J-20... The New Generation Fighter II
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Are you kidding? That has FAKE written all over it.
Anton Gregori
New Member
Re: Can a F-22 detect a J-20 beyond visual range? Probably not.
I can't comment on the rest of your post - don't know enough about the tech. But the amount of energy absorbed by the atmosphere will be a percentage of the total signal, not a constant number. So if the atmosphere absorbs 1% of the energy of a strong radar echo, it will also absorb 1% of the energy of a weak echo.
You're imagining the atmosphere as a barrier that requires a certain force to punch through, but a better way to think of it is that the signal consists of a certain number of photons - strong signals have a lot of photons, weak signals have a fewer. Each photon has the same (and independent) chance of being absorbed by the atmosphere.
The net effect is that the atmosphere really shouldn't matter all that much.
This also affects the scanning idea - of course you can scan a narrow beam rather than wide beam in the hope of generating a stronger signal. But if you scan a narrow beam quickly enough that it covers the same space as the wide beam, you're not going to get any more photons back. The only way you could get more signal is to dwell the beam on the same spot, but then you wouldn't be able to scan the same area.
No free lunch, I'm afraid.
I can't comment on the rest of your post - don't know enough about the tech. But the amount of energy absorbed by the atmosphere will be a percentage of the total signal, not a constant number. So if the atmosphere absorbs 1% of the energy of a strong radar echo, it will also absorb 1% of the energy of a weak echo.
You're imagining the atmosphere as a barrier that requires a certain force to punch through, but a better way to think of it is that the signal consists of a certain number of photons - strong signals have a lot of photons, weak signals have a fewer. Each photon has the same (and independent) chance of being absorbed by the atmosphere.
The net effect is that the atmosphere really shouldn't matter all that much.
This also affects the scanning idea - of course you can scan a narrow beam rather than wide beam in the hope of generating a stronger signal. But if you scan a narrow beam quickly enough that it covers the same space as the wide beam, you're not going to get any more photons back. The only way you could get more signal is to dwell the beam on the same spot, but then you wouldn't be able to scan the same area.
No free lunch, I'm afraid.
Martian
Senior Member
Re: Can a F-22 detect a J-20 beyond visual range? Probably not.
When a signal is excessively weak, I believe that you're incorrect. Let's assume that water droplets are present in the atmosphere. Each droplet of water can absorb a specific quantity of energy before becoming water vapor and drifting away.
Unless the radar energy beam contains sufficient energy to vaporize most or all of the intervening water droplets, the signal won't return to the transmitter. The other problem is that a weak signal that requires over 10 decimal places to quantify will be difficult to distinguish from the background noise. Another unknown is the sensitivity of military radars. We do not know the cut-off threshold when the receiver is incapable of detecting a signal.
We also don't know if a bird, hail, insect, or decoy (e.g. metal chaff or something more sophisticated) will give a stronger false-positive signal to mask the presence of a J-20 or F-22.
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You're wrong about the intensity. The formula for intensity is:
You can increase the intensity of the radar beam and likelihood of detection by increasing the power or decreasing the area being illuminated at one time. You already know that this is true. When you increase the power to a flashlight, it becomes much brighter and you can see objects more clearly.
When a signal is excessively weak, I believe that you're incorrect. Let's assume that water droplets are present in the atmosphere. Each droplet of water can absorb a specific quantity of energy before becoming water vapor and drifting away.
Unless the radar energy beam contains sufficient energy to vaporize most or all of the intervening water droplets, the signal won't return to the transmitter. The other problem is that a weak signal that requires over 10 decimal places to quantify will be difficult to distinguish from the background noise. Another unknown is the sensitivity of military radars. We do not know the cut-off threshold when the receiver is incapable of detecting a signal.
We also don't know if a bird, hail, insect, or decoy (e.g. metal chaff or something more sophisticated) will give a stronger false-positive signal to mask the presence of a J-20 or F-22.
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You're wrong about the intensity. The formula for intensity is:
You can increase the intensity of the radar beam and likelihood of detection by increasing the power or decreasing the area being illuminated at one time. You already know that this is true. When you increase the power to a flashlight, it becomes much brighter and you can see objects more clearly.
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cataphract
New Member
Re: Revised final estimate for J-20 canards' radar return energy is 1.035 x 10^-17
No offense but when you apply the amount of EM reduction from the claimed RAM figure, then virtually no shape other than perhaps corner reflectors would amount to anything detectable.
E.g., to start it simple, a metal ball(1m radius) has an rcs of 1m^2, if you apply ram, taking away even 99.684% of the EM reflected energy, you'll get an RCS of 0.00316m^2, that is on par with the F-35, with a what is possibly the worst stealth shape anyone can make(sphere).
