A question for radar/rf engineers

[MrOmnos]

When they say B2 spirit has RCS of 0.001 m² or F-22 Raptor has a RCS of 0.001, are these numbers valid for certain band radar or does it mean it will have the same RCS for any kind if EM radiation? What does having small RCS mean for an aircraft? Does it mean it can't be detected at all? Or does it mean, you can detect it but the RCS is so small, you can't home your missiles on it? I suspect it means, it can't be detected at all, else enemy would scramble jets.

[MrOmnos]

Sounds like almost meaningless PR / marketing-speak.
Technically a cross section is a unit of interaction area so if you had, say, a flat square metal plate with some area then the RCS of it is whatever the area of that plate would be facing the area it is looked at.  I suppose they could use spheres with similar hemispherical area or something like that in a similar definition or constrain the plate to be at the optimum incidence/reflection angle for the situation involved.

Anyway the basic point is that something like a perfectly conductive sphere or square plate of a certain area will be something close to an ideal reflector for its given dimensions.  Obviously the smaller area the plate the smaller reflection you get.  Just like if you shine a wide beam flashlight on the side of a big glossy white painted wall you will see more light coming back to you than if you shine the same light at the same distance onto a 20cm * 20cm square flooring tile painted with the same white paint, and obviously if you shine the light on a small painted coin at the same distance you'll get an even smaller amount of light back .  The amount of reflected light is proportional to the area of the reflector and the reflectivity of the surface and inverse-square-ly proportional to the distance between the light and the object.

So all they're saying is that in some case of frequency band and polarization and some case of incidence/reflection angles it appears no more reflective than some small object maybe like a small bird or something of that general area.
But that is just marketing-speak, the results may be much worse at some frequency or some polarization or some incidence/reflection angles so they just pick something like the best case to sound impressive without really telling you the "truth" about how good or bad it is in various realistic situations.

And anyway it may really not be so important since there are all kinds of other considerations as to how relevant or not that might even be.  So basically they are just advertising for PR and no other reason that is relevant.

Thanks for the answer:
I was doing some research on aircrafts in current military service all around the world and ones that will be coming in near future. "Stealth" has become keyword in every countries aircraft development program it seems. The whole F-35 program has been really expensive and when you look at the specs it is no better/similar to F-16s it is suppose to replace except for the part that it has small radar cross-section. In fact it is far behind in agility to russian Sukhois. That means almost all of the money went into making it  stealthy . So, if Uncle Sam has invested so much trust in stealth technology, there must be something that his enemies hate about it.   

[Voodoo 6]

This article will probably answer a lot of your questions about radar cross sections and the methods used: From Air Power Australia

http://www.ausairpower.net/APA-2011-03.html

We have a Ratscat facility at the White Sands Missile range here.

[MrOmnos]

This article will probably answer a lot of your questions about radar cross sections and the methods used: From Air Power Australia

http://www.ausairpower.net/APA-2011-03.html

We have a Ratscat facility at the White Sands Missile range here.

Just opened the post and saw pictures of j-20 and I saw j-20 pics back in 2014 and the first thin that cam to my mind was, who the hell sold raptor designs to Chinese? Lockheed was clearly hacked.

[Voodoo 6]

You might recall the 1999 shoot down of an American F117a in Yugoslavia, using an Sa-3 Goa air defense system. Stealth doesn't make it impossible just makes its a bit more difficult. Although, there might have been some hacks there is really no need to. The US defense industry is riddled with foreign agents from many nations.

[elgonzo]

Just opened the post and saw pictures of j-20 and I saw j-20 pics back in 2014 and the first thin that cam to my mind was, who the hell sold raptor designs to Chinese? Lockheed was clearly hacked.

The J-20 has canards and no rear elevators, whereas the F-22 has rear elevators but no canards. Basically, the aerodynamic design of the J-20 is entirely different from the F-22.
There is certainly a striking similarity in the general form of the fuselage, air inlets, canopy and such. It really is too close to be mere coincidence...

[MrOmnos]

You might recall the 1999 shoot down of an American F117a in Yugoslavia, using an Sa-3 Goa air defense system. Stealth doesn't make it impossible just makes its a bit more difficult. Although, there might have been some hacks there is really no need to. The US defense industry is riddled with foreign agents from many nations.

Actually, that's one of the reasons I asked this question, it was detected using Russian made VHF radar.(it still used vacuum tubes) The guy who shot it down said, he was able to detect the aircraft but SAM radars couldn't lock on. But when the pilot opened the bomb bay doors, SAMs obtained a lock and 3 missiles were fired. F-117 were clearly vulnerable to VHF.  So yeah, a VHF radar with giant Yagi antennas and receiver made out of tubes, helped bring down one of the most advanced bombers ever made.

[Voodoo 6]

The SA-3 Goa has made some pretty impressive hits considering it was introduced in 1961. Russian weapon designers are at the top of the game. Veniamin Yefremov (Deceased) , general designer of Almaz-Antei, is one of the super gurus when it came to systems design.

