Radar Cross Section

[Token]

Firstly: Orthogonal means at a right angle to the surface, correct?

Yes, square to the face and on axis.

Secondly: X-band seems to produce a greatly larger reflection: Do most modern air-to-air and surface-to-air radars use X-band, or other, provided it's not classified?

Higher frequency = larger RCS, it is not specific to X band. Many military air-to-air and surface-to-air radars are indeed in X-band, but there is a long series of events that has resulted in that.

So the optical region for a 12" diameter sphere would be 75.4"?

The optical region is defined by the frequency of the signal in comparison to the size of the sphere, signal wavelength to maximum dimension. So it would be defined in frequency.

What does "running rabbits" in the radar returns mean?

Running rabbits are asynchronous interference being shown on the radar display. Interference synchronized in time will be fixed and not moving (in range, range is time to a radar) on the display, interference that is not in sync will move on the display.

Wait, I'm confused. I thought the Mie range was where the wavelength was 1.5 x surface-area. This looks to be span/length x 1.5.

The Rayleigh, Mie, and Optical regions are not defined by area. I misspoke when I said "area", I meant "diameter" (I'll go back and correct that post after I post this one). And yes, span / length is the important part of what I said in that example of aircraft.

Fascinating

The B-26 designation confuses me because it was assigned to two different aircraft. Is this the B-26 Marauder or the B-26 Invader?

In this case it is almost certainly referring to the B-26 Marauder.

I believe P PFVA63 is quoting from Skolniks "Introduction to Radar Systems" (page 40, figure 2.16) with that polar plot (the top / first polar plot, not the smoothed lower polar plot). However, several, many even, sources have used that plot and data. I think the earliest publication I have seen using that plot/data is the MIT Radiation Laboratory Series, Volume 1 (page 76, figure 3.8), published in 1947. And that document quotes Radiation Laboratory Report No 931 (April 8, 1946), and No 914 (March 28, 1946) as the source. So I am pretty sure that data is from 1946.

I think the A-26 designation was not change to B-26 until 1948.

T!

 

[ThomasP]

From at least the late-1960s thru the early 2020s pretty much all of the major US fighter aircraft radars were X-band.
F-4J AWG-10(APG-59)
F-4S AWG-10A(APG-59)
F-14A AWG-9
F-14D APG-71
F-15A APG-63
F-15E APG-82
F-16A APG-66
F-16C APG-68
F-18A APG-65
F-18C APG-73
F-18E APG-79
F-22 APG-77 (X-band or X-band adjacent - ie H/I/J-bands)
F-35 APG-81 (X-band or X-band adjacent - ie H/I/J-bands)

NOTE You will sometimes see other designations for the X-band - ie NATO H-band and I-band

 

[Token]

From at least the late-1960s thru the early 2020s pretty much all of the major US fighter aircraft radars were X-band.
<snip>
F-22 APG-77 (X-band or X-band adjacent - ie H/I/J-bands)
F-35 APG-81 (X-band or X-band adjacent - ie H/I/J-bands)

NOTE You will sometimes see other designations for the X-band - ie NATO H-band and I-band

For those not familiar, there are multiple designation systems for frequency ranges. Of the several different systems used to discuss ranges of frequencies, especially when radar is involved, there are three primary schemes. The ITU Band designations, the IEEE Radar Band designations, and the ESM / NATO Band designations.

The ITU designations are as follows:

The IEEE Radar Band designations are here:

And the ESM / NATO Band designations are here:

You can see that there are some conflicts between these systems. For example, the IEEE and ITU systems both have "UHF", but define the upper limit differently. The IEEE and the ESM / NATO systems share the band designators S and C, but define each totally differently.

Yes, it can be confusing at times, but within specific communities they tend to stick to one or the other system (unless you feel like messing with someone), so it works out within each community. The ITU system is mostly used for communities outside radar, while it sometimes shows up in radar stuff it is very uncommon. Comms guys working in the microwave region and Radar guys who are not doing military radar tend to stick to the IEEE system. Radar guys who are doing mil radars end up using both IEEE and ESM, depending on who they are talking to. Within the military or in the ESM / ECM / ELINT communities they tend to use the ESM designations.

