was reading about different stealth technologies used by modern aircrafts to avoid radar detection. Wouldn't it be easier to have a receiver on the airplane listening on the radar frequencies and then re-transmit that same signal but phase shifted 180 degrees? (i.e. similar to how noise cancelling earphones work.)
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To get an idea of the difficulties, we will consider a similar problem with sound. There are places with names like "Echo canyon". If you shout, the sound reflects of a nearby canyon wall and you hear an echo. We want to prevent the echo.
One proposal is to put a person at every point on the cliff. Each person's job is to listen for a shout. If they hear one, they determine how loud the shout is and which direction it comes from. They quickly generate a matching shout of the same loudness and send it back toward the shouter. If they hear multiple shouters, they send a matching shout to each one. They avoid sending it in other directions to avoid alerting other people who aren't shouting.
Your ears don't tell you much about which direction sound comes from. But with a parabolic dish and an array of detectors, you can figure it out.
Ordinarily, shouting back would make the sound coming back louder. But sound is a pressure wave. If you are extremely fast, you can measure the precise time of the max and min pressures. If you are a very talented shouter, you can shout at just the right time so that your max pressures match the shouter's min pressure. If you do this just right, your shout cancels the reflected shout.
Let us apply this idea to a plane. A radar on the ground sends out a radar beam (which is light with a very long wavelength). If a plane flies through the beam, some is reflected back. It is proposed that the plane generate a radar beam to cancel any reflection.
You have to cover the smooth aerodynamic surfaces with parabolic dishes. This will affect how the plane flies. Perhaps you can be clever and just use a few dishes.
Radar frequencies are much higher than sound frequencies. So you have to be even faster to react.
People might counter this with a radar beam that switches frequency in a complicated way. This would be hard to reproduce on the fly.
The usual ways of avoiding trouble are easier.
Metal surfaces are mirrors to a radar beam. You carefully choose the angles of all the surfaces so the reflected beam does not go straight back to the radar. If it goes someplace else, there probably isn't a radar receiver there. If there is, the reflected beam won't stay on it more than an instant. You sometimes see a bright flash of reflected sunlight from a car window as it lines up just right with the sun. It would be like that.
You coat the plane with the equivalent of black paint. This reflects very little of the radar beam.
You might fly low so that if a radar sees you, you are quickly over the horizon. Or you might fly high so you are out of reach if they shoot at you. Either way, flying really fast helps.
As comments have pointed out, coating a plane with parabolic dishes is not how you would do it. A phased array would be better.
Think of coating the cliff with shouters. Each one closes his eyes and listens for a pressure wave. Pressure can be high or low at a spot. It doesn't have a direction.
When a wave arrives each shouter generates a counter pressure, which is also without direction. If you create a high pressure at a point, it spreads out in all directions. So how does this become a wave in one direction? It comes from all the shouters acting together.
A shout from Echo Point arrives at the cliff. Some parts of the cliff are nearer than others. The high pressure arrives at the nearest shouter first and moves on to more distant ones. As it passes, each shouter counters the pressure near themselves. The total effect is that the whole wave is negated. The shouters acting together have created the opposite of the arriving wave.
Suppose they repeated this performance when no wave was arriving. They would create the same opposite wave all by itself. This wave would travel in the opposite direction of the would-be arriving wave and arrive at Echo Point. This is how a phased array works.
It is a little counter intuitive. All the shouters created individual shouts that spread out in all directions. But the total adds to a wave in one direction. In all other directions, the individual shouts cancel each other. To perfectly recreate an incoming wave takes an infinite number of shouters at every point. With a finite number, you can still do a good job.
Control of phase is crucial to this. If the shouters do not match the phase of the incoming wave, sound does spread out in all directions, like the roar of a crowd in a stadium. Or perhaps like individuals standing at just the right time to make a wave circle around the stadium.
mmesser314
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The essential problem with that is that transmitting emits energy. You can cancel out the reflection in a particular direction this way, but in other directions it won't cancel. In effect, you're saying "here I am" to receivers monitoring the radar frequency. They don't even need to transmit, so you won't know they are there.
John Doty
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from https://doi.org/10.1109/WDDC.2007.4339412
Both answers are correct in explaining why cancelation is a bad scheme to avoid radar detection of an airplane. But repeating the incoming radar signal of the enemy to confuse its receiver is a standard EW (electronic warfare) technique. There are many versions of a repeat back jammer but none of them is likely to be applicable to a stealth aircraft trying to be flying under the radar, so to speak. If stealth is not an issue then a repeat back jammer by repeating the exact same radar signal delayed or phased can confuse the range estimate, of course, while assuming that the enemy receiver is prevented to receive its own pulse. Disadvantage is that pulse repetition can only be delayed not advanced, and so it is expected by the receiver.
A much more sophisticated and difficult to execute idea but more along the lines you were suggesting is the so-called cross-eye jammer. An airplane target is quite a large structure compared to a microwave radar's wavelength and it has, consequently, several and mostly independently phased reflection points. When the radar wants to estimate the angle of arrival these reflectors cause "glint", a kind of scintillation in the receiver, a jitter and "wandering" in angle. To add to that confusing image the target could apply this repeat scheme specifically aimed at a weakness in the most commonly used angle measurement technique that employs 2, 3 or 4 antennas in a so-called mono-pulse arrangement. If you try but cannot find much information in the public literature about "cross-eye" then you will know that it is because does work... Nevertheless it is not an easy thing to do it right.
hyportnex
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