[radar_macgyver]

My understanding is that "stealth" aircraft are optimized for low observability at X-band (~10 GHz) since that's the band used by most airborne fighter aircraft's targeting radar. They may also optimize for S-band (~3 GHz) from low elevations, since that's the band that's most commonly used for ground-based surveillance radars (that are part of a weapon system - longer range ground radars often use L-band instead).

Given this, it wasn't terribly surprising that the S-125 Pechora could detect an F-117. It also reportedly did so at close range. I found it a bit more impressive that one of these systems shot down a Predator, which would likely have an even smaller RCS, and likely be optimized for low observability to ground radar.

Radar bands are not merely bragging points (as in, "check out my pimped-out fighter with a Ka-band radar"), they're designed to a purpose. For example, radars that look for much lower RCS targets (ICBM RVs) are typically UHF or lower (GBR and TPY-2 are notable exceptions), since they tend to not be as limited by propagation losses. They are also ground-based, and are hence not size-limited.

[Voodoo 6]

Have to agree in everything Radar said, not to mention the integration of multiple bands on a single platform. like the x-aesa - l-aesa ect. As long as you can cool the elements, the sky is the limit so to speak.

[zl2wrw]

At short wavelengths (X, K, etc bands) it is practical to produce a tight beam from a small antenna (the sort that will fit into the nose of a jetfighter or a missile), this gives the radar good angular resolution, at the expense of higher propagation loss (atmospheric absorption generally increases with frequency) and shorter wavelengths are easier for a target to absorb (Radar Absorbent Material) or deflect away from the source (eg angular shape of F117).

Longer wavelengths (VHF or even the Duga or JORN over-the-horizon HF radar systems) require much larger antennae to form a tight beam (not really practical to put a VHF radar in a SAM!), but have less propagation loss, and tend to "see through" the radar absorbent coatings and special shapes of stealth aircraft (long wavelengths are hard to absorb with thin RAM layers and fancy shapes don't deflect returns away from their source when the features of those shapes are shorter than the wavelength in question).

I don't know if any military has done it yet (it would be secret), but I can imagine that an anti-stealth SAM system would probably use a low frequency radar to guide the rocket to the vicinity of the target (command guidance ala SA-2?) and then when the rocket was close enough to the target, it would use a plain old "all aspect passive IR" seeker for terminal guidance...

Dare I say it, stealth technology is probably of more use for an aircraft bombing someone else's country rather than stopping their aircraft from bombing your country.

[Bud]

Here is some news

http://www.extremetech.com/extreme/236167-china-claims-to-have-invented-quantum-radar-that-could-render-entire-us-stealth-fleet-obsolete

[Alex Eisenhut]

Isn't stealth just a way to make a jet fighter have the same impedance as free air? There's no reflections on a matched line ... so...

[radar_macgyver]

Isn't stealth just a way to make a jet fighter have the same impedance as free air? There's no reflections on a matched line ... so...

Not entirely. Some aspects of stealth (like radar absorbers) do this, but otherwise the geometry assumes monostatic radar (transmitter and receiver are co-located) so it attempts to reflect energy away from its source. Hence the funny shapes.

[Voodoo 6]

Basically what they have at the White Sands test stand is a pylon, its about 40 feet in the air. where you place your sample model on the top of, it gets zapped with every conceivable emission imaginable. The rcs is determined. Coatings and all. 

[zl2wrw]

Basically what they have at the White Sands test stand is a pylon, its about 40 feet in the air. where you place your sample model on the top of, it gets zapped with every conceivable emission imaginable. The rcs is determined. Coatings and all.

This site has some photos of an RCS test range:
http://www.otherhand.org/home-page/area-51-and-other-strange-places/bluefire-main/bluefire/radar-ranges-of-the-mojave/lockheed-martin-helendale-rcs-facility/

[Voodoo 6]

There is nothing strange about the test ranges at White Sands my friend, they are about a 1000 meters long and you can submit any model you wish to test with any parameters. We also have some nice places to eat, if you don't mind having the finest Tex-mex and a cold beer. 

[zl2wrw]

There is nothing strange about the test ranges at White Sands my friend, they are about a 1000 meters long and you can submit any model you wish to test with any parameters.

That's what I get for doing a google image search for "rcs test range" (finding that otherhand.org had some good photos, and thinking, "hey, these would be interesting to someone who has never seen an RCS test range").
Nothing strange to someone with any idea about radar, but quite possibly seems strange to someone who doesn't know much about radar (or physics) to whom radio seems "magic"...

[Voodoo 6]

Hey come on in and have a beer, and talk some radar cross sections. You can Google White Sands Missile Range. 

