Stealth Technology

1. INTRODUCTION
Stealth technology also known as LOT (Low Observability Technology) is a technology 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. The term “Stealth” is thought to have been coined in 1966 by Charles E. “Chuck” Myers, a combat pilot and later by an executive at Lockheed. The quest for a stealthy plane actually began more than 50 years ago during World War II when RADAR was first used as an early warning system against fleets of bombers. As a result of that quest, the Stealth Technology evolved. Stealth Technology is used in the construction of mobile military systems such as aircrafts and ships to significantly reduce their detection by enemy, primarily by an enemy RADAR. The way most airplane identification works is by constantly bombarding airspace with a RADAR signal. When a plane flies into the path of the RADAR, a signal bounces back to a sensor that determines the size and location of the plane.

Other methods focus on measuring acoustic (sound) disturbances, visual contact, and infrared (heat) signatures. Stealth technologies work by reducing or eliminating these telltale signals. Panels on planes are angled so that radar is scattered and no signal returns. Planes are also covered in a layer of absorbent materials that reduce any other signature the plane might leave. Shape also has a lot to do with the invisibility of stealth planes. Extreme aerodynamics keeps air turbulence to a minimum and cut down on flying noise. Special low-noise engines are contained inside the body of the plane. Hot fumes are then capable of being mixed with cool air before leaving the plane. This fools heat sensors on the ground. This also keeps heat seeking missiles from getting any sort of a lock on their targets. Stealth properties give it the unique ability to penetrate an enemy’s most sophisticated defenses and threaten its most valued and heavily defended targets.

2. HISTORY OF STEALTH AIRCRAFT
With the increasing use of early warning detection devices such as radar by militaries around the world in the 1930’s the United States began to research and develop aircraft that would be undetectable to radar detection systems. The first documented stealth prototype was built out of two layers of plywood glued together with a core of glue and sawdust. This prototype’s surface was coated with charcoal to absorb radar signals from being reflected back to the source, which is how radar detection systems detect items in the air. Jack Northrop built a flying wing in the 1940’s. His plane was the first wave of stealth aircraft that actually flew. The aircraft proved to be highly unstable and hard to fly due to design flaws. The United States initially ordered 170 of these aircraft from Northrop but cancelled the order after finding that the plane had stability Flaws. Then in 1964, SR-71 the first Stealth airplane launched. It is well known as “black bird’. It is a jet black bomber with slanted surfaces. This aircraft was built to fly high and fast to be able to bypass radar by its altitude and speed. The Blackbird was developed primarily for the Cold War between the United States and the U.S.S.R. SR-71. Aircraft is shown in figure below.

3. HOW DOES STEALTH TECHNOLOGY WORK?
The idea is for 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 an airplane invisible to radar. There are two different ways to create invisibility: > The airplane can be shaped so that any radar signal it reflects is reflected away from the radar equipment. > The airplane can be covered in materials that absorb radar signals.

3.1 AIRCRAFT DETECTION METHODS
The most common methods used today to detect an aircraft are: 3.1.1 RADAR
Currently the way to detect and even identify an aircraft is the use of RADAR (radio detection and ranging). This system invented during World War II, simply works by constantly sending bursts of radio waves of certain frequencies and measures the echoes of each burst. Objects are reflecting parts of the energy of radio waves. Depending on the material the object is made of, this echo is stronger or weaker, but there is an echo. By measuring the reflected energy as a function of position and time, computers can calculate what it is that reflects the energy, where it is in 3D space and also in what direction it moves. To get a proper overview of an area with RADAR, the transmitting and receiving antenna should rotate in angles of 360 degrees. RADAR works on the principle of echo and Doppler shift. Echo is the repetition of a note after the original note is dead and Doppler shift is the phenomenon of apparent change in the frequency of the radio wave whenever there is a relative motion between the source and the object.

3.1.1.1 SOURCES OF RADAR REFLECTION
a) Gaps and breaks in surface
b) Unshielded cockpit
c) External weapons
d) Exposed engines
e) Large, right-angled tail surfaces

3.1.1.2 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 to the power density that is intercepted by the target. RCS is the one single variable that is out of the radar designer’s control. The relationship of RCS to the detection range is not in direct proportion, because of the aforementioned conical beam and radial scattering effects. Detection range is in proportion to the fourth root of RCS. A conventional aircraft has a complex external shape, full of curves, flat panels and edges. As the airplane moves (rapidly, relative to a radar which is pulsing energy toward it), it throws off a constantly changing, scintillating pattern of concentrated reflections.

