Radar Active Cancellation: Another form of Stealth

[The SC]

According to the article by Bill Sweetman, published in Popular Science, French Rafale fighter may be using the so-called active cancellation technique to remain undetected by enemy radars. This technique is a special type of "stealthy" ECM based on the principle of wave superposition. According to the article, Rafale has two antennas separated by the length of the aircraft. The antennas emit electromagnetic waves half a wevelength out of phase with the reflected radar signal, thus effectively reducing its intensity, making the aircraft virtually disappear from the radar's screen. Active cancellation is a highly advanced and extremely complex process. This method is not known to be used on any military aircraft, though there were speculations that Russians may be using this technique on their S-37 and possibly MiG 1.42 prototype fighters. Also it is believed that the Northrop Grumman ZSR-63 defensive aids equipment installed on B-2 bombers may be using active cancellation. Just how complicated it is to cancel a reflected radar signal can be seens from the fact that the original incoming signal from the radar will be reflected from many spots on the aircraft's body. Each spot will produce an individual reflection with its own, often unique, amplitude and phase. The amplitude of the reflection (high amplitude means that the relection would be easier to pick up than the one with lower amplitude) would depend on many factors, such as incident angle, particular type of material, geometrical form of a certain location on the aircraft's body that produced the reflectio and some other factors. The phase shift will be dictated by the wavelength of the radar signal and the location (and geometrical form) of the particular spot that produced the reflection in question. The return signal, picked up by the radar, would look somewhat chaotic, consisting of background noise and "spikes". Background noise is produced by ionization levels of atmosphere, particular atmospheric conditions affecting scattering of electromagnetic waves of a given frequency, secondary reflections (weak signals). The "spikes" are produced by the strong reflections off the certain parts of the aircraftof a particular design. These "spikes" is, presumably, the main target for active cancellation system. By removing these "spikes" from the radar screen the aircraftmay blend in with the background noise, which is normally ignored by the radar operators. It is important to understand, however, that in case of an effective active cancellation system we are dealing with an immensly complicated issue. Something that can be popularly explained with a single wave sinusoidal signal will become progressively more complex in real life situations. The enemy radar does not, however, receive all of the reflected variations of the original signal as separate entities. It either selects the strongest return signal, or averages several strongest reflections. This simplification may be used to the advantage of the aircraft, since it will only need two antennas to transmit an simulated return signal averaged over the length of the aircraft, as we can see it in Rafale.

 

[randomradio]

According to the article by Bill Sweetman, published in Popular Science, French Rafale fighter may be using the so-called active cancellation technique to remain undetected by enemy radars. This technique is a special type of "stealthy" ECM based on the principle of wave superposition. According to the article, Rafale has two antennas separated by the length of the aircraft. The antennas emit electromagnetic waves half a wevelength out of phase with the reflected radar signal, thus effectively reducing its intensity, making the aircraft virtually disappear from the radar's screen. Active cancellation is a highly advanced and extremely complex process. This method is not known to be used on any military aircraft, though there were speculations that Russians may be using this technique on their S-37 and possibly MiG 1.42 prototype fighters. Also it is believed that the Northrop Grumman ZSR-63 defensive aids equipment installed on B-2 bombers may be using active cancellation. Just how complicated it is to cancel a reflected radar signal can be seens from the fact that the original incoming signal from the radar will be reflected from many spots on the aircraft's body. Each spot will produce an individual reflection with its own, often unique, amplitude and phase. The amplitude of the reflection (high amplitude means that the relection would be easier to pick up than the one with lower amplitude) would depend on many factors, such as incident angle, particular type of material, geometrical form of a certain location on the aircraft's body that produced the reflectio and some other factors. The phase shift will be dictated by the wavelength of the radar signal and the location (and geometrical form) of the particular spot that produced the reflection in question. The return signal, picked up by the radar, would look somewhat chaotic, consisting of background noise and "spikes". Background noise is produced by ionization levels of atmosphere, particular atmospheric conditions affecting scattering of electromagnetic waves of a given frequency, secondary reflections (weak signals). The "spikes" are produced by the strong reflections off the certain parts of the aircraftof a particular design. These "spikes" is, presumably, the main target for active cancellation system. By removing these "spikes" from the radar screen the aircraftmay blend in with the background noise, which is normally ignored by the radar operators. It is important to understand, however, that in case of an effective active cancellation system we are dealing with an immensly complicated issue. Something that can be popularly explained with a single wave sinusoidal signal will become progressively more complex in real life situations. The enemy radar does not, however, receive all of the reflected variations of the original signal as separate entities. It either selects the strongest return signal, or averages several strongest reflections. This simplification may be used to the advantage of the aircraft, since it will only need two antennas to transmit an simulated return signal averaged over the length of the aircraft, as we can see it in Rafale.

