Radar Cross Section

[Zipper730]

I got some questions regarding the subject of radar-cross section.

1. Is there any publicly available / publicly-listed data on the RCS of any of the following aircraft?
   • Bombers/Reconnaissance
     • He-111
     • B-17
     • Ju-88
     • B-24
     • De Havilland Mosquito
     • Avro Lancaster
     • B-29
     • B-36
     • B-47
     • B-52
     • Tu-16
     • Tu-95
     • M-4
     • Tu-22
     • XB-70
     • Tu-22M
     • B-1
     • F-117
     • B-2
   • Reconnaissance
     • Ki-46
     • SR-71
   • Fighters
     • Bf.110
     • J1N
     • P-61
     • F-86
     • MiG-15
     • MiG-17
     • MiG-19
     • F-100
     • F-104
     • MiG-21
     • Su-7
     • F-105
     • F-4
     • F-111
     • F-15
     • F-16
     • F-18
     • F-22
2. What effect does RCS have on detection distance? For example if RCS is reduced by 2 fold, what effect does that have?

While the arragnement seems odd, I put them in chronological order of first flight.

GTX , T Token X XBe02Drvr

 

[Joe Broady]

What effect does RCS have on detection distance? For example if RCS is reduced by 2 fold, what effect does that have?

I don't know the RCS of any plane, but the effect on range is easy. Look up "radar range equation" and you'll see detection range equals the 4th root of the product of several parameters, one of which is RCS. If you cut RCS in half but all other parameters are unchanged, detection range changes by the 4th root of 0.5, or 84%. This 4th root behavior is the reason a very large RCS reduction is necessary to get a big reduction in detection range.

 

[Joe Broady]

If you cut RCS in half but all other parameters are unchanged, detection range changes by the 4th root of 0.5, or 84%.

As soon as I posted that I realized it was badly worded. It gives the impression a 50% cut in RCS reduces detection range by 84%. I should have said detection range is still 84% of the original range, i.e., not a big reduction.

 

[MIflyer]

I got some questions regarding the subject of radar-cross section.

I am fairly sure that would depend on the frequency of the radar, and there is no reason to take measurements on any of those vintage aircraft. In WW2 there were radars operating at frequencies as low as 14 MHZ and as high as X-Band (10 GHZ).

 

[Zipper730]

M MIflyer

Wait, aircraft have different RCS in different frequencies?

 

[yosimitesam]

M MIflyer

Wait, aircraft have different RCS in different frequencies?

Yes they can, from what I read. In fact, in Germany during the late 1930's, scientists convinced the RLM (or whomever) that radar could NOT detect aircraft if their wavelength was smaller than about .5 meter. They had "calculated" that the amount of energy returned would not be enough for reliable detection, even if they could be made using higher powers. Of course, this was wrong. The smaller the wave length you use the more power is required to reach a given detection distance, but the Germans undoubtedly made some wrong assumptions in their "calculations." That's one of the reasons the "Rotterdam device" (H2S magetron) was a surprise to the Germans when they discovered it on a shot-down Brititsh bomber.

 

[MIflyer]

In fact, in Germany during the late 1930's, scientists convinced the RLM (or whomever) that radar could NOT detect aircraft if their wavelength was smaller than about .5 meter.

In most of the world the calculation was "A bomber is about 3 meters in diameter. Therefore a radar has to be broadcasting on frequencies of 2 meters of less in order for that bomber fuselage to act as a reflective antenna." Of course they were thinking of the fuselage diameter. That led to radars with frequencies of 200 MHZ and higher.

In England the calculation was, "A bomber's diameter is about 100 ft." Of course they were thinking of the wingspan. As for the fighters, nobody gave a rat's rump about them because they could not do significant damage to industries and cities. That meant that much lower frequencies could be used, as low 20 MHZ, given than the "Antenna" was the bomber's wings. And the lower frequency made it easier to generate lots of RF power, 100 KW and more.

So, when before the war the German sent a supposedly decommissioned airship on a secret ECM Ferret mission to the coast of the UK, they had receivers on board that looked for the frequencies they though were appropriate, the ones they were using. E.G, Giant Wurzsberg used 560 MHZ. And they did not find them. So they concluded the British had no radar.

 

[PFVA63]

Hi,
Here is a table from the paper at this link that notionally has a few planes listed.

 

[Zipper730]

Yes they can, from what I read. In fact, in Germany during the late 1930's, scientists convinced the RLM (or whomever) that radar could NOT detect aircraft if their wavelength was smaller than about .5 meter.