RCS is determined by first measuring, or calculating, the amount of radar energy reflected from a target toward an observer. RCS is based on the size of a reflective sphere that would return the same amount of energy. The projected area of the sphere, or the area of a disk of the same diameter, is the RCS number itself. The radar beam, it is important to remember, is a cone. The greater the range the greater the area illuminated by the radar, and the smaller the proportion if the energy which will be scattered by a target with a given RCS. The same effect results in the scattered energy returning to the radar. Therefore, at a longer range, the already reduced energy hitting the target is scattered over a wider area and less of it will be captured by the antenna.4

RCS depends on:
* Material of which the target is made, absolute size of the target * Relative size of the target (in relation to the wavelength of the illuminating radar) * 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) * Reflected angle (angle at which the reflected beam leaves the part of the target hit, it depends upon incident angle), * The polarization of transmitted and the received radiation in respect to the orientation of the target Factors affecting RCS:

* Size
* Material
* Radar absorbent paint
* Shape and orientation
* Smooth surfaces4,5
3.1.2 HEAT DETECTION
Another way of detecting if an aircraft is flying somewhere is by measuring the heat it radiates. Normally this heat is produced by the plane engines. There are two significant sources of infrared radiation from air breathing propulsion systems: hot parts and jet wakes. By modern heat image sensors (Infrared sensors) the difference can be seen between a flying object itself and the surrounding cold air. This is the same for the jet engine exhaust gases.

3.1.3 TURBULENCE DETECTION
Shape also has a lot to do with the ‘invisibility’ of stealth planes. Extreme aerodynamics keeps air turbulence to a minimum. Sophisticated Laser controlled turbulence sensors, which can measure paths of disturbed air, generated by an aircraft, which just passed.

3.1.4 VISUAL DETECTION
The exhaust of aircraft i.e., the white line in the sky caused by high- flying planes makes it easier to detect the aircraft even with the naked eye. Also the color of the aircraft is an important factor.

3.1.5 ACOUSTIC DETECTION
A very obvious source of detection is the noise, generated by jet engines. Several systems have been designed in the meantime to reduce the sound of jet engine exhausts to a minimum, making them harder to detect by just measuring sound waves.6,9

3.2 REQUIREMENTS TO BE STEALTHY
To make a stealthy aircraft, designers had to consider six key ingredients:
1. They need to reduce the imprint on the radar screen.
2. Turn down the heat of its infrared picture.
3. They need to reduce muffling noise.
4. They need to reduce the turbulence.
5. Making the plane less visible.
6. Stifle radio emissions.

3.2.1 RADAR ECHO REDUCTION
3.2.1.1 SCATTERING
The airplane can be shaped so that any RADAR signals it reflects are deflected away from the RADAR equipment. Most conventional aircraft have a rounded shape. This shape makes them aerodynamic, but it also creates a very efficient radar reflector. The round shape means that no matter where the radar signal hits the plane, some of the signal gets reflected back:

Fig 3.1 Conventional Aircraft-Very efficient radar reflector1 A stealth aircraft on the other hand, is made up of completely flat surfaces and very sharp edges. When a radar signal hits a stealth plane, the signal reflects away at an angle, like this:

Fig.3.2 Stealth Aircraft-Radar signal reflect away at an angle1

In addition, surfaces on a stealth aircraft can be treated so they absorb radar energy as well. The overall result is that a stealth aircraft like an F-117A can have the radar signature of a small bird rather than an airplane. The only exception is when the plane banks there will often be a moment when one of the panels of the plane will perfectly reflect a burst of radar energy back to the antenna.1,5 3.2.1.2 REDUCTION BY RAM:

A second way of stopping RADAR reflections is by coating the plane with material that soaks up Radar energy. Radar absorbing coatings can be applied to the surface of the body, which effectively drain the energy of the radar signal. For example, Radar Absorbent Material (RAM), coatings designed to suck in and dissipate the electromagnetic energy of radar wave instead of reflecting it back to the source.

RADAR ABSORBENT MATERIAL (RAM)
As its name implies, RAM is intended to reduce the scattered signal by absorbing some part of the incident radiation. Microwave energy is converted into heat energy with hardly any noticeable temperature rise because the energies involved are extremely small. Various kinds of materials can be made to absorb microwave energy by impregnating them with conducting materials such as carbon and iron. In the main, there are two currently used kinds of absorbers, called dielectric RAM and magnetic RAM. Addition of carbon products in an insulating material introduces electric resistance and changes the electrical properties.

Hence carbon-based absorbers are called dielectric RAM. Dielectric RAM is usually too bulky and fragile and not attractive where space is limited and severe mechanical vibrations exist. Magnetic RAM uses iron products such as carbonyl iron and iron oxides called ferrites. Iron effectively dissipates radar waves and has been used in aircraft paint. It is quite effective against the high frequency radars used in modern fighters. Unlike dielectric RAM, magnetic RAM is compact, thin and of adequate strength to withstand loads and an abrasive environment. Nevertheless, its thickness does rob volume from volume limited aircraft. Some important RAM’s used today are, (A) SALISBURY SCREEN

