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Author Topic: Mikoyan MiG-23 Avionics  (Read 77981 times)

Offline PaulMM (Overscan)

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Re: Mikoyan MiG-23 Avionics
« Reply #15 on: January 09, 2006, 12:37:32 pm »
The Sapfir-23 is generally a pulse radar that uses doppler processing of the signal. It uses "a method of moving target selection by external coherence", which is best described as clutter-referenced MTI rather than non-coherent MTI.

A true coherent pulse doppler radar produces identical trains of pulses, allowing you to process doppler information confident in the phase/frequency that the pulse was given by you. A Coherent-On-Recieve radar transmits varying pulses, but stores the information on the pulse so that on receiving the returning pulse it can be compared to the original transmitted pulse.

In contrast Sapfir-23 doesn't store the transmitted pulse information. Instead, it uses the clutter return as a reference signal.

Quote
To operate correctly the SDTs with external coherence technique needs to synchronize the target signal with the clutter phase. To simplify this task, the clutter signal received by the radar sidelobes in the second/third scan cycle is used to process the doppler filter.

I think that the use of sidelobe clutter return means that, at low levels, the problem outlined above in the quotes (targets can only be detected in the presence of clutter) should not be an issue, as the sidelobes should always be generating some ground clutter.
« Last Edit: January 09, 2006, 12:41:03 pm by overscan »
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Offline Dilbert

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Re: Mikoyan MiG-23 Avionics
« Reply #16 on: January 09, 2006, 12:45:04 pm »
It may be a matter of perspective, and in that case I'm probably biased by the Western definition, but as far as I understood, the whole point of "coherence" is to allow the integration of multiple pulses.  This can only be done when the phase between one pulse and the next is an integer number of wavelengths (i.e. the Western definition of "coherence").  Does the Sapfir-E actually perform MPRF or HPRF (Medium- or High-Pulse Repetition Frequency) pulse integration?  I don't understand how signals reflected from the ground can be argued to have the property of "coherence," when no such claim is made for the original transmitted pulses from the radar.   ???

From my perspective, it seems there was a Soviet theft or convenient re-definition of the words "coherent pulse-Doppler look-down/shoot-down" to describe "whatever radar technology we have right now" in 1970s (perhaps to claim parity with the West in some kind of negotiations?), that required them to invent a new label for what the west calls "coherent pulse-Doppler" when they finally invented that, in the 1980s, with N-019.  So, maybe this is where that strange Russian word "quazicontinuous" came from, that didn't appear in any English-language radar texts: Russian definition of "quazicontinuous" equals western definition of "coherent"?
« Last Edit: January 09, 2006, 01:01:09 pm by Dilbert »

Offline PaulMM (Overscan)

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Re: Mikoyan MiG-23 Avionics
« Reply #17 on: January 09, 2006, 12:57:52 pm »
"quazicontinuous" is best understood as "interrupted continuous wave".

I've never quite understood the difference between high prf pulse doppler with fm ranging and frequency modulated interrupted continuous wave. Aren't they basically the same thing?
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Offline Dilbert

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Re: Mikoyan MiG-23 Avionics
« Reply #18 on: January 09, 2006, 01:06:49 pm »
"quazicontinuous" is best understood as "interrupted continuous wave".

So is "coherence", by the western definition - in order for the pulse phase to be an integer number of wavelengths apart, it means that the pulses are simply on/off interruptions of a continuous wave.  Hence my suspicion that Western "coherence" = Russian "quazicontinuous", rather than Russian "coherence".

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I've never quite understood the difference between high prf pulse doppler with fm ranging and frequency modulated interrupted continuous wave. Aren't they basically the same thing?

I came to the same conclusion.

Offline Dilbert

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Re: Mikoyan MiG-23 Avionics
« Reply #19 on: January 09, 2006, 01:09:08 pm »
Quote
To operate correctly the SDTs with external coherence technique needs to synchronize the target signal with the clutter phase. To simplify this task, the clutter signal received by the radar sidelobes in the second/third scan cycle is used to process the doppler filter.

I think that the use of sidelobe clutter return means that, at low levels, the problem outlined above in the quotes (targets can only be detected in the presence of clutter) should not be an issue, as the sidelobes should always be generating some ground clutter.

Wow, you're on a roll today!  When I read that quote, I couldn't understand what it was saying at all.  I think you're right.

Offline PaulMM (Overscan)

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Re: Mikoyan MiG-23 Avionics
« Reply #20 on: January 09, 2006, 01:22:23 pm »
So I think we've got somewhere.

The Sapfir-23 is a pulse radar, uses doppler processing, and uses an external method of obtaining coherence. Hence, you could, at a stretch, say is is a "coherent pulse doppler" radar without actually lying.