That would require a wavelength around 600 MHz right?

In most of the world the calculation was "A bomber is about 3 meters in diameter. Therefore a radar has to be broadcasting on frequencies of 2 meters of less in order for that bomber fuselage to act as a reflective antenna." Of course they were thinking of the fuselage diameter. That led to radars with frequencies of 200 MHZ and higher.

So if the radar frequency is less than 2/3 the surface, it won't show up?

Hi,
Here is a table from the paper at this link that notionally has a few planes listed.

View attachment 821297

I greatly appreciate it, as well as the screencap (unfortunately the link won't load right, apparently "unusual activity" was detected from my end (whatever that means)

 

[pbehn]

M MIflyer

Wait, aircraft have different RCS in different frequencies?

Of course, and from different orientations.

 

[pbehn]

I got some questions regarding the subject of radar-cross section.

1. Is there any publicly available / publicly-listed data on the RCS of any of the following aircraft?
   • Bombers/Reconnaissance
     • He-111
     • B-17
     • Ju-88
     • B-24
     • De Havilland Mosquito
     • Avro Lancaster
     • B-29
     • B-36
     • B-47
     • B-52
     • Tu-16
     • Tu-95
     • M-4
     • Tu-22
     • XB-70
     • Tu-22M
     • B-1
     • F-117
     • B-2
   • Reconnaissance
     • Ki-46
     • SR-71
   • Fighters
     • Bf.110
     • J1N
     • P-61
     • F-86
     • MiG-15
     • MiG-17
     • MiG-19
     • F-100
     • F-104
     • MiG-21
     • Su-7
     • F-105
     • F-4
     • F-111
     • F-15
     • F-16
     • F-18
     • F-22
2. What effect does RCS have on detection distance? For example if RCS is reduced by 2 fold, what effect does that have?

While the arragnement seems odd, I put them in chronological order of first flight.

GTX , T Token X XBe02Drvr

A concept such as radar cross section is usually from scientists trying to rationalise the subject they are working in to compare things. In ultrasonics we compared signal strength to a drilled hole, in Thermography to a black box i.e. perfect emitter.

 

[Token]

M MIflyer

Wait, aircraft have different RCS in different frequencies?

Yes, for any shape other than a sphere the RCS of any target is, among other variables, dependent on frequency.

Lets take for example a small, square, flat aluminum plate, 12 inches on each side. Working only on the example of the radar being orthogonal to the surface. At 10 GHz, X-band, the RCS is about 120.5 square meters (20.8 dBsm), while at 3 GHz, S-band, the RCS is about 10.8 square meters (10.3 dBsm).

Once you get to a frequency high enough to be in the optical region (based on the diameter of the sphere), the RCS of the sphere is dependent on the radius of the sphere, and does not change with frequency. So that a 12" sphere has an RCS of ~0.3 square meters (-5.3 dBsm) at both 3 GHz and 10 GHz.

Getting down into the Mie region of scattering (physical size ~1.5 wavelengths or less), or worse yet, the Rayleigh region (physical size ~0.1 wavelengths or less), these calculations become more difficult, as small variations in frequency can cause large excursions in RCS as you get into and out of resonance zones.

T!

 

[Token]

Hi,
Here is a table from the paper at this link that notionally has a few planes listed.

View attachment 821297

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

T!

 

[Token]

So, when before the war the German sent a supposedly decommissioned airship on a secret ECM Ferret mission to the coast of the UK, they had receivers on board that looked for the frequencies they though were appropriate, the ones they were using. E.G, Giant Wurzsberg used 560 MHZ. And they did not find them. So they concluded the British had no radar.

The airship was the LZ-130, Graff Zepplin II. Absolutely true that they focused on too high a frequency range, however, several sources state that they actually did find the Chain Home radars, and never realized it. The British radars use the mains frequency as the PRF (50 Hz in this case), allowing the radars at various locations to be in sync with each other, and not cause running rabbits in the radar returns. The German ELINT / ESM operators heard various transmissions in the 25 MHz region with 50Hz hum and assumed those were failed BBC or other shortwave transmitters. This kind of failure, and sound, is common if a filter cap in the high voltage supply breaks down.

T!

 

[Token]

So if the radar frequency is less than 2/3 the surface, it won't show up?

It is not that simple.

As a general statement, for a given shape and material, the higher the frequency the better almost anything shows up. This is assuming the target does not include any LO coatings or materials (those are generally tailored for higher frequencies). This is one of the basics of RCS.