Its construction consists of a conductive carbon coated “lossy” fabric, separated from a conductive ground plane by a low dielectric foam core. (B) FOAM MATERIALS
Different foam materials are,
a) Single layer foam
b) Multi layer foam-made of 3 single layers
c) Reticulated foam
d) Weather proof foam

(C) MAGNETIC ABSORBERS
The magnetic absorbers are elastomeric moulded sheets loaded with magnetic filler. The use of the magnetic filler provides the best performance at the minimum thickness. Different magnetic absorbers are, a) Tuned frequency magnetic absorbers

b) Surface wave absorbers
c) Multiband absorbers
(D)CORE MATERIAL
Core material is a broadband microwave absorbing honeycomb core. Normally uses either aramid or fiberglass honeycomb core and applies a lossy coating to it. (E) PIFRAM (POLY CRYSTALLINE IRON FIBRE RAM)

It is the only electromagnetic Radar Absorbing Material that may be retrofitted to existing material because of its low weight and very low thickness.6,10,16 3.2.2 ECHO CANCELLATION
Metal component such as the engine, which produces significant radar reflections, can be shielded using a metal and plastic sandwich whose layers are spaced in such a way as to create a standing wave, canceling out any radar reflections. 3.2.3 HEAT RADIATION REDUCTION

Infrared radiation (heat) should be minimized by a combination of temperature reduction and masking. The main body of the airplane has its own radiation, heavily dependent on speed and altitude, and the jet plume can be a most significant factor, particularly in after burning operation. The engines are buried deep in the fuselage. These have got shallow ‘platypus’ exhausts, which cool and deflect the exhaust gases upward to minimize heat emissions.

3.2.4 TURBULENCE DETECTION REDUCTION
By optimizing the aerodynamics of the stealth plane, the eye visible turbulence trail in the air, can be kept to a minimum. This way it becomes harder for the very special laser equipment to detect the trail and trace it back all the way to the plane which created it. 3.2.5 VISUAL DETECTION REDUCTION

3.2.5.1 HIDING SMOKE CONTRAILS (JET WAKE)
Reducing smoke in the exhaust is accomplished by improving the efficiency of the combustion chambers. Tests have been done using exotic chemicals to be inserted in to the engine outlet gases to modify infrared signature as well as to force water molecules in the exhaust plume to break up in to much finer particles, thus reduce or even eliminate contrails. One of the chemicals used for this was chloro-fluoro-sulphonic acid.

3.2.5.2 LOW VISIBILITY
An aircraft at low to medium altitudes tends to be a black dot against the background of the sky. To avoid this, the plane is given a special medium gray color. The gray, when combined with light scattering at low to medium altitudes ensures about as low observability as can be possible or a reduction to 30% in visibility. 3.2.5.3 LOW LEVEL FLIGHT

Another technique used by aircraft to avoid radar is to fly at very low levels where there is a great deal of “ground clutter” because of radar reflections given off by buildings and other objects. Low level aircraft can go undetected by most radar systems.6,10,15

4. ADVANTAGES
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 ever 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 crewmembers.11,13,14

5. DISADVANTAGES
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.11,13,14

6. LIMITATIONS

There are limits to the utility of stealth techniques. Since the radar cross-section of an aircraft depends on the angle from which it is viewed, an aircraft will typically have a much smaller RCS when viewed from the front or rear than when viewed from the side or from above. In general stealth aircraft are designed to minimize their frontal RCS. But it is not possible to contour the surface of an aircraft to reduce the RCS equally in all directions, and reductions in the frontal RCS may lead to a larger RCS from above. Thus while a stealth aircraft may be difficult to track when it is flying toward a ground-based radar or another aircraft at the same altitude, a high-altitude airborne radar or a space-based radar may have an easier time tracking it.

Another limitation of stealth aircraft is their vulnerability to detection by bi-static radars. The contouring of a stealth aircraft is designed to avoid reflecting a radar signal directly back in the direction of the radar transmitter. But the transmitter and receiver of bi-static radar are in separate locations indeed, a single transmitter may be used by radar receivers scattered over a wide area. This greatly increases the odds that at least one of these receivers will pick up a reflected signal. The prospects for detection of stealth aircraft by bi-static radar are further improved if the radar transmitter is space based, and thus viewing the aircraft from above, the direction of its largest radar cross section.

Several analysts claim stealth aircraft such as the ATF will be svulnerable to detection by infrared search and track systems (IRST). The natural heating of an aircraft’s surface makes it visible to this type of system. The faster and aircraft flies, the warmer it gets, and thus, the easier to detect through infrared means. One expert asserts “if an aircraft deviates from its surroundings by only one degree centigrade, you will be able to detect it at militarily useful ranges.” Stealth aircraft are even more vulnerable to multiple sensors used in tandem.11,13,14

7. 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 Russian belt 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 anti radiation 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 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.3,12

8. REFERENCES

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16. http://www.calvin.edu/~pribeiro/courses/engr302/Samples/wilson-paper.doc