"X band" is a term that is part of what is now called the IEEE Radar Band designations, and covers the frequency range of 8 - 12 GHz. You can see that this is the I and lower part of J band in the ESM / NATO system. Modern radars, especially AESA's (Active Electronically Scanned Arrays) tend to be wider banded. That leads to @ThomasP comment about the bandwidth of the APG-77 and 81, and "H/I/J bands", this basically implies those radars can each work from below 8 GHz to above 12 GHz..

You might notice that the ITU and ESM bands seem structured and predictable, while the IEEE band designators are somewhat chaotic. This is on purpose.

What we now call the IEEE Radar Bands were originally defined during WW II, as the "Radar Bands", and were initially classified. They did not use sequential letters for the bands so that their relationship to each other could not be guessed. It was not until some time after WW II (I think in the mid 1950's, but not sure) that this system was declassified, and (I have been told) that only because it was so widely used that while still technically classified it was in regular public use.

In the early days of radar it was also very common to use the approximate wavelength as a band designator. I.E. "10 meter" meant a radar near 30 MHz, "10 cm" meant a radar near 3 GHz, "3 cm" meant a radar near 10 GHz, etc.

Initially the ESM bands were also classified, but I think that was for a very short time.

By the way, the ESM / NATO bands system can be further broken down, to define frequency. This is seldom done today, but at one time, and in certain communities, was common. Each band can be broken into 10 sub-bands, and each of those into 10 sub-bands. I band is 8 - 10 GHz. But, I1 (letter I followed by number one) is 8.000 to 8.199 GHz, I2 is 8.2 to 8.399 GHz, I3 is 8.4 to 8.599 GHz, etc. And this can further be broken up into 10 more increments, designated A to J. I8A covers 9.400 - 9.41999 GHz, I8B covers 9.420 - 9.43999 GHz, I8C covers 9.440 - 9.45999 GHz, etc.

So I band covers 8 - 10 GHz. I8 covers 9.400 - 9.5999 GHz. I8C covers 9.440 - 9.45999 GHz. So a radar on 9450 MHz (9.45 GHz) would be said to be in I8C. While seemingly cumbersome, yes, there are reasons to do this.

T!

 

[Zipper730]

"The main echo regions that characterize RCS signature are the nose, broadside, and tail. A simple approximation for the nose-on aircraft RCS is = σ 0.01L2 where L is the aircraft length.

So for head-on, the wingspan would be providing the figures? If I do this I get a RCS of 18.47 m^2 for a B-29 and 49.15 m^2 for the B-36. Those numbers seem a little small.

This next table from the same source shows nose aspect ratio data for several targets.
View attachment 821822

Some of these numbers are all over the place. At least in some cases it's clearly specified (such as the 707) that it's owing to different wavelenghts (C & X band appear to be the most likely used for radar as ATC uses C-band and military aircraft use X-band), but some numbers seem strange.

• The 720/727 being lumped together is very strange: They might have been designed for short-haul but the 720 was basically a 707 with a shortened fuselage (8 feet) and a modified inboard leading-edge and the 727 is a three engined T-tailed aircraft.
• The broadside figures for the 737 seem to match an X-band radar based on the image below which displayed a 727 or medium attack bomber aircraft from a broadside perspective, but the 727's figures seem kind of low based on these figures.

I'm guessing all the military aircraft indicated are in X-band since the military would be the most likely to track such an aircraft.

Yes, square to the face and on axis.

Understood

Higher frequency = larger RCS

Except with stealth it seems

Many military air-to-air and surface-to-air radars are indeed in X-band, but there is a long series of events that has resulted in that.

Understood. Weren't X-band radars already being used in WWII?

The optical region is defined by the frequency of the signal in comparison to the size of the sphere, signal wavelength to maximum dimension. So it would be defined in frequency.

So a sphere with a diameter of 12" would have an optical region of 18", and that would correspond to 535.4 MHz?

