23-04-2011, 11:43 AM
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1. HISTORY
Almost since the invention of radar, various techniques have been tried to minimize detection. Rapid development of radar during World War II led to equally rapid development of numerous counter radar measures during the period; a notable example of this was the use of chaff.
In the late 1950's the American, Central Intelligence Academy (CIA) began sending Lockheed U-2 'spy-planes' over the Soviet Union to take intelligence photographs. The U-2's were little more than jet-powered gliders built of plastic and plywood flew at 80,000ft (24,000m) to be out of range of anti-aircraft fire, but it then became clear that radar was not detecting them.
It was not until May 1960, after more than four years of over-flights, that the Russians shot one down using new radar equipment belonging to SA-2, surface-to-air missiles and even then the U-2 did not receive a direct hit.
The success of the U-2s led to highly classified research work in the US, known as 'Stealth', to create a military aircraft that was invisible to radar. The U-2 had gone undetected for so long because it was made of non-metallic materials which absorbed radar waves rather than reflecting them back to the radar ground station, as normally happens.
The Stealth program aimed at designing high-performance military aircraft incorporating, among other features, a minimum of metal and with the exterior clad in highly absorbent tiles. The aircraft would be almost invisible to radar and could make most radar-controlled anti-aircraft systems obsolete.
After being developed under a blanket of secrecy, the high-tech B-2 Stealth bomber was unveiled at the Northrop Company’s manufacturing plant in Palmdale, California, in November 1988. An audience of invited journalists and guests was kept well away from the plane, which was designed to slip through enemy radar defenses without being detected and the drop up to 16 nuclear bombs on key targets.
Modern submarines are coated in a thick layer of a top-secret resin which is highly absorbent acoustically, and reflects only a minute amount of the energy transmitted by sonar detectors.
2. INTRODUCTION
“STEALTH”, as seen in dictionaries is ‘the act of moving, proceeding, or acting in a covert way’. Stealth technology (also known as LOT, Low Observability Technology) is a sub-discipline of 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 and other detection methods.
A mission system employing stealth may well become detected at some point. With in a given mission, such as when the target is destroyed, but current use of stealth systems should seek to minimize the possibility of detection. Attacking with surprise gives the attacker more time to perform is mission and exit before the defending force can counter-attack. If a surface-to-air missile battery defending the target observes a bomb falling a summarizes that there must be a stealth aircraft in the vicinity, for example, it is still unable to respond if it cannot get a look on the aircraft in order to feed guidance information to its missiles.
Radar is a system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain. The term RADAR was coined in 1941 as an acronym for Radio Detection and Ranging. In infrared (IR) detection method, the IR signature of exhaust plume is the prime contributor.
Stealth is not a single technology but is a combination of technologies that attempt to greatly reduce the distances at which a vehicle can be detected in particular reducing the Radar Cross Section (RCS).
3. ABOUT RADARS
Radar is an object-detection system which uses electromagnetic waves — specifically radio waves — to determine the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish, or antenna, transmits pulses of radio waves or microwaves which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the transmitter.
3.1 Principles of radar
It mainly works on the two principles:
1. ECHO and
2. DOPPLER SHIFT
Echo is used to detect time and distance of target
Doppler shift is used to detect the speed of target approaching
3.2 Echo and Doppler Shift
Echo is something you experience all the time. If you shout into a well or a canyon, the echo comes back a moment later. The echo occurs because some of the sound waves in your shout reflect off of a surface (either the water at the bottom of the well or the canyon wall on the far side) and travel back to your ears. The length of time between the moment you shout and the moment that you hear the echo is determined by the distance between you and the surface that creates the echo.
Doppler shift is also common. You probably experience it daily (often without realizing it). Doppler shift occurs when sound is generated by, or reflected off of, a moving object. Doppler shift in the extreme creates sonic booms (see below). Here's how to understand Doppler shift (you may also want to try this experiment in an empty parking lot). Let's say there is a car coming toward you at 60 miles per hour (mph) and its horn is blaring. You will hear the horn playing one "note" as the car approaches, but when the car passes you the sound of the horn will suddenly shift to a lower note. It's the same horn making the same sound the whole time. The change you hear is caused by Doppler shift
4. STEALTH PRINCIPLES
4.1 Vehicle shape:
One of the important factors 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.