Radar (or any RF) wavelength is dependent on frequency. λ=c/f. λ is the wavelength, c is the speed of light, and f is the frequency. If you use c in km/s then you can use f in kHz, or if you use 300 for c you can use frequency in MHz. So, to find wavelength in meter, λ=300/(f MHz).

How well a radar signal is reflected, how well it scatters, is dependent on many variables. One of them is size in relationship to λ. Once an objects size becomes small enough it now falls in the Mie region (roughly 1.5 λ, although some sources will put this at 10 λ), the reflections can be harder to define. Small changes in frequency can cause large changes in reflectivity as the relationship between object size and frequency slide in and out of resonance. In these cases a smaller object, in resonance, can have a larger RCS than a physically larger object. Until you get down to the Rayleigh region, then it is all down hill the smaller an object gets.

Roughly, once something is smaller than half a wavelength, the reflectivity goes down significantly. It still reflects, it just has a smaller RCS.

So, put very crudely, a fighter, with a wingspan (or fuselage length, whichever is greater) of 10 meters, will be fairly well seen (for the most part, with a few holes in frequency) down to about 15 MHz. Below roughly that frequency the RCS falls off quickly.

All of the above, taken together, is part of the reason that almost every major nation today (Australia, US, UK, Russia, China, Iran, India, etc) has HF OTHR (High-Frequency Over-The-Horizon-Radar) in use (HF is the frequency range 3 - 30 MHz). In general, LO (stealth) coatings don't work well in the freq ranges and the physical size of the targets (rockets, aircraft, boats, etc) makes them pretty detectable.

T!

 

[PFVA63]

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

T!

Hi,

In general I believe that it is more useful, when trying to help someone understand a complex topic like aircraft RCS, to try and show what the limits of some provided data might be rather than dismiss things as meanlingless. Specifically, while data may not be of use as a "specific value" for determining a real world solution to a radar range equation type calculations they still hold value in relative terms for understanding how different targets may compare. And the source document of the table was provided to give insite on the specifics of the numbers listed, if the reader wants to try and dive deeper into the meanings of the numbers provided.

Specfically, tracing back through the source document, and the references that they are based on one can see that the table provided is based on information from the book "Radar Technology Encyclopedia", which is available online as a PDF. In this document there is information provided which is in turn based on infromation from the books "Airborne Early Warning Radar" by WC Morchin, "Introduction to Radar Systems" by Merrill Skolnik, and "Radar Design Principles" by Nathanson, Reilly and Cohen. Although the Morchin book does not appear to be available on lin, PDFcopies of the latter two do appear to be readily available. From these source documents it can be seen that;

"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.

The classical picture of RCS pattern of a B-26 bomber showing the lobe structure of this pattern for two different frequencies are shown in Fig. R69."

This figure is of particular interest for anyone trying to understand some of the basics of radar cross sections of complex targets like an airplane in that it helps really show the variation of RCS with viewing angle and how some aspects really dominate at specific frequencies.

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

Next, here are two additional tables from "The Radar Tachnology Encyclopedia" helping better define how values can vary for aspect ratio and radar Band as well as relative values for the nose aspect for various targets, with an additional table providing data on how such data varies for a couple targets in various bands and relative aspacts/viewing angles

These next two tables show similar information from Nathanson, et al.

And finally, here also from Skolnik here is a table showing data for a T-38 trainer with landing gear up and down, that also might be of help to someone trying tobettr understand RCS issues for complex targets like aircraft

Regards

 

[PFVA63]

PS. Herre are links to some of the documents referenced above

https://selasl.wordpress.com/wp-con...rtech-house-radar-technology-encyclopedia.pdf
https://selasl.wordpress.com/wp-content/uploads/2011/08/mcgraw_hill_-_radar_design_principles.pdf
https://soaneemrana.org/onewebmedia/INTRODUCATION TO RADAR SYSTEM BY MERRIL, I SKLOINK (4).pdf

 

[Token]

In general I believe that it is more useful, when trying to help someone understand a complex topic like aircraft RCS, to try and show what the limits of some provided data might be rather than dismiss things as meanlingless. Specifically, while data may not be of use as a "specific value" for determining a real world solution to a radar range equation type calculations they still hold value in relative terms for understanding how different targets may compare. PS. Herre are links to some of the documents referenced above

I was not being dismissive, the purpose of my saying what I did, "These numbers mean nothing without the specific frequency they were calculated or measured at. And for aircraft, also the aspect angle" was to inform people not familiar with the concept that you have to take more into account than a simple table. While the data presented has some value for relative comparison, it has no real meaning without a good bit more info.