Running rabbits are asynchronous interference being shown on the radar display. Interference synchronized in time will be fixed and not moving (in range, range is time to a radar) on the display, interference that is not in sync will move on the display.

So running rabbits would mean the return would rapidly zip all over the place in range?

In this case it is almost certainly referring to the B-26 Marauder.

I didn't know the book was published that far back. That said, in retrospect, the silhouette does look more like a B-26 than an A-26 owing to the wing (the A-26 has higher aspect ratio wing with less taper).

For those not familiar, there are multiple designation systems for frequency ranges. Of the several different systems used to discuss ranges of frequencies, especially when radar is involved, there are three primary schemes. The ITU Band designations, the IEEE Radar Band designations, and the ESM / NATO Band designations.

What does ITU, IEEE and ESM (I know what NATO means) mean?

 

[MIflyer]

Except with stealth it seems

The longer wavelengths may be able to detect stealth aircraft that are designed to evade microwave radars. But the longer wavelengths are also much less useful in supplying precise information on just where the aircraft is located, a problem obviously complicated by the fact that airplanes tend to move along a rather good clip.

So a longer wavelength radar may tell you that there is an aircraft over say, South Carolina, but that does not enable you to home in on it.

Decades ago on the TV show 60 Minutes they had some guy who had flown B-25's in WW2 saying that stealth bombers would not be of much use because you could just look around and find them. That man was a complete idiot. I can tell you that even in the daytime it is hard to see aircraft, even when they are not trying to evade being seen. We have enough mid-air collisions with light aircraft to prove that. And at night and in weather, it's a whole lot harder.

Point enough RF power at an stealth airplane and you will see it. But how close do you have to be in order to accomplish that? And is the radar info good enough, steady enough to enable you to shoot something at it?

During testing of the YB-49 they found it was very hard to see it on radar, even though no particular effort had been made to make it stealthy. My theory is that the B-49 production was killed and all examples destroyed in such a bizarre manner because the Soviets had penetrated Northrop (which we know to be the case), had copied the B-29, and had developed the atomic bomb. That adds up to a long range bomber able to carry a nuke and evade radar.

 

[Token]

So for head-on, the wingspan would be providing the figures? If I do this I get a RCS of 18.47 m^2 for a B-29 and 49.15 m^2 for the B-36. Those numbers seem a little small.

No, it is not that simple. You are calculating simple size and area, not RCS. While physical size plays a part in RCS, shape is just as important. Go back to the flat plate I mentioned before. Lets change the size to 1 meter by 1 meter, for a physical area of 1 m^2. But, the on-axis RCS at X band is about 41.4 dBsm, or about 13963 m^2.

Physical size comes into play as a larger component when you get very low in frequency, as that size may determine what is the minimum frequency that has a reasonable chance of detecting the target. And even then, it ain't that simple. There are literal books written on what we are stating in single sentences and paragraphs here.

The best we can hope for here, without re-writing said books, is the general information involved. And generalities are almost always wrong at some level, the more technical the subject the more likely generalizations are to be wrong.

I'm guessing all the military aircraft indicated are in X-band since the military would be the most likely to track such an aircraft.

Not at all. In fact I would bet that most of the military aircraft in such charts are not actually measured values, but guesstimates. Remember that most of these charts are not from military sources. The newer the aircraft, the more likely that is to be. I would bet a pretty fair quantity that no unclassified actual measurements, nothing that would show up on a forum like this, of modern examples of the F-22, F-35, B-2, B-1, etc, aircraft exist. In the public domain at best someone has modeled them, and made some assumptions of the coatings used.

And X band is not all military aircraft have to worry about. It is true that many, maybe even most, threat radars (radars that can track and shoot deadly things at you) are in X band, but far from all threats are there. Also, the radars that hand off the target to the threat radar, things like acquisition, search, EW, radars, they might not be X band (many are) but you still want to hide from them also. So you need to know how easily you can be seen in every threat band, not just one of them.