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 aircrafts use a different arrangement, tilting the tail surfaces to reduce corner reflections formed between them.
Stealth design must also 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 that – weapons, fuel tanks, and other stores must not be carried externally. Any stealthy vehicle becomes un-stealthy when a door or hatch is opened.
a. Propulsion subsystem shaping:
Fluidic nozzles for thrust vectoring with aircraft jet engines, and ships, will have lower RCS, due to being less complex, mechanically simpler, with no moving parts or surfaces, and less massive (up to 50% less). Fluidic nozzles divert thrust via fluid effects. Tests show that air forced into a jet engine exhaust stream can deflect thrust up to 15 degrees.
b. Non-metallic airframe:
Dielectric composites are relatively transparent to radar, whereas electrically conductive materials such as metals and carbon fibers reflect electromagnetic energy incident on the material's surface. Composites used may contain ferrites to optimize the dielectric and magnetic properties of the material for its application.
c. Radar absorbing material (RAM):
RAM, often as paints, is used especially on the edges of metal surfaces. One such coating, also called iron ball paint, contains tiny spheres coated with carbonyl iron ferrite. Radar waves induce alternating magnetic field in this material, which leads to conversion of their energy into heat.
Previously, neoprene-like tiles with ferrite grains embedded in the polymer matrix were used, now RAM paint is applied directly. The paint must be applied by robots because of problems of solvent, toxicity and tight tolerances on layer thickness.
Similarly, coating the cockpit canopy with a thin film transparent conductor helps to reduce the aircraft's radar profile because radar waves would normally enter the cockpit, bounce off something random and possibly return to the radar, but the conductive coating creates a controlled shape that deflects the incoming radar waves away from the radar. The coating is thin enough that it has no adverse effect on the pilot's vision.
1. HISTORY
Almost since the invention of radar, various techniques have been tried to minimize detection. Rapid development of radar during World War II led to equally rapid development of numerous counter radar measures during the period; a notable example of this was the use of chaff.
In the late 1950's the American, Central Intelligence Academy (CIA) began sending Lockheed U-2 'spy-planes' over the Soviet Union to take intelligence photographs. The U-2's were little more than jet-powered gliders built of plastic and plywood flew at 80,000ft (24,000m) to be out of range of anti-aircraft fire, but it then became clear that radar was not detecting them.
It was not until May 1960, after more than four years of over-flights, that the Russians shot one down using new radar equipment belonging to SA-2, surface-to-air missiles and even then the U-2 did not receive a direct hit.
The success of the U-2s led to highly classified research work in the US, known as 'Stealth', to create a military aircraft that was invisible to radar. The U-2 had gone undetected for so long because it was made of non-metallic materials which absorbed radar waves rather than reflecting them back to the radar ground station, as normally happens.
The Stealth program aimed at designing high-performance military aircraft incorporating, among other features, a minimum of metal and with the exterior clad in highly absorbent tiles. The aircraft would be almost invisible to radar and could make most radar-controlled anti-aircraft systems obsolete.
After being developed under a blanket of secrecy, the high-tech B-2 Stealth bomber was unveiled at the Northrop Company’s manufacturing plant in Palmdale, California, in November 1988. An audience of invited journalists and guests was kept well away from the plane, which was designed to slip through enemy radar defenses without being detected and the drop up to 16 nuclear bombs on key targets.
Modern submarines are coated in a thick layer of a top-secret resin which is highly absorbent acoustically, and reflects only a minute amount of the energy transmitted by sonar detectors.
2. INTRODUCTION
“STEALTH”, as seen in dictionaries is ‘the act of moving, proceeding, or acting in a covert way’. Stealth technology (also known as LOT, Low Observability Technology) is a sub-discipline of 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 and other detection methods.
