Craig Alpert wrote:A Stealthier Rafale?
Active cancellation means preventing a radar from detecting a target by firing back a deception signal with the same frequency as the reflection, but precisely one-half wavelength out of phase with it. Result: the returned energy reaching the radar has no frequency and can't be detected.
It's quite as difficult as it sounds. Some reports have suggested that the so called SP-3 or ZSR-62 "radar jamming device" planned in the early days of the B-2 program was an active cancellation system. It did not work and was scrapped in 1987-88. In 2005, Northrop Grumman paid $62 million to settle a False Claims Act case involving the system.
This may not be the first French attempt to implement AC on the Rafale. At the Paris air show in 1997, I interviewed a senior engineer at what was then Dassault Electronique, about the Rafale's Spectra jamming system. He remarked that Spectra used "stealthy jamming modes that not only have a saturating effect, but make the aircraft invisible... There are some very specific techniques to obtain the signature of a real LO aircraft." .....................
The major problem with the SPECTRA-like method of active cancellation is that the system must sample a portion of the seeking radar's pulse train... to generate a fake dummy , out of phase dummy pulse.
Check Here http://en.wikipedia.org/wiki/Radar_sign ... cteristics
The radar pulse train is a form of square wave, the pure form of which consists of the fundamental plus all of the odd harmonics. The exact composition of the pulse train will depend on the pulse width and PRF, but mathematical analysis can be used to calculate all of the frequencies in the spectrum. When the pulse train is used to modulate a radar carrier, the typical spectrum shown on the left will be obtained.
Basically...The transmitter turn itself on...Wait for a specified period of time...Then turn itself off. The rise and fall in power level constitute a pulse. Several pulses in a sequence make up a pulse train. How long is that sequence depends on the radar's intention and design. The appropriate analogy is a real locomotive with the gaps between sections.
A sample of several pulses must be studied. If the system take too short a sample in order to study the pulse train's signal characteristics to create a credible cancellation, the rest of the pulse train or next pulse train will reveal the aircraft,
or may be its possible to neutralize this pulse by sending a out of phase pulse from Spectra along with reflected wave back , given processing of radar pulse by Spectra is ultra-quick and spectra generates a out of phase dummy pulse.
Remember the active cancellation wave that Spectra is sending also contains location of Rafale itself, if wave dosent cancels the returning radar signal , both combine in phase to give an even more accurate position of aircraft (Rafale in this case)
If the system take too long a sample, then the aircraft will be revealed anyway by the current pulse train. The seeking and therefore hostile radar can change the characteristics of each pulse train from one to the next.
Pulse repetition frequency (PRF)
In order to build up a discernible echo, most radar systems emit pulses continuously and the repetition rate of these pulses is determined by the role of the system. An echo from a target will therefore be 'painted' on the display or integrated within the signal processor every time a new pulse is transmitted, reinforcing the return and making detection easier. The higher the PRF that is used, then the more the target is painted. However with the higher PRF the range that the radar can "see" is reduced. Radar designers try to use the highest PRF possible commensurate with the other factors that constrain it, as described below.
The hostile radar can change the PRF from one train to the next, forcing the SPECTRA-like system to constantly recalibrate itself. The hostile radar is employing the tactic called 'PRF jittering'. The 'jittering' technique is common when the radar is operating in an electronically dense environment BUT the 'jittering' sequence is predictable. The radar using this technique will remember the exact sequence of the many PRFs and will perform the appropriate correlation to eliminate unwanted signals that may come from other radars in the vicinity. Civilian airports are places of where predictable 'PRF jittering' technique is used.
But for military purposes like ECM and ECCM, predictability is not always desirable and a 'non-cooperative target' is always looking for predictability. If the 'PRF jittering' sequence is known, this SPECTRA-like system will work as advertised. If the 'PRF jittering' sequence is not known, the system will create many anomalous echoes for the seeking radar, one moment the system successfully canceled the pulse train but with the new pulse train with a different pulse train characteristics the system must resample, which at the very least will make the seeking radar operator suspicious. The goal is to make the operator unaware, not suspicious.
Dassault has tried to sniff these particular Pulse sequence from SU30mki/F15E during Redflag 08, and update SPECTRA's so called digital threat library.
Now if you compare - SPECTRA's Digital threat library is regularly updated but, this library can only hold A fixed information of characteristic Pulse pattern and Jittering sequence. That means Radar such as SLOT-Array or PESA's which always operate at particular fixed pattern bcoz there is portioning of Single Frequency beam
When you talk of modern AESA radar where each sub array / TR module operate in different independent freq changing every milli sec its impossible for SPECTRA to hold information of each wave pattern and generate new dummy pulses.Finally to conclude
- A SPECTRA-like system is a good idea in application , but not against first-tier militaries or Some second-tier militaries who can manage to purchase first-tier radars(LPI AESA) and they will find the aircraft. The balance between sampling and response is what make the SPECTRA system difficult to employ precisely because of the unpredictability of potential adversaries.