He got the concept right, but he got the location of the emitters wrong. The Rafale has three emitters, not two.

This is where the three emitters are.

The Mig-1.44 and S-37 were supposed to be LO aircraft with active cancellation. But unlike Rafale, they had internal weapons bays.

I have always felt the lack of IWBs were a drawback on the Rafale. Had they designed it with the bays, the aircraft would have been relevant even in the 2040s.

 

[The SC]

He got the concept right, but he got the location of the emitters wrong. The Rafale has three emitters, not two.

This is where the three emitters are.

The Mig-1.44 and S-37 were supposed to be LO aircraft with active cancellation. But unlike Rafale, they had internal weapons bays.

I have always felt the lack of IWBs were a drawback on the Rafale. Had they designed it with the bays, the aircraft would have been relevant even in the 2040s.

You can have active cancellation with two emitters, but with 3 that is needed in Spectra, an all ESM/CM system, .(two similar signal transmitter in front of the air intakes)

 

[randomradio]

You can have active cancellation with two emitters, but with 3 that is needed in Spectra, an all ESM/CM system, maybe the 3rd emitter helps the main ones in detecting radar emissions as early as possible..

Emitters are not receivers.

The Rafale has 3 emitter locations for ECM and 3 receiver locations for ESM.

 

[The SC]

Emitters are not receivers.

The Rafale has 3 emitter locations for ECM and 3 receiver locations for ESM.

An air defense radar is transmitting at a certain frequency; the signal is bouncing off the aircraft; a receiver aboard the aircraft picks up the signal and a computer analyses its base frequency and modulations and an out-of-phase signal is generated by onboard systems to cancel out the enemy radar signal.
The main problem is that the incoming signal is complex and the reflection off the surface of the aircraft is even more complex. How do you cancel it out? How do you process so much information so quickly? But most importantly, how do you position transmitting antennae aboard the aircraft to cover the entire aircraft (since the enemy radar signal is reflected from a multitude of points on the airframe and it's reflected differently from every one of them).

 

[randomradio]

An air defense radar is transmitting at a certain frequency; the signal is bouncing off the aircraft; a receiver aboard the aircraft picks up the signal and a computer analyses its base frequency and modulations and an out-of-phase signal is generated by onboard systems to cancel out the enemy radar signal.
The main problem is that the incoming signal is complex and the reflection off the surface of the aircraft is even more complex. How do you cancel it out? How do you process so much information so quickly? But most importantly, how do you position transmitting antennae aboard the aircraft to cover the entire aircraft (since the enemy radar signal is reflected from a multitude of points on the airframe and it's reflected differently from every one of them).

The beamwidth of radar signals are large, they cover the entire aircraft. What the Rafale does is uses its shaping to force the radar signals into specific points on the aircraft where they generate strong returns or spikes and then cancel out those spikes, so that way the radar will only pick up background noise.

 

[The SC]

The beamwidth of radar signals are large, they cover the entire aircraft. What the Rafale does is uses its shaping to force the radar signals into specific points on the aircraft where they generate strong returns or spikes and then cancel out those spikes, so that way the radar will only pick up background noise.

One major, massive, insourmountable problem with it is this - active radar cancellation can only deal with one radar transmitter and reciever.

However, aren't more modern radar systems not one transmitter but multiple reciever nodes? To deal with this, you'd need cancellation transmitters installed in every part of the airframe to cancel the local wave reflection and not cancel the global reflection in one direction.

 

[randomradio]

One major, massive, insourmountable problem with it is this - active radar cancellation can only deal with one radar transmitter and reciever.

No. It can deal with as many as the emitter hardware allows. The GaN upgrade to Spectra will increase the number of radar targets attacked. There is still a point of saturation, but that would mean the enemy needs a lot of radars watching the Rafale, but that also give the Rafale the opportunity to attack those radars.

That's also why the Rafale flies below radar, it will have to deal with lesser radars then.

However, aren't more modern radar systems not one transmitter but multiple reciever nodes? To deal with this, you'd need cancellation transmitters installed in every part of the airframe to cancel the local wave reflection and not cancel the global reflection in one direction.

No. For that to happen, the aircraft should be very close to the radar, which makes the radar less useful.