A mission system employing stealth may well become detected at some point. With in a given mission, such as when the target is destroyed, but current use of stealth systems should seek to minimize the possibility of detection. Attacking with surprise gives the attacker more time to perform is mission and exit before the defending force can counter-attack. If a surface-to-air missile battery defending the target observes a bomb falling a summarizes that there must be a stealth aircraft in the vicinity, for example, it is still unable to respond if it cannot get a look on the aircraft in order to feed guidance information to its missiles.
Radar is a system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain. The term RADAR was coined in 1941 as an acronym for Radio Detection and Ranging. In infrared (IR) detection method, the IR signature of exhaust plume is the prime contributor.
Stealth is not a single technology but is a combination of technologies that attempt to greatly reduce the distances at which a vehicle can be detected in particular reducing the Radar Cross Section (RCS).
3. ABOUT RADARS
Radar is an object-detection system which uses electromagnetic waves — specifically radio waves — to determine the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish, or antenna, transmits pulses of radio waves or microwaves which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the transmitter.
3.1 Principles of radar
It mainly works on the two principles:
1. ECHO and
2. DOPPLER SHIFT
Echo is used to detect time and distance of target
Doppler shift is used to detect the speed of target approaching
3.2 Echo and Doppler Shift
Echo is something you experience all the time. If you shout into a well or a canyon, the echo comes back a moment later. The echo occurs because some of the sound waves in your shout reflect off of a surface (either the water at the bottom of the well or the canyon wall on the far side) and travel back to your ears. The length of time between the moment you shout and the moment that you hear the echo is determined by the distance between you and the surface that creates the echo.
Doppler shift is also common. You probably experience it daily (often without realizing it). Doppler shift occurs when sound is generated by, or reflected off of, a moving object. Doppler shift in the extreme creates sonic booms (see below). Here's how to understand Doppler shift (you may also want to try this experiment in an empty parking lot). Let's say there is a car coming toward you at 60 miles per hour (mph) and its horn is blaring. You will hear the horn playing one "note" as the car approaches, but when the car passes you the sound of the horn will suddenly shift to a lower note. It's the same horn making the same sound the whole time. The change you hear is caused by Doppler shift
4. STEALTH PRINCIPLES
4.1 Vehicle shape:
One of the important factors 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.
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 aircrafts use a different arrangement, tilting the tail surfaces to reduce corner reflections formed between them.
Stealth design must also 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 that – weapons, fuel tanks, and other stores must not be carried externally. Any stealthy vehicle becomes un-stealthy when a door or hatch is opened.
a. Propulsion subsystem shaping:
Fluidic nozzles for thrust vectoring with aircraft jet engines, and ships, will have lower RCS, due to being less complex, mechanically simpler, with no moving parts or surfaces, and less massive (up to 50% less). Fluidic nozzles divert thrust via fluid effects. Tests show that air forced into a jet engine exhaust stream can deflect thrust up to 15 degrees.
b. Non-metallic airframe:
Dielectric composites are relatively transparent to radar, whereas electrically conductive materials such as metals and carbon fibers reflect electromagnetic energy incident on the material's surface. Composites used may contain ferrites to optimize the dielectric and magnetic properties of the material for its application.
c. Radar absorbing material (RAM):
RAM, often as paints, is used especially on the edges of metal surfaces. One such coating, also called iron ball paint, contains tiny spheres coated with carbonyl iron ferrite. Radar waves induce alternating magnetic field in this material, which leads to conversion of their energy into heat.
Previously, neoprene-like tiles with ferrite grains embedded in the polymer matrix were used, now RAM paint is applied directly. The paint must be applied by robots because of problems of solvent, toxicity and tight tolerances on layer thickness.
Similarly, coating the cockpit canopy with a thin film transparent conductor helps to reduce the aircraft's radar profile because radar waves would normally enter the cockpit, bounce off something random and possibly return to the radar, but the conductive coating creates a controlled shape that deflects the incoming radar waves away from the radar. The coating is thin enough that it has no adverse effect on the pilot's vision.