In addition, I have measured airframe RCS values that change significantly serial number to serial number.

https://selasl.wordpress.com/wp-con...rtech-house-radar-technology-encyclopedia.pdf
https://selasl.wordpress.com/wp-content/uploads/2011/08/mcgraw_hill_-_radar_design_principles.pdf
https://soaneemrana.org/onewebmedia/INTRODUCATION TO RADAR SYSTEM BY MERRIL, I SKLOINK (4).pdf

I am familiar with those publications. And I worked with Dr. Skolnik.

T!

 

[Zipper730]

Yes, for any shape other than a sphere the RCS of any target is, among other variables, dependent on frequency.

Lets take for example a small, square, flat aluminum plate, 12 inches on each side. Working only on the example of the radar being orthogonal to the surface. At 10 GHz, X-band, the RCS is about 120.5 square meters (20.8 dBsm), while at 3 GHz, S-band, the RCS is about 10.8 square meters (10.3 dBsm).

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

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?

Once you get to a frequency high enough to be in the optical region (area of the sphere ~1.5 wavelengths of the radar or greater) the RCS of the sphere is dependent on the radius of the sphere, and does not change with frequency. So that a 12" sphere has an RCS of ~0.3 square meters (-5.3 dBsm) at both 3 GHz and 10 GHz.

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

The airship was the LZ-130, Graff Zepplin II. Absolutely true that they focused on too high a frequency range, however, several sources state that they actually did find the Chain Home radars, and never realized it. The British radars use the mains frequency as the PRF (50 Hz in this case), allowing the radars at various locations to be in sync with each other, and not cause running rabbits in the radar returns. The German ELINT / ESM operators heard various transmissions in the 25 MHz region with 50Hz hum and assumed those were failed BBC or other shortwave transmitters.

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

It is not that simple.

As a general statement, for a given shape and material, the higher the frequency the better almost anything shows up. This is assuming the target does not include any LO coatings or materials (those are generally tailored for higher frequencies). This is one of the basics of RCS.

Radar (or any RF) wavelength is dependent on frequency. λ=c/f. λ is the wavelength, c is the speed of light, and f is the frequency. If you use c in km/s then you can use f in kHz, or if you use 300 for c you can use frequency in MHz. So, to find wavelength in meter, λ=300/(f MHz).

I would have just calculated for meters/second, which comes out to around 300,000,000 (I think the exact figure is 299792458 m/s) to calculate for Hz and then just move the decimal point as need be for order of magnitude (300,000,000 Hz or 300 MHz).

How well a radar signal is reflected, how well it scatters, is dependent on many variables. One of them is size in relationship to λ. Once an objects size becomes small enough it now falls in the Mie region (roughly 1.5 λ, although some sources will put this at 10 λ), the reflections can be harder to define. Small changes in frequency can cause large changes in reflectivity as the relationship between object size and frequency slide in and out of resonance. In these cases a smaller object, in resonance, can have a larger RCS than a physically larger object. Until you get down to the Rayleigh region, then it is all down hill the smaller an object gets.

Does this have to do with the fact that the object is low enough in mass that it vibrates more readily with the energy of the beam, or is this something else?

Roughly, once something is smaller than half a wavelength, the reflectivity goes down significantly. It still reflects, it just has a smaller RCS.

Understood

So, put very crudely, a fighter, with a wingspan (or fuselage length, whichever is greater) of 10 meters, will be fairly well seen (for the most part, with a few holes in frequency) down to about 15 MHz. Below roughly that frequency the RCS falls off quickly.

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.

All of the above, taken together, is part of the reason that almost every major nation today (Australia, US, UK, Russia, China, Iran, India, etc) has HF OTHR (High-Frequency Over-The-Horizon-Radar) in use (HF is the frequency range 3 - 30 MHz). In general, LO (stealth) coatings don't work well in the freq ranges and the physical size of the targets (rockets, aircraft, boats, etc) makes them pretty detectable.

Fascinating

The classical picture of RCS pattern of a B-26 bomber showing the lobe structure of this pattern for two different frequencies are shown in Fig. R69."[/I]

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?
ratio data for several targets.

 

[MIflyer]

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?

ATC radars are C band, and airborne weather radars are X band. If someone had a big airplane that had a big radar designed to spot other airplanes then it might use X-band but I am not saying that such a thing exists.

By the way, a concept touted for some time is the use of broadcast commercial VHF TV and FM radio signals for tracking at much lower costs. This was considered for tracking rockets at Cape Canaveral but was found not to be useful because rockets tend to be pointed "up" and the commercial signals are not polarized in that direction.