[Srbel]

When they say B2 spirit has RCS of 0.001 m² or F-22 Raptor has a RCS of 0.001, are these numbers valid for certain band radar or does it mean it will have the same RCS for any kind if EM radiation? What does having small RCS mean for an aircraft? Does it mean it can't be detected at all? Or does it mean, you can detect it but the RCS is so small, you can't home your missiles on it? I suspect it means, it can't be detected at all, else enemy would scramble jets.

It is just propaganda. Such a huge plane to have radar reflection of 0,0001m2 is a pure fantasy. It can be detected  with a ground based radad, like Soviet P-18.

[zl2wrw]

It is just propaganda. Such a huge plane to have radar reflection of 0,0001m2 0.001m2 is a pure fantasy. It can be detected  with a ground based radad, like Soviet P-18.

If Australian Air Power site previously linked to by Voodoo6 (repeated below) regarding China's J20 is anything to go by, whilst the B2 (which is now an old aircraft) may have a very low RCS at microwave frequencies, it probably has a significantly larger RCS at VHF and HF frequencies:
http://www.ausairpower.net/APA-2011-03.html

So yes, you probably could detect a B2 with a P18 (VHF) radar, but if your (microwave radar guided) SAM can't lock on and track the B2 (RCS is too small to produce a useful radar return, unless the B2 is close, in which case it is probably carrying a bomb with your name on it  ), you would have to find some other way of shooting at it...

[CatalinaWOW]

There isn't a binary answer to this.  Of course any stealth vehicle can be detected.  But if the detection range is cut in half it has advantages and consequences.  Reaction time is cut in half.  You might have to build twice as many radars to cover a perimeter, or four times as many to cover an area.  Coordinating between systems at different frequencies to achieve a system solution is hard, and makes your system vulnerable to disruptions in the communications. 

Who has won this interchange between many elements is only clear when actual combat occurs.  The answer is frequently a surprise to all of the "experts".  According to Aviation Week the shootdown of the F-117 was a result of many factors.  Clever use of resources by the air defenses, including using ground observers at the launch airports with telephones to warn of raid times was part of the solution as was coordination of systems at different frequencies.  Overconfidence of the attacking forces which led to using the same routes over and over allowed positioning of air defenses within their detection and engagement ranges was another contributor.  Whether one successful intercept of a first generation stealth aircraft indicates the end of stealth usefulness will be debated for years.

One hopes that combat doesn't actually occur, and that if it does, that your side wins.  The uncertainty in that answer is one of the best levers keeping combat from happening, but it obviously isn't always sufficient. 

[Voodoo 6]

Absolutely right Catalina, some folks refer to it as the fog of war, which cannot be computed.

When they say B2 spirit has RCS of 0.001 m² or F-22 Raptor has a RCS of 0.001, are these numbers valid for certain band radar or does it mean it will have the same RCS for any kind if EM radiation? What does having small RCS mean for an aircraft? Does it mean it can't be detected at all? Or does it mean, you can detect it but the RCS is so small, you can't home your missiles on it? I suspect it means, it can't be detected at all, else enemy would scramble jets.

Its great asking a question like this because it sparks so many different paths of discussion. The basic concept is long wavelengths have a better chance of detecting a low rcs. Now the question becomes once detected, can we hit it. Thats the magic.

[max_torque]

Sort of "On Topic" this, BTW, is a brilliant book, and includes a lot of stuff about the development of Stealth:



Highly recommended!   

[CatalinaWOW]

Absolutely right Catalina, some folks refer to it as the fog of war, which cannot be computed.

When they say B2 spirit has RCS of 0.001 m² or F-22 Raptor has a RCS of 0.001, are these numbers valid for certain band radar or does it mean it will have the same RCS for any kind if EM radiation? What does having small RCS mean for an aircraft? Does it mean it can't be detected at all? Or does it mean, you can detect it but the RCS is so small, you can't home your missiles on it? I suspect it means, it can't be detected at all, else enemy would scramble jets.

Its great asking a question like this because it sparks so many different paths of discussion. The basic concept is long wavelengths have a better chance of detecting a low rcs. Now the question becomes once detected, can we hit it. Thats the magic.

You missed the magic of the answer.  Stealth vehicles can be detected at any frequency.  The magic is how hard it is to do.  It is easier at long wavelengths, but hypothetically still harder than for a non stealth vehicle.  So more resources are required to achieve the same defensive effect.   Stealth isn't a magic cloak of invisibility, it is more like very effective camoflauge.

It is kind of like personal armor.  It started with people wearing corsets or tunics of hardened leather.  Did it make them invulnerable?  No.  Did it make them harder to disable or kill? Yes.  Even when armor had advanced to custom fitted steel mail with chain mail undergarments those wearing the armor weren't invulnerable.  And when longbows and firearms were invented armor didn't become totally ineffective.  It still would deflect a glancing shot, and reduced the damage of those shots that penetrated.  As it turns out the effort of building armor and training those who wore it was larger than the effort of building projectile weapons and training their users so armor dropped out of use for a while.  But it is coming back as technology has brought more effective materials that are cheaper to make and which have simpler requirements for the wearers.