Can someone explain
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What aircraft radar cross section is
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How the F-117 and other stealth aircraft can have radar cross sections as small as a fly
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How some aircraft can have radar cross sections larger than the aircraft’s own cross section
Can someone explain
What aircraft radar cross section is
How the F-117 and other stealth aircraft can have radar cross sections as small as a fly
How some aircraft can have radar cross sections larger than the aircraft’s own cross section
Comments
Radar cross section is a measure of how detectable something is to radar. Radar works by radio waves being emitted, hitting an object, then bouncing back to the receiver, bigger objects or objects that reflect more waves straight back to the receiver have a bigger cross section than smaller objects or objects that can absorb/redirect the radiowaves.
This is not true, other air craft have radar cross sections also incredibly small. But it’s also a function of the aircraft itself, like the F-117 is TINY compared to a B-2 Bomber.
Because RCS is a fictitious area. It’s an estimate based on how things show up on radar screens on average. It isn’t directly tied to the size of the object. Like for example, a normal plane isn’t going to bounce ALL the radiowaves back to the receiver. But if you had a device that INTENTIONALLY bounced as much of the radiowaves as possible back to the receiver, that device would appear “bigger”.
Radar works by bouncing radio waves off of things, and detecting the waves that bounce back.
The characteristics of the return ‘bounce’ picked up by the radar receiver determine the radar cross section. It doesn’t correspond directly to actual physical size, and what the returned signal looks like to the radar depends on a few different things.
That said, the size of the object is of course a big factor, as a large object naturally has more surface for the radio waves to potentially bounce off of.
The answers to your other two questions are related.
Stealthy aircraft have small radar cross sections because they’re specially designed to reduce their radio wave ‘bounciness’. They do that in multiple ways, such as by having shapes that make the radio waves scatter away instead of bouncing back the way they came, and special surface materials that absorb some of the energy from the waves. And probably also technological countermeasures that can destructively interfere with the radar signal, but someone more knowledgeable about such things would have to chime in on that front.
So for an object to have a radar cross-section bigger than expected for its physical size, the opposite would be true. Maybe they have an extra ‘bouncy’ shape that reflects a greater proportion of the radio waves, or are made from an especially radio-reflective material.
There’s a Wikipedia article for radar cross section that goes into quite a bit of detail about it.
Radar bounces off stuff and you measure how much returns to you. Complicated shapes like an airplane will bounce some of the waves into the sky or into the floor or absorb a portion of it, so you only get some of the radio waves you sent out to return to you. However a hypothetical giant flat sheet would return almost all of the waves right back to you.
So the amount of radar that bounces back from a given airplane is the same amount of radar that would bounce back from a flat sheet of X area.
A way to quantify how much radar will return to the source after interacting with an object. It’s relative to an equivalent flat sheet that perfectly reflects all the waves that hit it in the objects current position.
If you shape your plane to deflect those waves or use materials that absorbs those waves the radar source will only receive a fraction of those waves back. That might end up being equivalent to what a small bird or bug returns in certain circumstances.
Same way that you can deflect waves you can concentrate waves. Because RCS is relative to a flat surface if the object reflects more than that shape the RCS is now bigger than its physical 2d shape
Finally I will say that RCS is a tool to help visualize how stealthy an airplane is. In reality engineers use decibels as that’s much easier to mathematically compare how much energy is sent out from an emitter and how much energy the source gets back.
So, the radar cross section (RCS) is a way to put a number on how much does an object stand out on radar
So, firstly, how radars work, is that they shoot out tiny pulses of radiowaves into the sky, wait for them to reflect back, and if the pulse ever does come back, there’s something there, and we can look at the time it took and a bunch of other processing to find out stuff like distance and speed. If nothing ever comes back – it must be empty sky out there
Now, no machine is perfect, and if there’s something out there, but the reflection is small and weak and frail enough, the radar won’t notice the reflection, and will think it’s empty sky out there, when there’s maybe a fly 100km away actually
Now, how far away we can detect a thing depends on how much it reflects. RCS is a way to put a number on that reflectivity. Essentially, the idea is: “If this object was replaced by a perfect sphere, how big would it need to be to reflect the same amount?”. Or specifically, the cross-section of that sphere, hence radar cross-section
Now, onto the stealth aircraft. The main trick they rely on is reflecting the radar pulse everywhere except back to the radar. That’s why say F-117 is so angular in its shape. A radar pulse comes from the front, and the plane reflects it left, right, top, down – but not back forward, as much as the engineers could manage. Which means that like, a random cloud to the left gets a lot of that radar pulse – but the radar itself sees very little of it
There’s also some high tech stuff like absorbent coating and other classified tricks to squeeze out even more stealth, but the core principle remains specific shapes to reflect, essentially, sideways
Now, aircraft with RCS larger than themselves are essentially doing the opposite – being really good at reflecting the radar pulses exactly where they came from instead of just evenly all around.