Except with stealth it seems

Higher frequencies = larger RCS unless you are actively trying to change that equation, such as with stealth. Reduced RCS is not magic, and it is not a cloaking device. It only reduces the range at which you can be detected. For military applications, if you can't see me until after I have killed you, then I might as well be invisible.

Remember what I said above, "while physical size plays a part in RCS, shape is just as important". One thing not discussed so far is coatings.

To reduce RCS you can shape the object so reflected energy goes anyplace but back towards the radar. This will reduce the monostatic RCS (but leave you vulnerable to bistatic technologies). You can also coat it with materials that are not reflective in the frequency range (wavelengths) of interest. This will also reduce the RCS in both monostatic and bistatic applications.

Take the example of a flashlight in a flat black painted room. You shine the light around and the reflections, back to the person holding the flashlight, are weak, not a lot of light comes back to the source from the flat black painted walls.

Now put a mirror in the room pointed at a 45 degree angle to the the light source. The light will reflect off the mirror, but not back towards the source, and much of it will get lost in the black painted room. Still a weak reflection, from the aspect of the person holding the light. This is a monostatic scenario. But, if someone happens to be where the light is redirected to by the mirror, the light will be bright. This is bistatic.

Now point the mirror towards the light source. The same surface, but now reflecting all the light back to the source. Bright reflection.

Now paint the mirror flat black, what happens?

The angle mirror is shape, the flat black paint on the mirror is coating. In a very rough way.

Understood. Weren't X-band radars already being used in WWII?

X band radars existed by the end of WW II, yes. However, they were still a minority. Most radars in use during WW II were S band and lower.

So a sphere with a diameter of 12" would have an optical region of 18", and that would correspond to 535.4 MHz?

If you abide by the 1.6 lambda definition, yes. For the purposes of calculating the RCS as a calibration target it is not useful as some variability will be there until you get up into the 10 wavelength area.

So running rabbits would mean the return would rapidly zip all over the place in range?

Yes, running rabbits are moving (in range) false targets / interference. Two radars tuned to about the same frequency and not synchronized (in pulse timing) would produce such false targets. If the false targets or interference are in sync, not running, then it is easier for the operator to differentiate fakes from real. Two radars that are synchronized (in pulse timing) would produce such fixed false targets.

What does ITU, IEEE and ESM (I know what NATO means) mean?

ITU = International Telecommunications Union

IEEE = Institute of Electrical and Electronic Engineers

ESM = Electronic Support Measures

T!

 

[yosimitesam]

These numbers mean nothing without the specific frequency they were calculated or measured at. And for aircraft, also the aspect angle.

T!

Indeed. If you pointed a radar beam at an aircraft and tracked it through its path (like using a spotlight), you would notice it "twinkling" on the scope like a star as a wing dips slightly or it makes a slight change in course, etc. takes place. This is due to the variable reflection (RCS) as the aircraft's exact aspect to the radar unit changes.

 

[Zipper730]

The longer wavelengths may be able to detect stealth aircraft that are designed to evade microwave radars. But the longer wavelengths are also much less useful in supplying precise information on just where the aircraft is located, a problem obviously complicated by the fact that airplanes tend to move along a rather good clip.

So a longer wavelength radar may tell you that there is an aircraft over say, South Carolina, but that does not enable you to home in on it.

So it's a fuzzier image without as much angular accuracy?

During testing of the YB-49 they found it was very hard to see it on radar, even though no particular effort had been made to make it stealthy. My theory is that the B-49 production was killed and all examples destroyed in such a bizarre manner because the Soviets had penetrated Northrop (which we know to be the case), had copied the B-29, and had developed the atomic bomb. That adds up to a long range bomber able to carry a nuke and evade radar.

Well, there seemed to be corruption that surrounded the YB-35 program from 1946 on at minimum (and this might have predated the first flight) so it seems that the opposition to the aircraft predated the difficulty in detecting it on radar.

Regardless, even if we produced the YB-49 (first flight was in 1947), it would have been some time before the Soviets would have been able to copy the key aspects of the design that would have made it hard to detect and we didn't realize they were as far along with a nuclear weapon. From what I recall we expected they'd have a nuclear bomb by around 1953 or so (obviously wrong)

No, it is not that simple. You are calculating simple size and area, not RCS.