No offense but when you apply the amount of EM reduction from the claimed RAM figure, then virtually no shape other than perhaps corner reflectors would amount to anything detectable.
E.g., to start it simple, a metal ball(1m radius) has an rcs of 1m^2, if you apply ram, taking away even 99.684% of the EM reflected energy, you'll get an RCS of 0.00316m^2, that is on par with the F-35, with a what is possibly the worst stealth shape anyone can make(sphere).
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Maggern
Junior Member
Re: Revised final estimate for J-20 canards' radar return energy is 1.035 x 10^-17
Though I by no means control the math in this area, I did take a medium course in physics, and I agree that when we're talking about electromagnetic radiation, atmospheric interference is practically close to zero. Perhaps if you're looking through a severe thunderstorm or a hurricane, it will have noticeable effects. And I feel that when we're talking almost down to quantum level, the energy of a radar would, even if miniscule, persist in some form throughout the atmosphere (even if almost unreadable).
Now what is really the point in this discussion about atmospheric absorption rates?
Though I by no means control the math in this area, I did take a medium course in physics, and I agree that when we're talking about electromagnetic radiation, atmospheric interference is practically close to zero. Perhaps if you're looking through a severe thunderstorm or a hurricane, it will have noticeable effects. And I feel that when we're talking almost down to quantum level, the energy of a radar would, even if miniscule, persist in some form throughout the atmosphere (even if almost unreadable).
Now what is really the point in this discussion about atmospheric absorption rates?
Re: Revised final estimate for J-20 canards' radar return energy is 1.035 x 10^-17
Regarding attenuation (loss), at the popular X-band employed by air traffic controllers and most military systems, rain drops are considered 'volumetric' targets, meaning the X-band system will qualify a meteorological phenomenon call a 'rain' mass but not a single rain drop. What this mean is that the interactions between reflections off individual rain drops are 'constructive interference' and that summation/amplification is what give the scope an area that we call 'rain'.
Another 'volumetric' target is vegetation. A good example is the F-111 terrain-following-radar (TFR). The system in 'hard TF' flight and at minimum safe altitude have collided with trees on hilltops. The TFR's operating freq could not distinguish vegetations on hilltops but can fully detect the hill itself regarding its slopes and commanded pitch accordingly. As it crested the hilltop in a pitch curve intended to keep the aircraft below the enemy's radar horizon, the aircraft will not see those trees and will brush them. I have seen those evidences myself during my years stationed at RAF Upper Heyford.
Another 'volumetric' target is an insect mass. Most radar systems will not detect individual bugs but will detect the cloud or mass the insects created. If individual bugs begins to drop from the mass, eventually the mass will disappeared off the scope.
Another 'volumetric' target is a bird formation. But avian targets are unique in that quite often we can detect individual birds as well as the formation as a unit and more sensitive systems can display both when selected. The reason why avian targets are unique are because of their hard and geometric beaks. So it is not difficult to see why the pelican would be the largest avian target of all birds.
Why beaks but not their bodies...???
Just something to complicate the discussion.
Incorrect. Atmospheric attenuation is present at at all times. The degree of loss depends on the freq employed. It may sound counter-intuitive, but we can look at that 'loss' is how we detect objects. A 'loss' came from direct interference, meaning the wave collided with a physical object, be it a rain drop, a bird, a leaf, or an aircraft. The finer the freq employed, meaning is the wavelength metric (HF,VHF, UHF), or finer as in centimetric and millimetric, the finer the target resolutions, such as speed, altitude, and heading. The transmission method is also important, such as pulsed or continuous wave (CW).
Regarding attenuation (loss), at the popular X-band employed by air traffic controllers and most military systems, rain drops are considered 'volumetric' targets, meaning the X-band system will qualify a meteorological phenomenon call a 'rain' mass but not a single rain drop. What this mean is that the interactions between reflections off individual rain drops are 'constructive interference' and that summation/amplification is what give the scope an area that we call 'rain'.
Another 'volumetric' target is vegetation. A good example is the F-111 terrain-following-radar (TFR). The system in 'hard TF' flight and at minimum safe altitude have collided with trees on hilltops. The TFR's operating freq could not distinguish vegetations on hilltops but can fully detect the hill itself regarding its slopes and commanded pitch accordingly. As it crested the hilltop in a pitch curve intended to keep the aircraft below the enemy's radar horizon, the aircraft will not see those trees and will brush them. I have seen those evidences myself during my years stationed at RAF Upper Heyford.
Another 'volumetric' target is an insect mass. Most radar systems will not detect individual bugs but will detect the cloud or mass the insects created. If individual bugs begins to drop from the mass, eventually the mass will disappeared off the scope.
