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From the internet or any other medium of information I came to know that some cutting edge stealth aircraft have radar cross sections of a bird (or something similarly small).

I don't have the in depth technical know how of how modern radars, air surveillance and detection systems work (obviously). But judging from what they are required to be, here is my conclusive query:

If you want to detect an enemy aircraft (either using a centralized monitoring system or in an anti aircraft missile), you want the detection in the 3D cartesian (x, y, z) axis location of it. If that is possible, and an aircraft with a bird size of RCS, is seen as a bird or small object on radar, still a small object at heights of 30,000 - 40,000 feet moving at a speed of near Mach 1 or higher Mach numbers, would not that be something very obviously alarming?

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    $\begingroup$ Gotta love Heisenberg's Uncertainty Principal. Radars are a perfect example of this coming into play. Radars cant perfectly see both location and speed. The more accurate they are on one, the less accurate they are on the other. A smaller RCS dramatically complicates this. $\endgroup$
    – David S
    Oct 27 at 16:46
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    $\begingroup$ @DavidS Err, what? How do you work that out? A slow sweep-rate will affect both; a faster sweep will surely make both readings more precise? $\endgroup$ Oct 28 at 15:10
  • $\begingroup$ @MikeBrockington Its a balancing act among multiple things. Too fast of sweep and you lose accuracy on speed readings. Think of a single pulse. One ping only. Great for distance and location. But that second ping is needed to update location, or obtain speed. $\endgroup$
    – David S
    Oct 28 at 15:38
  • $\begingroup$ @DavidS I think, modern systems must be having redundant but optimized systems to compensate for these difficulties. $\endgroup$
    – lousycoder
    Oct 30 at 8:08
  • $\begingroup$ @DavidS - Why don't you think radars can't "perfectly see both location and speed"? Modern day pulse-doppler radars can do both, and on a single pulse if the SNR is good enough. $\endgroup$
    – SteveSh
    Oct 30 at 18:05
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Its all about detection distance.

Take a car and a beer can. You're 10 miles away, on an open plain. From there, you can't see either. Get closer...5 miles. Still can't see either.

At 3 miles, you can see the car, but not the beer can. So there, you can shoot or avoid the car.

You can't see the beer can until you are 100 meters away. But the beer can is actually the same physical size of the car, and just as deadly.

A smaller radar signature gives me time to get closer, without being detected.

To bring this out to the real world, B-52 and B-2. The radar signature of the B-52 is B-52 size. The B-2, bird size. Much much smaller.

So... You have radar sites all around your border. A B-52 is flying toward you. You can detect him 500 miles out.

Scramble your interceptors. They find him 200 miles out, and either convince him to go away or shoot him down.

The B-2, however...you might not detect him until he is only 5 miles out.

By the time you scramble your interceptors and they find him, he is already inside your perimeter, doing unspeakable things to your countryside.

Detection distance.

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    $\begingroup$ @WayneConrad how about: "but - SURPRISE - the beer can turns out to be the same size as the car, despite how it looks" $\endgroup$
    – Chris H
    Oct 26 at 9:34
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    $\begingroup$ A bit on the beer can/car analogy. This holds assuming both have the same reflectivity. But what if they don't? Assume they're in the desert. If the car is painted desert camouflage, while the beer can has a mirror-like finish, then you may be able to detect the beer can at a greater distance than the car. This is what RCS is all about - minimizing the returns (reflections) back to the radar site. And the size of the object is only one part of that. $\endgroup$
    – SteveSh
    Oct 26 at 10:53
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    $\begingroup$ @SteveSh you can take the analogy further - your shiny can helps only if it catches the sunlight, or if facing directly towards you and you shine a light on it in the dark (that last being basically the same as radar of course) $\endgroup$
    – Chris H
    Oct 26 at 11:25
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    $\begingroup$ @SteveSh that kind of thing is what is being measured by "cross-section". If the car is as easy to see as a beer can then it has "the cross-section of a beer can". Doesn't matter whether you achieved that with special paint, special geometry, or a slightly torn invisibility cloak $\endgroup$
    – user253751
    Oct 26 at 11:37
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    $\begingroup$ @WayneConrad - Instead of car+beer can, consider 2x pickup trucks, with a .50 CAL mounted in the bed. We sprinkle some magic dust on one of them, known as Stealth Paint, and it then appears to the naked eye as the size of a beer can. The gun is still just as deadly. But you are in range of its gun, long before you can see it. $\endgroup$
    – WPNSGuy
    Oct 26 at 21:48
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Besides the problem of whether you can see it at all far enough away to have time to do something about it, you also have the problem that if you do build a radar capable of spotting bird-sized objects at 100 miles, then you get to see every bird-sized object within 100 miles of you on your radar screen. And there are lots of birds in the world!

Every time the radar sweeps round you get zillions of dots, and then it sweeps round again a few seconds later, and you get zillions of dots again in different places, and you have to figure out which pairs of dots are from the same object, moving in some unknown direction, and which are different. Over and over again, with every sweep. Join-the-dots is fairly easy if you only have a dozen big chunky dots on the screen and they're well spaced out. The more sensitive you make the radar, the more dots there are, the more closely spaced they are, and the harder it is to tell if they're the same thing. An aircraft moving at 330 m/s might be half a mile away by the time the radar beam sweeps round again. How many birds, reflections off moving ocean waves, or waving tree branches, glint off buildings, mountains, cliffs, etc., can you see within half a mile around each and every dot?

