile (SAM) system on the open market, stealth aircraft are likely to be very important in a high intensity conflict in order to gain and maintain air supremacy. Stealth technology .in future, would be required for clearing the way for deeper strikes , which conventional aircraft would find very difficult .For example ,China license-builds a wide range of SAM systems in quantity and would be able to heavily defend important strategic and tactical targets in the event of some kind of conflict .Even if antiradiation weapons are used in an attempt to destroy the SAM radars of such systems, these SAMs are capable of shooting down weapons fired against them. The surprise of a stealth attack may become the only reasonable way of making a safe corridor for conventional bombers. It would then be possible for the less-stealth force with superior weaponry to suppress the remaining systems and gain air superiority. The development and the deployment of the Visby’s- the first commissioned Stealth ships has raised new threats in the maritime boundaries. The sudden appearance of sea clutters on the radar at a region may be these ships. The plasma stealth technology raises new hopes of engineering brilliance. As plasma is said to absorb all electromagnetic radiation the development of a counter stealth technology to such a mechanism will be a strenuous task. Well to conclude the current scenario appears something similar to the cold war both sides are accumulating weapons to counter each other and each side can be termed as “Stealth Technology” and the other as “Anti-Stealth Technology”. It’s an arm race except it isn't between specific countries. “It’s a fight between Technologies”.

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16. REFRENCES
1. http://www.totalairdominance.50megs.com/articles/stealth.htm 2. http://en.wikipedia.org/wiki/Stealth_technology 3. http://en.wikipedia.org/wiki/Radar 4. http://en.wikipedia.org/wiki/Stealth_ship 5. http://www.resonancepub.com/images/stealth_ship.gif 6. http://images.google.co.in/images 7. http://science.howstuffworks.com/question69.htm 8. http://www.espionageinfo.com/Sp-Te/Stealth-Technology.html 9. http://www.airplanedesign.info/51.htm 10. http://www.hitechweb.genezis.eu/stealth4f_soubory/image013.jpg 11. http://www.geocities.com/electrogravitics/scm.html 12. http://www.razorworks.com/enemyengaged/chguide/images/lo- reflecting.gif 13. htp://www.x20.org/library/thermal/pdm/ir_thermography.htm 14. http://en.wikipedia.org/wiki/Plasma_stealth 15. http://www.military-heat.com/43/russian-plasma-stealth-fighters/ 16. http://homepage.mac.com/ardeshir/Anti-StealthTechnology.pdf 17. http://www.scribd.com/doc/7393272/Anti-Stealth-Technology 18. http://www.megaessays.com/essay_search/wartime_coalition.html 19. http://www.termpapersmonthly.com/topics/Advantages%20and%20Disadvant tages %20of%20Technology/160 20. http://www.marinetalk.com/articles-marine-companies/art/StealthTechnology-for-Future-Warships-BAE00120817TU.html 21. http://www.fighter-planes.com/info 22. http://robocat.users.btopenworld.com 23. http://www.absoluteastronomy.com 24. http://www.williamson-labs.com/ltoc/ship-stealth-tech.htm 25. http://www.aticourses.com/wordpress-2.7/weblog1/index.php 26. http://iron-eagles.tripod.com/articles/active.htm 27. http://www.scribd.com/13992535-Technical-PaperStealth-Technology 28. http://www.scribd.com/Stealth Technology – Infrared Signature Studies 29. Naval Infrared Stealth Technology- Davis 30. Wavelet based acoustic detection of moving vehicles-Amir Averbuch Valery Zheludev Neta Rabin and Alon Schclar ,School of Computer Science ,Tel Aviv University, Tel Aviv 69978, Israel

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