SPECTRA again...???
Thales Spectra - Wikipedia, the free encyclopedia
Active cancellation is supposed to work by sampling and analysing incoming radar and feeding it back to the hostile emitter out of phase thus cancelling out the returning radar echo.
The highlighted is significant because it is the foundation for radar detection and what SPECTRA does.
But first, we must get back to basics...
The above is an illustration of a basic sine wave. Physical wavelengths are not to actual scale but only for comparisons between operating freqs.
The above is an illustration of a basic pulse train and its major components.
Each pulse can be seen as a packet of energy. Each pulse have timestamps, as in the time indexes of when leading and trailing edges are produced. These timestamps are crucial because data processing uses them to calculate these vital target resolutions:
- Altitude
- Speed
- Heading
- Aspect angle
The shorter each pulse, the lower the energy, obvious enough. The lower the energy that impact the target, the lower the energy that will return to the seeking radar. This is not taken into consideration attenuation (losses) incurred during travel to/from target. Attenuation (losses) are physical interference such as dust and/or hydrometeors and they are a data processing category by themselves. In designing a radar system, there are dedicated engineers just for atmospheric losses.
The shorter the pulse, the closer the time indexes are to each other, creating more and more precise and accurate target resolutions as the target translate thru 3D space. In other words, showing a body moving from 1.0001 to 1.0002 seconds is better than from 1 to 2 seconds.
Therein lies the compromise. Longer pulses means more energy on target but lower resolutions. Shorter pulses means higher resolutions but lower echo energy to create those resolutions and to create a composite profile of the target that is called 'radar cross section' (RCS).
Coarse resolutions are good for long range volume search, like those in AWACS. Fine resolutions are good for target management, aka 'missile targeting'. That is why all radar guided missiles uses centimetric or millimetric freqs with very short pulses and pulse trains. If the radar in this missile can record the target translating from 1.0001 to 1.0002 seconds, it can predict where the target will be, not might be, for the next .0001 second, and the next .0001 second, and so on. The more coarse the resolutions, the more the radar will move the target from predictive to probabilistic. For a missile, this is not good.
So from the basics of radar detection, pulses are obviously crucial.
Q: How long is a pulse train, if there is such a thing as a 'typical' pulse train ?
"Sequence and waveform set design for radar and communication systems" by Jiann-Ching Guey
Since the performance of this signaling technique is most promising for very long pulse train, we further investigate its application to the Synthetic Aperture Radar in which thousands of pulses are usually used.
A: There is no such thing as a 'typical' pulse train.
Each radar system must be specifically designed for an intended mission category. Some uses trains in the thousands, some in the tens of thousands, some even more.
Agilent Technologies : Techniques and Tools for Complete Pulsed RF Waveform Analysis : Article
Time interval analysis is necessary not for a single pulse but an entire train of pulses numbering from the thousands to the millions. This analysis enables finding issues and discrepancies within a given pulse train, such as a missing or misplaced pulse, pulse repetition interval (PRI) variations and pulse width jitter. Additionally, when there are multiple emitters present, there may be a need to sort through captured signals and categorize emitters depending on their characteristics, like the operating pulse repetition frequency and variations in signal level.
Getting back to the SPECTRA...
For ease of understanding, the seeking radar will be called 'hostile emitter' and the SPECTRA is 'defensive emitter'. The word 'emitter' is appropriate because that is exactly what the SPECTRA do.
What the system does is to sample an X percentage of the incoming pulse train and create a similar, can never be exact, echo pulse. This false 'echo' pulse contains just enough pulse characteristics with slight modifications intended to mislead the hostile emitter.
It is in the sampling process that is both strength and weakness for the defensive emitter, SPECTRA or any label from anyone.
In basic radar detection, if a pulse train of 100 pulses are sent, how much of the return pulses must the system process in order to classify something as a 'target' ? Too low a sampling and there will be many false targets. Too high a sampling and crucial time is lost as the system works to determine if that 'something' is a 'target'. This leads back to the point that a radar system must be specifically designed for a specific mission category because raindrops do not maneuver as jet fighters do, so a lower sampling rate can be justified for raindrops.
Likewise for the SPECTRA, the system must have a good idea of what kind of pulses and pulse trains hostile emitters are and might be using in order to have a credible sampling rate so it can create a credibly misleading false echo. This is SPECTRA's strength.
For the hostile emitters, one way to confuse SPECTRA's sampling process is to continuously modify the pulses and pulse trains.
The above is only an example of the unknown methods and quantities of pulse trains modifications that hostile emitters can employ.
For the hostile emitter, it must remember its transmissions. One train can contain pulses of dissimilar amplitudes, one train can contains pulses of dissimilar intervals, one train can contains pulses of dissimilar freqs, one train can contains combinations of all components in different orders, and so on. In a train that might contains thousands, tens of thousands, hundreds of thousands, or even millions of pulses, this can overwhelm SPECTRA's sampling process. This is SPECTRA's weakness.
Anything beyond the basic explanations above will venture into the 'classified information' region. SPECTRA's sampling processes are no doubt secret, but then so is equally secret any first world AESA system of those countermeasure capabilities. There is no way for SPECTRA to know with absolute certainty of all hostile emitters out there to create a comprehensive library of SIGINT threats. All Thales can do is speculate on what are the most likely pulse train characteristics hostile emitters may use and create one common sampling process or several processes to counter potential threats. This educated guess is actually not that difficult to do since the knowledge on what kind of body produces what kind of RCS signatures are public information anyway.
But against a potential hostile emitter from the likes of US...Pfffttth...
Can it warm my Bun Kabob in its Microwave oven as well only 2/5th of the wavelength