The more sensitive you are forced to make your radar, the harder it is to process all the zillions of tiny objects you can now see, and make sure none of them is an approaching threat. And the more reliable you have to make your processing. If you have a hundred radar tracks, and 0.01% error rate per sweep on each track (99.99% correct sounds pretty good, right?), you would get about 1 false alarm every couple of minutes. If you have ten thousand radar tracks, you would get 100 false alarms every couple of minutes!

Making the radar sensitive enough that it can see birds at very long range makes the radar's job much, much harder.

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  • $\begingroup$ Well, this only applies when Doppler shift of the reflected signal is in the range you would get from the bird, which is sometimes, but not always. Same for the altitude, I think. $\endgroup$ Oct 26 at 6:38
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    $\begingroup$ @EugeneRyabtsev but Doppler radar has its own downsides - you need to dwell on an area of sky for longer and getting the frequency of every reflected signal still isn't easy when they're close together. The Doppler effect can be small, even zero if the target is moving tangentially $\endgroup$
    – Chris H
    Oct 26 at 9:38
  • $\begingroup$ "Making the radar sensitive enough that it can see birds at very long range makes the radar's job much, much harder." No, that's not entirely true. There are signal processing and filtering techniques that can be used to remove the returns from thousands of birds, without dialing down the sensitivity of the radar system. I can't think of any military radars that deliberately compromise sensitivity, except in very specific situations. $\endgroup$
    – SteveSh
    Oct 26 at 11:55
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    $\begingroup$ @SteveSh signal processing is great, but this answer already points out that it is harder than not having to do it at all. $\endgroup$ Oct 26 at 12:12
  • $\begingroup$ @MikeBrockington This gives a rather ancient notion of how a military radar works. If it was just about connecting the dots, ECM would be so much more effective. $\endgroup$ Oct 26 at 12:26
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A radar's antenna sends out a radio signal that is reflected off of the target. That reflection, the "radar return," is received by the radar's antenna. The radar has a certain minimum amount of energy it must receive before it can register the return. Returns that carry too little energy won't be registered. They won't show up on the scope.

That means that there are a couple of ways to keep your airplane from showing up on the radar. One is that you can be too far away. As an airplane gets farther away, less radar energy gets to it to be reflected back to the radar. And, the less of that return is received by the radar's antenna. The radar signal is attenuated by the inverse square law twice, so the energy of the radar return is inversely proportional to the 4th power of the distance. If my airplane gets far enough away, the radar doesn't see it.

Another way to keep the airplane from showing up on radar is by having a lower radar cross section. The lower the radar cross section, the less the radar signal is reflected by the plane.

If I can make my plane's radar cross section be very small, then I can get much closer to the radar before I am seen than if my radar cross section was large.

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Even if two objects, say a bird and a plane, have the same RCS and so can be detected at similar ranges, there are ways to distinguish between the two of them by the way they interact with or reflect the radar signal back to the radar receiver.

Modern day radars can not only distinguish between a bird and a plane, but in many cases can distinguish between types of planes.

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    $\begingroup$ Fortunately, they can distinguish between a bird and a plane, but can they identify Superman? $\endgroup$
    – FreeMan
    Oct 25 at 16:48
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    $\begingroup$ Yes! By the flapping of his cape ;-). $\endgroup$
    – SteveSh
    Oct 25 at 16:57
  • $\begingroup$ One more reason why capes are a serious safety issue. No capes! $\endgroup$
    – Jason
    Oct 26 at 6:18
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    $\begingroup$ I would hope that modern day radars have the option to filter out a Superman return, because other than the novelty of seeing Superman, there isn't anything you can do about it. Unless you have Kryptonite. $\endgroup$
    – Mohair
    Oct 26 at 15:37
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Your question is valid and you are right to suspect that in many situations having a reduced RCS is not useful. Speaking as someone with experience in military radar research I can describe some of the basic factors:

(1) Having a reduced RCS simply increases the chance an aircraft might go undetected and does not eliminate the possibility of detection.

(2) Reducing RCS is very effective against intercepting aircraft because such aircraft have weak radars that operate head on. This makes it difficult for interceptors to find your bomber or to target it with air-to-air missiles.

(3) Reducing RCS is effective against weak ground based radars and reduces the range at which they can detect the bomber. This makes it a lot easier for a bomber to penetrate the borders of a country without being noticed.

(4) If the bomber is operating in a limited area that has air defenses or in an area where the enemy expects the bomber to pass, then the reduced RCS is more or less useless. We saw the effects of this in the 1999 F-117 shootdown.

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  • $\begingroup$ That is an interesting incident and an informative article! But, how can it be concluded that RCS is useless here? The wiki doesn't share any info on how precisely were they able to pin point the plane and take it out? Are there any similar incidences involving modern 4th or 5th gen stealth aircrafts? $\endgroup$
    – lousycoder
    Oct 31 at 19:41
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    $\begingroup$ @lousycoder In that situation the lowered RCS was useless because the Serbs knew the path the bomber would take and they also knew approximately when it would arrive because they had spies in Aviano that watched it take off. So, when the aircraft arrived in the designated defense zone they just turned on their radar and shot it down. The fact that it had a lowered RCS didn't matter because once it is close enough to the radar it can still be detected. $\endgroup$ Oct 31 at 20:34

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