Invisible spear and shield

  In many literary and film and television works, invisibility is often an important ability of the protagonist. The invisibility cloak, one of the three sacred artifacts in “Harry Potter and the Deathly Hallows”, was an important help for Harry Potter to defeat Voldemort in the end. In reality, stealth technology is usually applied in the military field, and its professional definition refers to the specialized technology that seeks to reduce the detectability of weapons and equipment during the development process to make them difficult to be found, tracked, and attacked by the enemy. Since there is a “spear”, there is bound to be a “shield”. With the development of stealth technology, anti-stealth technology is also advancing rapidly, and there has been a situation where stealth and anti-stealth technology compete and rise in cycles.

The mystery of “invisibility cloak”

  In the first air strike of the Gulf War, the number of F-117A stealth fighters deployed only accounted for 2% of the total number of aircraft deployed by the multinational force, but they completed 40% of the total air strikes without any loss. The reason was that the Iraqi army at that time The radar can’t detect them at all. So, how does F-117A behave like no one else? Let us uncover the mystery of stealth technology.
  Stealth technology is used in the military field by reducing the observation characteristics of sound, radar, infrared, and photoelectric in the overall design. Weapons and equipment using this technology can not only “hidden the truth” to save themselves, but also “show falsehood” to confuse the opponent. The emergence of stealth technology is the result of the combined effect of the operational requirements to improve the survivability of combat weapons, the development of electromagnetic theory and electronic technology.
  At present, the technologies to achieve stealth mainly include radar stealth technology, infrared stealth technology, electronic stealth technology, visible light stealth technology, and acoustic wave stealth technology. Among them, radar stealth technology and infrared stealth technology are the focus of current development.
Deception radar “clairvoyance”

  The radar stealth technology takes a variety of measures to reduce the target’s reflection characteristics of radar electromagnetic waves, so that the target echo signal energy received by the radar is greatly reduced, and the detection range of the target is greatly reduced. The radar cross section of a weapon is related to factors such as its shape, material, and radar wave incidence angle. One of the stealth measures is to design a stealth shape. Using modern design methods and manufacturing processes, under certain restricted conditions, optimize the equipment and its shape design to minimize the radar cross section while maintaining certain performance. The more classic stealth shapes are the “diamond-shaped” layout of the American F-117A stealth fighter and the “flying wing” layout of the B-2 stealth bomber. The second measure is to use stealth materials. By using paint or composite materials that can absorb or transmit radar waves, the radar waves emitted by the enemy can come back and forth, more and less, so as to achieve the purpose of being difficult to detect. The B-2 stealth bomber uses both stealth paint and stealth layout, so that its radar cross-sectional area is only the size of a window. The third measure is to use plasma stealth. Plasma generators or radioactive isotopes are used to form a plasma cloud on the surface of the weapon. By designing the characteristic parameters of the plasma, some of the radar waves irradiated on the plasma cloud are absorbed, and the other part of the radar waves change the direction of propagation, thereby making the return The energy to the radar receiver is very small, achieving the purpose of stealth. According to relevant reports, the probability of an aircraft using plasma stealth technology being discovered by the enemy can be reduced by 99%. The fourth measure is to hide through electronic means. Adaptive loading technology, electronic countermeasures, active cancellation technology, etc. can all reduce the radar cross section of the weapon, thereby inhibiting the radar’s reception of the reflected wave of the target.
  In addition, many weapons and equipment use a variety of technology combinations to enhance their stealth performance. For example, the stealth technology of the American F-22 stealth fighter includes shape optimization, electromagnetic and thermal signal shielding, coverage of stealth paint on key parts, installation of electronic deception, electronic interference and other means. Russia’s C-37 “Golden Eagle” fighter jet, the first flight time is only 18 days from the F-22 first flight time. Although the stealth performance is slightly inferior to the F-22, it adopts a smooth and streamlined design similar to the F-22. The forward-swept wings are almost entirely made of composite materials, and the body is coated with radar absorbing materials, and its stealth performance ranks among the top in the world.
The secret of infrared stealth

