Radar Cross Section question...

[Singular Intellect]

From my very limited understanding of radar principles, stealth aircraft like the F22 Raptor have reduced 'cross sections' that can be picked up on radar. As I understand it in layman's terms, it's basically the surface area of the craft that reflects radar signals back to the source.

Now, radar absorbent material and such aside, why doesn't radar systems including classification of all flying objects based upon actual speed negate a reduced cross section counter measure?

You may have the virtual cross section of a bird, but if radar can tell it's flying at jet fighter speeds, why wouldn't that set it off, regardless of size?

Or am I mistaken with the assumption that radar can detect objects of that size?

Perhaps I'm missing something obvious here?

[Dark Flame]

From what I understand, it's not like the radars pick up a steady bird-sized signal. They pick up a small occasional scattering of radar waves that add up to bird size. This signal probably isn't even strong enough to determine if it's all from one object.

However, everything I know about this I've read from Tom Clancy novels, so I'm probably more in the dark than you!

[Count Chocula]

If you're in an F-22, the inverse square law is your friend. From what I recall, RCS is calculated on the ground against test airframes, using a variety of radar sets to determine the cross-sectional area of an aircraft that reflects radar waves back to the receiving antenna. A bird-sized RCS means that only a few tenths of a square meter of the aircraft will reflect radar waves back to the emitter. By comparison, an aircraft like an F-4 or F-15 has a radar cross section of around 5 square meters. So now we get back to the inverse square law.

If a plane 50 nautical miles from a ground radar is at the edge of detection of that set, at 100 miles, it may well be invisible, since the radar set would need to emit four times the radar energy at 100 miles to get the same return it sees at 50 miles. A lower RCS on a warplane, in this example, reduces the detection range of an enemy radar set because it reduces the amount of energy reflected back to the radar receiver, making detection of low-energy returns more difficult. It's not simply that a radar operator notices a bird moving at 600 miles per hour; the return is of such a low energy that it may not be detected, or filtered out by the radar set's declutter logic.

[Sea Skimmer]

From my very limited understanding of radar principles, stealth aircraft like the F22 Raptor have reduced 'cross sections' that can be picked up on radar. As I understand it in layman's terms, it's basically the surface area of the craft that reflects radar signals back to the source.

Radar cross section is supposed to be a measure of equivalence against a specific band of radar, not actual surface area. It compares how reflective an aircraft is to that band of radar in terms of what the equivalence would be if you just had a flat steel panel. So a 10 meter square RCS means the plane would have the same RCS as a 10 square meters of steel placed in front of the radar. The actual frontal area of the plane will probably be a whole lot bigger then that, even in non stealthy aircraft.

Normal heavyweight fighters have a RCS of around 10 square meters, smaller ones closer to 5 square meters on average. The F-22 is estimated based on what little has been said of it and open source analyst to be something like .00001 meters square. Some older aircraft have unusually large RCS because no attention was paid to reducing signature during design. Many modern planes like the F-16C have minor tweaks to reduce RCS.

RCS is reduced by several means. The most basic and important is shaping, the surfaces of the plane are blended together and angled so that they scatter radar energy off to the sides, rather then reflecting it right back at the radar. Composite materials are used in the structure meanwhile, to inherently provide less mass that can reflect radar energy. Areas which would reflect radar really well like the cockpit and engine intakes are covered by screens just like the screens on the window of a microwave. Lastly special radar absorbent material is used to simply soak up the radar energy and convert it from radio waves into heat. All of those has been known about since the second world war, it just took a very long time and a couple hundred billion dollars in accumulated R&D to make it all work well.

However no matter how good a job you do, RCS changes dramatically with angle, and will be different against different radar wavelengths. Stealth planes are optimized to defeat 3-10cm radars, which are normally used by fighters and SAM batteries for targeting. Stealth will always be best on the frontal arc, how much it drops off on the sides and rear depends on the design

The actual detection range at which radar sees an aircraft depends on its radar cross section, the power output of the radar, the radar wavelength, and the size of the radar antenna. Stealth works not by making the plane invisible to radar, but rather it makes the RCS so small that the radar cannot see the aircraft at a useful range. A stealth plane only a couple miles away from a radar site is still going to be detected. Indeed a radar may see a stealth plane 20-30 miles away or even more, but at that point the stealth plane can already release a guided bomb to attack the radar site. The radar site dies, now the path is open for other aircraft to pour through the gap. Alternatively, the stealth plane can simply fly in-between radar sites spaced too far apart and strike targets behind them, this is exactly what the F-117 did to Iraq in 1991.

The defenses can to an extent counter stealth aircraft by using bigger, more powerful radars, spacing radars more closely together, and using less precise wavelengths which stealth doesn’t work so well against.

Now, radar absorbent material and such aside, why doesn't radar systems including classification of all flying objects based upon actual speed negate a reduced cross section counter measure?

You can’t classify what you cannot detect. A stealth plane works because it does not reflect enough energy to the radar to be detected at long range. As the range decreases more and more energy will be reflected until at last, the radar does achieve detection. This works with none stealthy planes too. A B-52 bomber will be detected at much greater ranges then an F-16 fighter. However normally, even that small fighter will still be seen at 200 miles away or more, more then good enough for the defenders to react and engage it before it engages them or bypasses the defenses.

You may have the virtual cross section of a bird, but if radar can tell it's flying at jet fighter speeds, why wouldn't that set it off, regardless of size?

Radars don’t arbitrarily detect objects at any distance regardless of size. This seems to be the main flaw in your thinking. Radar isn’t some magic wand of perception like movies and games make it out to be.

[Kuroneko]

The term "cross-section" doesn't have much to do with physical size, although it is in principle jointly determined by physical size and a variety of other factors. It's just a term describing the proportion of the the relevant radar-band radiation scatters of the object, and having it sufficiently small means the object will not be detected because the reflected signal doesn't have enough power.

Just as an analogy to a recent thread, consider the drag equation:
[1] F = (1/2)ACρv²,
where A is the physical cross-section, ρ is the density of the fluid, v is the velocity of the object, and C is some dimensionless number dependent on the geometry of the object and various other things, e.g., Reynold's number. We could instead let σ = AC/2 and say
(2) F = σρv²,
and call σ is the "drag cross-section". It's affected by physical size, sure, but by itself it is of no indication how large the object is.

That sort of situation actually comes up a lot in physics. For example, the Thompson cross-section of an electron describes how it scatters electromagnetic radiation. However, physically, both electrons and photons are point-particles and thus have no positive size in the first place.