I kind of got the vibe the formula was an early formula that was more of a rule of thumb than a modern estimate. I still calculated the numbers out of curiosity, and things looked off (I didn't think about flat-plate area, admittedly).

Physical size comes into play as a larger component when you get very low in frequency, as that size may determine what is the minimum frequency that has a reasonable chance of detecting the target. And even then, it ain't that simple. There are literal books written on what we are stating in single sentences and paragraphs here.

Good point.

Not at all. In fact I would bet that most of the military aircraft in such charts are not actually measured values, but guesstimates.

There is a certain degree of logic to that.

And X band is not all military aircraft have to worry about. It is true that many, maybe even most, threat radars (radars that can track and shoot deadly things at you) are in X band, but far from all threats are there. Also, the radars that hand off the target to the threat radar, things like acquisition, search, EW, radars, they might not be X band (many are) but you still want to hide from them also. So you need to know how easily you can be seen in every threat band, not just one of them.

And you'd have to be able to reliably jam most of those frequency bands as well, I would guess?

For military applications, if you can't see me until after I have killed you, then I might as well be invisible.

That makes enough sense, and I guess as long as an F-22 could put an AIM-120 through an opponent, or a B-2 could drop a bomb on a target before anybody detected it it would have served its purpose.

One thing not discussed so far is coatings.

To reduce RCS you can shape the object so reflected energy goes anyplace but back towards the radar. This will reduce the monostatic RCS (but leave you vulnerable to bistatic technologies). You can also coat it with materials that are not reflective in the frequency range (wavelengths) of interest. This will also reduce the RCS in both monostatic and bistatic applications.

Take the example of a flashlight in a flat black painted room. You shine the light around and the reflections, back to the person holding the flashlight, are weak, not a lot of light comes back to the source from the flat black painted walls.

Now put a mirror in the room pointed at a 45 degree angle to the the light source. The light will reflect off the mirror, but not back towards the source, and much of it will get lost in the black painted room. Still a weak reflection, from the aspect of the person holding the light. This is a monostatic scenario. But, if someone happens to be where the light is redirected to by the mirror, the light will be bright. This is bistatic.

Now point the mirror towards the light source. The same surface, but now reflecting all the light back to the source. Bright reflection.

Now paint the mirror flat black, what happens?

The angle mirror is shape, the flat black paint on the mirror is coating. In a very rough way.

That's a good analogy. While I heard of bistatic radars, I didn't know much about what they were used for, though that seems like a solid reason.

X band radars existed by the end of WW II, yes. However, they were still a minority. Most radars in use during WW II were S band and lower.

For some reason I though the SCR-720 was an X-Band but it was S-Band (I just did a search).

If you abide by the 1.6 lambda definition, yes.

Understood

For the purposes of calculating the RCS as a calibration target it is not useful as some variability will be there until you get up into the 10 wavelength area.

So that's where the 10 lambda figure came from?

Yes, running rabbits are moving (in range) false targets / interference. Two radars tuned to about the same frequency and not synchronized (in pulse timing) would produce such false targets.

Is this because the radar can't tell if the return it's getting is from it's own transmitter or the other?

 

[Token]

I kind of got the vibe the formula was an early formula that was more of a rule of thumb than a modern estimate. I still calculated the numbers out of curiosity, and things looked off (I didn't think about flat-plate area, admittedly).

Not sure exactly which formula you are referring to as "early".

RCS values were, until recent decades, mostly actual measured numbers, not calculations. For example, the B-26 RCS plot shown earlier in this thread was an actual measured plot, not calculated. For simple shapes, plates, spheres, cylinders, etc, of homogeneous materials the formulas are relatively simple, and those have been around a long time. It has only been in the later decades that computing power, formulas, and modeling fidelity were robust enough to start yielding good, reliable, numbers for complex shapes and varying materials.

Is this because the radar can't tell if the return it's getting is from it's own transmitter or the other?