  As the name implies, infrared stealth is a technology that reduces or changes the infrared radiation characteristics of an object to achieve low detectability of the target. In recent years, its main technical measures include two: One is to change the infrared radiation characteristics of objects. That is, by changing the infrared radiation band, adjusting the infrared radiation transmission process (including the direction and characteristics), so as to make the infrared detector invalid. For example, the exhaust pipe of the F-117 stealth bomber is transferred to the back of the aircraft. The second is to reduce the intensity of infrared radiation. It is mainly to reduce the temperature of the object and use effective paint, or the engine tail nozzle adopts a closed loop cooling system to reduce the target’s radiation signal and increase the difficulty of infrared detection equipment detection. But many of these measures are not perfect. For example, the anti-infrared radiation coating of the B-2 stealth bomber must be repainted after every flight, and it is expensive.
Hidden in “Fantasy Light”

  Many readers must have seen the Hollywood blockbuster “Mission Impossible 4”. In the play, the protagonist Ethan Hunt (starring Tom Cruise) in order to complete a task, dressed as a Russian general, sneaked into the Kremlin with the cooperation of his teammates. In order to deceive the guards, he used a projector to display the background of the environment on the screen, deceived the guards with falsehoods, and sneaked into the secret room. Although this is the “black technology” that appears in the movie, the use of visible light to achieve stealth has become a reality. At present, the most widely used is the use of appropriate colors to achieve stealth. As far as airplanes are concerned, under normal lighting conditions, the difference between the brightness of the airplane and the sky background is closely related to the flight altitude of the airplane. The higher the flight altitude, the greater the difference in brightness. Paint different colors on airplanes of different flying heights to realize the visible light stealth of the airplane. For example, the cruise altitude of the B-2 stealth bomber is more than 10,000 meters, and its bottom is painted dark gray, and it is equipped with a visible light sensor. Based on the information given by this sensor, the pilot raises or lowers to a suitable flight altitude to make the bottom color of the aircraft. It is basically consistent with the sky background. There are also warships that use the same principle to become invisible. According to their actual conditions, warships of various countries will use off-white and dark gray. Similarly, the camouflage uniforms worn by the soldiers are also made with colors close to the environment, which has the effect of blending with the surrounding environment to a certain extent to reduce visibility.