Another 'volumetric' target is a bird formation. But avian targets are unique in that quite often we can detect individual birds as well as the formation as a unit and more sensitive systems can display both when selected. The reason why avian targets are unique are because of their hard and geometric beaks. So it is not difficult to see why the pelican would be the largest avian target of all birds.
Why beaks but not their bodies...???
Aside from the bird's body, its feathers and the layers are natural absorbers. Feathers are essentially 'hair'. Various grades of hairs, from human to animal, have been known for decades for their performance as radar absorbers.
Just something to complicate the discussion.
Engineer
Major
I have done link-budget calculation for satellites, and when I look at Martian's calculation, bells are ringing in my head that indicate something is wrong. I didn't have time to go through his steps in details, but the problem which stands out the most is the way he made assumption; it's too superficial. For example, the way he claims how canards reflect radar signal is only his assumption. Reality may be quite different and we have no way to verify his assumption. Another problem is the final result; we still don't have any estimate of the RCS, and we can't gauge whether Martian's methodology is correct because we can't say "oh, this is close to the publicly claimed RCS of the F-22, so the value is in the ballpark". Furthermore, so what if the returned signal is in nanowatts? If the noise is a few order of mangitudes lower, then the signal is still detectable, hence the plane is still detectable. What you actually want is the signal-to-noise ratio, because the absolute power of the return signal is completely meaningless.
during the mid-70's US professor come up with "Track-before-detection" ,design to track very weak signal return.
Israel G-550 AWACS already incorporate TBD algorithms design to track cruise missile.
other solution was third gen. IRST.latest gen. of fighter aircraft,from mig-35 to F-35 incorporate DAS,since third gen. of focal plane array could detect even the "cooler" iR band.this allow the pilot to detect LO or stealth aircraft.
Israel G-550 AWACS already incorporate TBD algorithms design to track cruise missile.
other solution was third gen. IRST.latest gen. of fighter aircraft,from mig-35 to F-35 incorporate DAS,since third gen. of focal plane array could detect even the "cooler" iR band.this allow the pilot to detect LO or stealth aircraft.
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Anton Gregori
New Member
Re: Can a F-22 detect a J-20 beyond visual range? Probably not.
Egads, no! Even an incredibly powerful radar is not vapourizing water droplets! Never mind that most of the water is already in the form of vapour anyway - you're not moving the water droplets either.
Photons interact with matter at the atomic level, and at that level even solid objects are mostly empty space. Shooting a photon through the atmosphere is sort of like shooting a spaceship through the solar system. You will occasionally hit something, but you have to be pretty unlucky.
This doesn't change the basic problem that your sensor needs to collect a certain number of signal photons to distinguish the signal from the background noise (as you just alluded to). I already conceded that you will get more photons back from the target if you dwell the narrower bream on the same spot. But if you're searching, then the only way to do this is to search less space.
A different way of thinking about it: your formula is correct - you can increase the intensity by decreasing the area illuminated. But decreasing the area illuminated implies searching in a smaller area (by definition - the sentence even sounds a little silly because this is a truism). You can't get out of this by scanning the beam because you're then spreading out your photons again, the same as if you had a wider beam to begin with.
I'm not sure if this makes things clearer or just confuses them more, but you should also consider that wide beams are still scanning. They're just able to take in more area in a single sweep. You need the scanning behaviour because your receptors aren't able to focus all that tightly - you wouldn't be able to tell where the return signal was coming from if the outgoing signal didn't scan.
Egads, no! Even an incredibly powerful radar is not vapourizing water droplets! Never mind that most of the water is already in the form of vapour anyway - you're not moving the water droplets either.
Photons interact with matter at the atomic level, and at that level even solid objects are mostly empty space. Shooting a photon through the atmosphere is sort of like shooting a spaceship through the solar system. You will occasionally hit something, but you have to be pretty unlucky.
This doesn't change the basic problem that your sensor needs to collect a certain number of signal photons to distinguish the signal from the background noise (as you just alluded to). I already conceded that you will get more photons back from the target if you dwell the narrower bream on the same spot. But if you're searching, then the only way to do this is to search less space.
A different way of thinking about it: your formula is correct - you can increase the intensity by decreasing the area illuminated. But decreasing the area illuminated implies searching in a smaller area (by definition - the sentence even sounds a little silly because this is a truism). You can't get out of this by scanning the beam because you're then spreading out your photons again, the same as if you had a wider beam to begin with.
I'm not sure if this makes things clearer or just confuses them more, but you should also consider that wide beams are still scanning. They're just able to take in more area in a single sweep. You need the scanning behaviour because your receptors aren't able to focus all that tightly - you wouldn't be able to tell where the return signal was coming from if the outgoing signal didn't scan.
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