Radar Cross Section Estimates

[Krishty]

Amateurs estimate the radar cross sections of various current fighters:

• Rafale
• J-20
• Su-57
• F-35A
• F-16A
• F-35 vs. J-20 vs. Su-57 Summary

And also weapon systems:

• SPEAR
• AARGM-ER

The whole blog is gold, with highly interesting details on how radars and optical systems work.

[mikew]

Certainly nicely presented.

I've always been a bit sceptical about the effort put into stealth aircraft after hearing some anecdotes about the F-117 not being particularly difficult to track over 30 years ago with 1980s era radars.
Since then, I would assume that developments in Radar signal processing technology would have outpaced aircraft design by a large margin. While a plane may only have the RCS of a bee, the radar just need to track the bees moving over 500mph.

[Krishty]

I've always been a bit sceptical about the effort put into stealth aircraft after hearing some anecdotes about the F-117 not being particularly difficult to track over 30 years ago with 1980s era radars.
Since then, I would assume that developments in Radar signal processing technology would have outpaced aircraft design by a large margin. While a plane may only have the RCS of a bee, the radar just need to track the bees moving over 500mph.

Haven’t read all articles on the site yet, but

It is a common misconception that stealth technology is short live and as radar get more powerful, soon, they will be able to out range weapon engagement envelop , thus renders all money spend on RCS reduction a waste. This impression is inaccurate because any technology that can increase a radar peak power or gain will also benefit a jammers in the same ways. And stealth has a synergy relationship with jamming .

[…]

Which mean when RCS is reduced to 1/100th the original value, the required jamming power is also reduced to 1/100th the original value

The argument is, there’s already enough bees to track, and if jamming adds 200 more supersonic bees to the echo (which requires just as much energy as emitting two fake non-stealth planes) then everything is back where it started.

The distance where you can start distinguishing the fake bees from the real ones is called burn-through distance, and it says

even if adversary radar can see through jamming of conventional assets from 400 km aways, a stealth asset can still get within 40 km of such radar using exactly same jamming system

[mikew]

Where's the jamming coming from? If it's the stealth aircraft, then it's blown its cover.
I'm just considering a 1v1 situation with a Radar vs a stealth aircraft. It's usually more complicated than that of course.

[Krishty]

Where's the jamming coming from? If it's the stealth aircraft, then it's blown its cover.

I haven’t finished the articles, but I don’t think that’s how radar works.

From my understanding, the radar receiver is always stupid and receives anything from a wide angle. It’s the radar emitter that creates a focused beam. But the receiver cannot see where its signals come from, and I think the reason is just physics.

If you wanted to see where exactly a radar signal comes from, then you’d need an aperture. The larger an aperture relative to the wavelength it observes, the better the resolution. Visible light is in the 400 THz spectrum, with wavelengths in nanometer range, so all animals develop eyes with several millimeters to several centimeters radius.

But radar is only in the GHz spectrum (search radar in the MHz) with the wavelength being 1,000–100,000× longer. So your “eye” needs to be 1000 times larger. We actually have radar receivers with excellent angular resolution, but they look like this:


Obviously, you cannot carry this around on a battle field.

So radars work by emitting a narrow beam in a certain direction, listening to an echo from the approximate direction, and if one comes, the assume the beam hit something.

Now if you send a beam towards North, but a jammer sends a fake answer from NNE, then your radar will assume a target N and the jammer’s position is not revealed.

You might choose more, narrower beams to survey the sky but the article lists lots of things that go wrong then. Specifically, it drastically reduces your range. And you get more interference from side lobes, refraction, ground reflection; reduced Doppler accuracy etc.

Maybe you’re right after all, but I don’t think these stealth plane people are stupid either.

[mikew]

For a parabolic antenna like in your picture, the receiver will have the same beam shape as the transmitter where the receiver will be most sensitive to signals coming from the same point in space as gets the maximum power on the transmit side. By rotating the antenna, you create a 'synthetic aperture' where you can determine the direction of the target by seeing where the received signal 'peaks'.
For 3D coverage you need to rotate the antenna both horizontally and vertically.

These days, instead of a parabolic dish, you have a planar array of individual Transmit/Receive elements. By controlling the phase and amplitude of the RF transmitted /received at each element, you can create multiple transmit and receive beams which can be rapidly 'swept' across an area of space.

For cost/technology reasons you may have a hybrid system combining those approaches.

[Krishty]

These days, instead of a parabolic dish, you have a planar array of individual Transmit/Receive elements. By controlling the phase and amplitude of the RF transmitted /received at each element, you can create multiple transmit and receive beams which can be rapidly 'swept' across an area of space.

Right, and Wikipedia has this awesome animation for it:


Also you compress the signal, randomize it, play with polarization, etc. to make jamming harder. The article goes into it in great detail.

How well such an array is suited for high-resolution receiving is unknown to me though.

[mikew]

In that animation, you can think of the receive beam in the same way where the received echo will be received best when the beam is pointing at that direction in space. The transmit and receive beams don't necessarily have to be the same shape though.
You can have more receive beams, so you can simultaneously receive the same echo from multiple independent channels which makes localization easier by comparing them.

It's that signal processing, particularly on the receive side, that must have been the largest advance in recent decades.
Making this stuff is much cheaper now as largely the same technology is used for cellular phone networks.

[Krishty]

This article about Radar Warning Receivers gives some numbers on phased array antennas:

Interferometers are used when high accuracy DF measurement is important and it is possible to achieve high accuracy of the order (on the order of 0.1 to 1º).Interferometer DF accuracy is determined by the widest baseline pair. Typical cavity-backed spirals, track to 6 electrical degrees, and associated receivers track to 9º, resulting in an rms total of 11º. At a typical 16 dB signal to noise ratio, therms phase noise is approximately 9 electrical degrees. For these errors and an emitter angle of 45º, a spacing of 25 wavelength is required for 0.1º rms accuracy while a spacing of 2.5 wavelength is needed for 1º accuracy. For high accuracy, interferometer spacings of many feet are required. In airborne applications, this usually involves mounting interferometer antennas in the aircraft’s wingtips.

I.e. you still need an aircraft’s wingspan for a somewhat-accurate direction measurement (can be done with ground-based mobile radars) but you also need irregular receiver placement to support different wavelengths and polarizations, etc.