They say that some planes and some submarines are stealthy: they remain invisible on enemy radars. Not all objects can become invisible, so don't hope to add stealth mode to your car to avoid police radar, it's not the same. To understand aircraft stealth mode, let's start by learning how radar works, because that's what a stealth aircraft tends to hide from. Radar Radar, in aviation terms, is a device that detects what is in its field of detection using radio waves. The term "Radar" comes from the acronym for RAdio Detection And Ranging, which means "detecting and estimating distance by radio waves." Just as a flash camera can pick up light reflected from objects, radar uses radio waves on the same principle. First, the radar will emit an electromagnetic wave toward the sky (in case we want to detect airplanes). This wave will propagate, hit the aircraft and be reflected by the aircraft. If part of this reflection, called a radar echo, passes through the radar antenna, the latter detects the aircraft and can display information on the screen:
Not all aircraft are the same size, so each pattern of aircraft has a wave signature that is peculiar to it. Thus, radar can be used to tell if an airliner, a small glider or a fighter jet is involved. By analyzing the time it takes for the wave reflection to return to the radar, you can find out how far away the aircraft is. By repeating the operation you can find out its direction and speed. Hidden mode If we do not want the radar to detect us, we should not return to it the reflection of the wave it sends you. There are several solutions for this: Be transparent to the waves: the wave passes through you without being reflected; Send the wave in a different direction from where it came from: it will not go to the radar; Absorb the wave: the wave will not be reflected; Send an identical wave, but 180° out of phase so that the incident wave and the reflected wave overwhelm each other. As for transparency: usually (any) waves are reflected from objects whose size approaches the wavelength of the emitted wave. To detect something large, a large wavelength is enough. To detect a bird or even rain, you need rather short wavelengths. All objects, of any material, can reflect electromagnetic waves, and can therefore become visible on some radars. Needless to say, under these conditions, it becomes difficult to create an aircraft that is invisible to all waves: it is enough for the enemy to use a wave that fits the size of the aircraft to be sure that he has an echo radar. So that leaves three other solutions: deflect the wave, absorb it, or cancel it with another wave. Many of these methods are usually used simultaneously. Already, stealth aircraft have a very peculiar geometry. This geometry is here precisely to reflect the wave elsewhere than where it comes from. In some cases, the radar wave also hits the angular protrusions of the aircraft, and the cockpit ends up dissipating the energy completely:
Stealth aircraft are coated (usually directly with paint) to absorb radio waves. This coating is designed to absorb a wide range of frequencies, making the aircraft invisible to many different radars. This coating can then absorb the wave chemically or physically. In the first case, the wave turns into heat because it reacts with the metal inclusions in the paint. Metals, and especially metals with high electrical permeability or magnetic permeability (e.g., Permalloy or Mu-metal) absorb electromagnetic waves very well. In the second case, the wave is suppressed by the thin film interference effect due to the thickness of the coating. In the latter case, the received wave is reflected 50% by the coating and 50% by the cab below, ideally with a half phase wavelength phase shift: so the two reflected waves compensate for each other. The disadvantage is that this only works well for one particular wavelength. In addition to these passive methods, there are also active wave jammers. This means that the aircraft will emit its own waves, or cancel out the waves it receives by emitting waves offset by 180°, which will then cancel out the radar wave. Here the aircraft has a sensor that detects the nature of the incident radar signal and then emits a signal that cancels it out. Finally, some aircraft can release anti-radar decoys, a bit like thermal decoys, but for radar and radar-guided missiles. The radars then detect thousands of different signatures and no longer know which one is the real plane.
In conclusion about stealth mode It should be noted that it is materially impossible to be completely transparent to all radar waves, mainly because there is no material that absorbs or reflects the entire electromagnetic spectral range. Then part of the wave will always be absorbed by the aircraft, which means that the wave turns into heat, and the aircraft gets a little warm. What the aircraft gains invisibility on radar, it loses in thermal signature and on thermal imaging cameras, which also equip enemy defense devices. Rather, the idea of stealth mode is to delay the enemy's response. If a plane manages to pass behind a bird or a cloud on the radar screen, it will not raise an alarm: the result will be positive for the plane, even if it is only a few minutes away from detection. It is the same for submarines. It should be noted that in the water, noise should also be taken into account: in addition to radars, sonars are used. The principle is the same, just we use mechanical waves such as sound, and plus radio waves (which are very bad in water).
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