A radar never knows if a return is from its own transmitter. But a return that is in sync (not moving in time) will at least appear stable on a display. Time is range to a radar, varying time (as in unstable or not synced time) means varying range.

T!

 

[MIflyer]

A radar never knows if a return is from its own transmitter.

I recall seeing on a TV weather report that as the radar swept another circle cut across the display. The TV WX guy explained that was due to another WX radar sweep.

 

[ThomasP]

re

A radar never knows if a return is from its own transmitter

I believe the usage of pulse coding in radars allows the individual radar to recognize its own emissions vs the emissions from other radars. The idea is that any particular radar, while using the same general frequency band as other radars in the area, will individually code its own pulses (sometimes each pulse will be the same for the specific radar, sometimes each pulse will be different from the one before and after for better ECCM).

Heavy noise jamming can degrade this ability, and very fast/very sophisticated types of deception jammers may fool slower and less sophisticated pulse coded radars. The noise jammers make it difficult for the radar to pick out the signal from the noise/clutter generated by the jammer, while the deception jammers (if they are fast enough/sophisticated enough) may actually be able to generate pulse coded signals that mimic the emitting radars pulse code and (under some conditions) cause the emitting radar to be unable to determine which signal is the original.

Yes? No?

 

[Token]

re


I believe the usage of pulse coding in radars allows the individual radar to recognize its own emissions vs the emissions from other radars. The idea is that any particular radar, while using the same general frequency band as other radars in the area, will individually code its own pulses (sometimes each pulse will be the same for the specific radar, sometimes each pulse will be different from the one before and after for better ECCM).

Heavy noise jamming can degrade this ability, and very fast/very sophisticated types of deception jammers may fool slower and less sophisticated pulse coded radars. The noise jammers make it difficult for the radar to pick out the signal from the noise/clutter generated by the jammer, while the deception jammers (if they are fast enough/sophisticated enough) may actually be able to generate pulse coded signals that mimic the emitting radars pulse code and (under some conditions) cause the emitting radar to be unable to determine which signal is the original.

Yes? No?

Yes, pretty much on the money, but probably used a bit less often than one might think.

Yes, pulse coding is a thing, and reasonably common. Sometimes as an ECCM technique (as you have described), but most often as a pulse compression technique, sometimes both. Don't get me wrong, it essentially always has the ECCM potential, but many radars that use such coding do not use it because they are planning on being in a noisy environment, instead they use it for its other advantages. Assuming it is for an ECCM application it will identify what it _assumes_ is its own returns, based on the code. So sure, in that case, a coded pulse being used by the radar for ECCM, not compression, another radar should not interfere, or at least cause minimal interference.

In a benign world (no ECM present) this is true, and all is good. But with something like a DRFM based box (pretty much everything designed in recent years) you just repeat the code back to it, duplicated as many times as you want and delayed / advanced / Doppler shifted as you see fit. With DRFM there is no question of "fast enough", and as I said, DRFM is everywhere today, it is pretty much the standard.

Under conditions like those (any modern box) a radar does not know if a return is from its own transmitter, but it assumes that everything with the right code on return is its own. And you (assuming you are the jammer) can use this assumption to your advantage.

T!

 

[Zipper730]

Not sure exactly which formula you are referring to as "early".

PVFA63 mentioned the following in a quote "The main echo regions that characterize RCS signature are the nose, broadside, and tail. A simple approximation for the nose-on aircraft RCS is = σ 0.01L2 where L is the aircraft length.

I figure this was like a WW2 era rule of thumb.

RCS values were, until recent decades, mostly actual measured numbers, not calculations. For example, the B-26 RCS plot shown earlier in this thread was an actual measured plot, not calculated. For simple shapes, plates, spheres, cylinders, etc, of homogeneous materials the formulas are relatively simple, and those have been around a long time.

Understood

It has only been in the later decades that computing power, formulas, and modeling fidelity were robust enough to start yielding good, reliable, numbers for complex shapes and varying materials.

I assume people did make estimates when a new aircraft was found to exist, however?