The secret to countering invisibility

  In 1991, the F-117A stealth fighter, which was unparalleled in the sky over Iraq, was shot down by the Yugoslavian old Sam-3 missile over Eastern Europe in 1999, which directly led to the bleak end of the F-117A’s disastrous retirement shortly after its debut. Why has the F-117A’s super stealth ability shattered? Let’s take a closer look at the technical level of anti-stealth.
  Anti-stealth technology mainly includes radar anti-stealth technology and passive detection technology. There are two main types of the former: one is to suppress the target’s stealth technology, to improve the target’s radar cross section, such as the use of resonant radar technology; the other is to improve the radar detection capability, such as increasing the radar radiation power to enhance the strength of the echo signal.
  The anti-stealth ability of F-117A’s natural enemy VHF radar comes from its longer wavelength. When the incident wavelength is close to the target size, the reflected wave will resonate with other traveling waves (such as diffracted waves) on the target. , Resulting in a strong echo signal. The wavelength of the VHF radar is 1.65 to 1.90 meters. At this time, the main components on the aircraft may produce electromagnetic resonance. The longer the wavelength, the more difficult it is to use the coated absorbing material to achieve stealth.
  VHF radars appeared during the Cold War, when the Soviet Union had already developed the P-14 “Tall King” and P-12/P-18B/D radars (code-named “key holder”). It was the Soviet Union’s P-12/18 series radar and the SNR-125 “Low Blow” digital upgrade radar that the Yugoslavia discovered and destroyed the US F-117A. At present, Russia is the country with the most developed VHF radar technology in the world.
  However, VHF radar also has many deficiencies, such as poor sensitivity, poor performance of clutter echo suppression when tracking low-altitude targets, and susceptibility to interference from signals such as TV, FM or portable radio. The most powerful measure to deal with VHF radar is to eliminate the smaller parts in appearance. For example, the smallest component sizes on the B-2 bomber are measured in meters, and the most critical parts of the fuselage are made of thick-section multilayer stealth materials.
  The combination of long and short is always suitable. Double base (DB) radar, as the name implies, is that the radar’s transmitting and receiving are not at the same place. Even if the stealth aircraft changes the reflection angle through the shape design to cause the reflected wave to deviate from the transmitting source, it is still unavoidable that the echo will be received by the dual-base radar.
  Modern stealth aircraft are aimed at conventional centimeter-wave radars operating in the 1-20 GHz frequency band, but they are powerless in the face of meter-wave and millimeter-wave radars. The working principle of the meter wave radar is that when the wavelength is close to the target size, resonance will occur, thereby increasing the intensity of the echo, but the disadvantage is that the clutter interference is large and the detection accuracy is low. It is mainly used for warning, but the operating distance is long. Because of the short wavelength of millimeter wave radar, when irradiated on the surface of the target, any uneven part on the target will act as a reflector, forming strong electromagnetic reflection, resulting in an increase in the radar cross-section, which makes the stealth performance great. discount. Millimeter-wave radar has the advantages of high resolution, strong anti-interference ability, and good low-altitude performance. It is very suitable for detecting some small targets. It can provide more accurate information on the target’s speed, distance, and angle, but the disadvantage is that the operating range is short.
  If the long-wave radar and the short-wave radar are combined to form a dual-base radar, the detection capability of the radar can be greatly improved. But the dual-base radar is not always effective, because the plane is flying in the air and the reflection direction of the radar wave is changing at any time, so the signal obtained on the radar is always flickering. Currently, Russia has successfully developed a giant radar antenna array.
  The clairvoyance of clairvoyance The over-the-horizon (OTH) radar is also called “over-horizon radar”, and its detection frequency is completely outside the range of the standard radar band. It usually works in the HF frequency band with a wavelength of 5 to 150 meters. Using the antenna propagation mechanism, it can detect targets with a distance of 900 to 4000 kilometers. It can overcome the limitation of the earth’s radius of curvature, and realize the detection and tracking beyond the horizon by reflecting the target signal from the earth’s ionosphere. Radar waves reflected by the ionosphere can detect targets at a distance of about 5,000 kilometers. There are generally two types of over-the-horizon radars: one is to use ionospheric reflection and is called “sky-wave over-the-horizon radar”; the other is to use ground diffraction, which is called “ground-wave over-the-horizon radar.”
  Another important function of over-the-horizon radar is that it can deal with aircraft with absorbing coatings. Its shortcomings mainly include low accuracy, only the approximate azimuth information of the target can be obtained, in addition, the radar system is very large, there are many interference factors, and it is greatly affected by climate change. In the medium wave and short wave bands, the spectrum is crowded.

Schematic diagram of over-the-horizon radar

  Scanner above the Nine Sky Space-based radar, also known as “spaceborne radar” or “space radar.” It is set on a synchronous satellite with an altitude of 3,600 kilometers. The principle is to use an antenna system with a diameter of about 30 meters to radiate the transmission power provided by solar energy to the ground, and then the ground antenna array to receive the signal of the moving target, forming a large area Scope of search.
  The working frequency bands of space-based radars mainly include L, C, and X. It has the advantages of multiple functions, high resolution, all-weather and global coverage, and can obtain real-time information of static and dynamic targets and accurate three-dimensional images. In recent years, countries such as the United States, Japan, France, Germany, the United Kingdom, India, and China have all been developing space-based radar systems.
  Silent Monitor Passive detection technology has been listed by the United States as the most effective anti-stealth method. Its biggest feature is that it does not emit electromagnetic waves, but instead relies on passively receiving electromagnetic waves radiated by the target to discover and track the target. In the process of penetration, the stealth aircraft must use its own radar to radiate electromagnetic waves in order to search for targets, navigate and locate, command and contact, etc., so that it can be detected by passive radars. Passive detection technology has a high degree of concealment, wide adaptability, wide time domain, and accurate target recognition characteristics. It can receive enough information from the opponent’s radar to identify and locate it. In addition to being affected by heavy rain, passive detection technology is basically not affected by other climatic conditions and battlefield smoke and dust, and can work all-weather and all-day.
  Stealth and anti-stealth technologies, this pair of “spears and shields”, have continuously improved over the course of decades of offensive and defensive development. The development of stealth and anti-stealth technologies mutually restrict and promote each other. No matter which side has a new breakthrough, it will cause major changes in the other side. Countries around the world will also vigorously research and develop stealth and anti-stealth technologies, not only in the military field, but also in other fields to facilitate people’s lives.

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