For example, imagine I built a radar lens that is specifically designed to catch a radar pulse, and send all of it back in as focused fashion as possible. Now, come back to the definition of RCS – a sphere that’s the right size to reflect just as much as the object in question. I’d need a huge sphere to bounce as much of the pulse back to the radar as a dedicated lens could. It’d be way bigger than the lens. Therefore, the lens’ cross section is bigger than itself
At least, as long as the lens is actually focusing on the radar. This is also a demonstration of how RCS can be directional – that lens would reflect loads at wherever it if focused, but maybe not as much from the side on
Cross section is the equivalent frontal area in terms of radar signature. For example if your plane has an RCS of 1 square meter then it has the same signature as a flat plate with 1 square meter of area. As in you put a flat plate with x area on the radar test range and the airplane under the same conditions they will have an equal return strength.
The design has materials that absorb radar energy and reflect it at angles other than directly back at the transmitter-receiver.
You can think of RCS like aerodynamics. A car can have a frontal area of 4 square meters but with a 0.25 coefficient of drag it has the same drag as a 1 square meter flat plate. A parachute has a coefficient of drag >1 meaning it produces more drag for its shape than a flat plate would. Radar RCS is similar, the shape and materials produce a signature which a flat plate of a certain area would also produce this signature. This equivalent flat plate can be smaller, bigger, or equal in size to the object.
RCS is also angle and wavelength dependent. A high frequency radar and a low frequency radar may have a different RCS for an object because the object is better at being stealth against one wavelength over the other.
The way it’s actually measured is similar to sound and light. The amount of energy coming back. Essentially: when we point a radar at this thing, how much of an echo in decibels or reflection in lumens do we get off of it?
The radar cross section in square meters is there as relatable scale. Known-sized objects are used as a baseline against which others things can then be compared.
E.g. a 3sqm mirror reflects a lot of light. Angle it away, or cover it in a sheet and the mirror remains the same physical size but it reflects as much light back to you as a much smaller object.
Much (but not all) of the explanation is is how reflection happens for different shapes. Right angles have the property of reflecting waves back the way they came, which is great for radar.
If you go into a bathroom that has two mirrors meeting at right angles, notice you can always see a reflection of yourself right in the corner where the mirrors meet. If you had a third, horizontal mirror, at right angles to the other two, then the trick works in 3D. That’s how many “cats’ eye” reflectors work on roads, and explains how brightly they reflect back their headlights to the driver in the dark. They’re called “corner cubes” because it’s like the way three faces of a cube meet at a corner. Sometimes large metal corner cubes are used as radar reflectors, to make something stand out brightly.
Most aircraft have surfaces that meet at right angles, e.g., where the wings meet the fuselage and where the engines are mounted to the wings. This makes them easy to spot on radar. Notice how stealth aircraft try hard to avoid right angles. Stealth navy vessels do the same, including sloping the sides of the ship inward to avoid making a right angle with the surface of the water.
Having flat surfaces can also help, because they will reflect all at the same angle. A curved surface will reflect over a range of angles, so it’s more likely that some will go back the way it came.
RADAR works basically the same way road sign reflectors work with light. Your head lights emit photons which hit the road signs that have tiny shapes built in that bounce the light directly back to the source. Radar works much the same way
If you’re sitting on the second floor of a bus, you won’t see the road signs reflecting the headlights because the light is bouncing and returning to the source, not your eyes. This is similar to how an F-117’s cross section works. It’s angled so there are no right angles, and the radio waves don’t bounce back to the source, so it doesn’t ‘light up’ like the sign doesn’t light up for you