# What is a Radar Cross Section (RCS) on a fighter jet?

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Everyone’s talking about how the F-35 Lightning II has a RCS the size of a Golfball but I don’t know what it means or how significant it is.

In: 5

When we send out a radar wave, we listen for an echo. How long it takes to hear the echo tells us how far away the thing is. How loud the echo is tells us roughly how big the thing is, but more precisely, how much of our radar is bounced back to us listening for it.

Stealth aircraft don’t have a very loud echo. So it would be the same as getting an echo from a much smaller object. Like a golfball.

ELI5?

Radar shows a little dot for everything it sees. Big stuff, like a passenger jet or a fighter or whatever usually makes a BIG dot. Little stuff like say a bird, a much smaller one.

Stealth fighters aren’t invisible, but they make a much smaller dot than things their size usually would. In this case, a dot roughly the same size as the one a golfball would produce.

RCS roughly means the size of the dot.

Radar reflects of radar-reflecting surfaces, like metal. By making a fighter jet out of composite materials that are invisible to radar, angling surfaces at certain angles, so that radar bounces off in a different direction, and using radar absorbing coating, we make the plane reflect much less radar than it would otherwise if it was made out of metal with rounded surfaces.
So those combined measures together, make it so that when radar hits the plane, it either passes through, bounces at different angle than back to radar station, or absorbed. The ammount of radar reflected back into radar dish is equivalent to what it would see if it scanned a golf ball.

Radar works by bouncing light off an object and the light needs to bounce just perfectly enough to do a 180 turn and go back to the radars “eye” which sees the light which means it sees the object.

Just like your eye can see a distant thing and get a feel for the objects size radar can do the same thing. It can tell understand how far away an object is, how quickly the object is moving, AND roughly how big the object is.

So how do you fight this? Well easiest are 2 things – 1 – make your plane such a funky shape that it’s almost impossible for light to do that 180 turn around back to the eye, that would make it invisible to the radar. and 2 – coat your airplane in a material that’s the radar equivalent of black paint, it absorbs the light instead of reflecting it and again the radar eye can’t see it.

So nothing’s perfect, but an F-35 is a big friggen metal object, roughly 50x15x35 feet and you *know* a good radar operator would get worried if the radar says there is a tractor trailer sized hunk of metal moving in the sky at Mach 1, I mean, what else could that possible be other than an airplane. But the F-35 has the right shape and coating that only a tiny bit of the light gets back to it eyes, roughly the same amount of light that a golfball sized hunk of metal would reflect. So the radar operator doesn’t see a flying tractor trailer, they see a flying golfball. I mean, it could be bird, or maybe even just a cloud or something. So they ignore it.

Everything has a radar cross section.

Radar works by sending out an electro magnetic pulse and then listening for a bounce back. By counting the milliseconds between when you sent out the pulse and when you got a bounce back you now have a range, and by observing from what direction you got the bounce from, you now have a bearing (there are two types of radars, directional and non directional, but Im gonna skip that for now).

So anything that can reflect the frequencies that various radars send out their pulses in has a radar cross section. The term “cross section” stems from the historical early radars where in most cases the size of the “blip” on the radar scope was really just a reflection of the thing’s size. Big ship or airplane == bigger blip.

Now, there’s a few things that influence how big your bip is to a radar: size is obvious, but there’s also:

– incident deflection: if you form the body of your plane with flat surfaces angled away from the radar, it will bounce the radar pulses alright, but it will angle them away from the radar, so it never sees the return pulse.
– this same effect can come from interesting aspects of your plane. The turbine blades inside the jet engine for one – huge reflectors of radar pulses. But if we can _hide_ the blades inside a duct or deep within a nacelle, the radar won’t get a bounce back from the blades.
– radar absorbing paints and materials: can’t send back a radar bounce if you absorb it.

Large flat rounded surfaces have _Huge_ radar cross sections: there literally is a huge area to bounce a radar signal back from. So do large straight seams between panels. So stealth aircraft will make a door or a panel with a zig-zag shaped door so the edge of the door or panel has lots of little angles that will bounce a radar signal off in different directions instead of all back in one direction (if it happens to be in the direction of the radar).

Old warships have a lot of flat vertical walls that make up the superstructure. When the angle to the radar is right, those will just bounce the whole radar pulse right back to the radar receiver. MOdern warships have flat panels that are angled away from vertical so any radar pulses are bounced up and hopefully away from the radar receiver.

So an airliner has all these large flat or rounded body panels, they reflect huge radar signals. Stealth aircraft will reflect very little, and this is on top of what their skin absorbs using special materials.

So when they say the F-35 has the RCS of a golfball, they literally mean, at the same distance, the F-35 will reflect the same about of radar reflection as a literal golf ball. THat’s pretty good. The RCS of a 747 is about the same as a barn.