The Sapfir-23 was continuously developed, the later versions used greater numbers of doppler filters for better range accuracy, and introduced additional modes to pick out targets in different situations. Later versions are better in lookdown, but still not comparable to a true pulse-doppler radar.

The N-019 is a pulse-doppler radar with true internal coherence, and hence is an interrupted continuous wave ("quazicontinuous") radar.
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Offline PaulMM (Overscan)

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Re: Mikoyan MiG-23 Avionics
« Reply #21 on: January 09, 2006, 02:21:13 pm »
Quote
Thus Lincoln Laboratory researchers attempted to make signals passing through the MTI canceler coherent by locking a coherent local oscillator (or COHO) to the phase of the return from a single range-gate sample of clutter. The COHO then ran at its own frequency until reset in phase by the clutter sample following the next transmission. This technique did not work well because of the finite width of the main-beam clutter spectrum. The COHO locked onto a spread of Doppler frequencies and produced ugly radial streaks on the PPI. A solution for the streaking problem was found by deriving the COHO phase from the average phase of a large number of clutter range elements. This technique, which became known as Time-Averaged Clutter-Coherent Airborne Radar (TACCAR), was conceived by one of the authors, Melvin Labitt

A quick search founds dozens of PDFs about "TACCAR". I'm sure they might be illuminating. Its the basis of the original E-2 Hawkeye radar.

Source


« Last Edit: January 09, 2006, 02:23:51 pm by overscan »
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Offline mrdetonator

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Re: Mikoyan MiG-23 Avionics
« Reply #22 on: January 09, 2006, 04:15:30 pm »
From my perspective, it seems there was a Soviet theft or convenient re-definition of the words "coherent pulse-Doppler look-down/shoot-down" to describe "whatever radar technology we have right now" in 1970s (perhaps to claim parity with the West in some kind of negotiations?), that required them to invent a new label for what the west calls "coherent pulse-Doppler" when they finally invented that, in the 1980s, with N-019.  So, maybe this is where that strange Russian word "quazicontinuous" came from, that didn't appear in any English-language radar texts: Russian definition of "quazicontinuous" equals western definition of "coherent"?

I was thought that the "quasicontinuous wave" consists of pulses with high repetition frequency. So, at some aspect you can say the wave appears as a continuous but in real it is not, thus the word "quasi=nearly"continuous or in russian (Квазинепрерывный режим излучения-KNI).  The Sapfir radar is thus quasicontinuous wave, a pulse radar with high repetition frequency, which at last enabled them to utilize various doppler filtering techniques.
The meaning of the word "coherence" is the same for both the east(russian) and west world, but in case of the Sapfir radar, the difference is here merely how/where the coherence is achieved. 
Then the N-019 is a radar with quasicontinuous coherent wave or "true" coherent pulse doppler or "импульсно-допплеровские РЛС с квазинепрерывным излучением".....etc
Does it make sense to you at all, because sometimes I`m confused by your explanations.... :o

Offline Dilbert

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Re: Mikoyan MiG-23 Avionics
« Reply #23 on: January 09, 2006, 05:08:41 pm »
The meaning of the word "coherence" is the same for both the east(russian) and west world, but in case of the Sapfir radar, the difference is here merely how/where the coherence is achieved.

From Stimson's "Introduction to Airborne Radar":

Quote
Coherence

By coherence is meant a consistency, or continuity, in the phase of a signal from one pulse to the next.

Are you sure that this is the Russian definition of "coherence" also?  How can reflections from the earth maintain the same phase from one pulse to the next?  I would expect the ground clutter reflections to be filled with phase noise - i.e. to have completely random phase from one pulse to the next.

Offline PaulMM (Overscan)

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Re: Mikoyan MiG-23 Avionics
« Reply #24 on: January 09, 2006, 11:52:04 pm »
NOTE: I am not a radar engineer. This is my opinion ;)

Given that the "MV" mode has a fixed, known scan pattern, the average phase of the ground clutter return will vary with the speed of the host aircraft. If you therefore take the average of a number of ground clutter returns over a reasonably short period of time, during which the velocity and altitude of the host aircraft can be regarded as constant, the average phase shift is likewise constant (hence 'time-averaged clutter coherent'). This defines a baseline "ground return" which you can compare to any individual returned pulse. A return from a moving target that is moving significantly differently from the ground will be a "spike" away from this 'average noise'.

Paul

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Quote
COHERENCE is the concept generally applied to harmonic oscillations:
u(t) = V0 sin(wt + y)
Two or more harmonic oscillations are termed coherent at the interval Tc if the phase shift between them is constant for the whole interval Tc. In radar, coherence is considered in a broader sense, and typically the signals are considered to be coherent if their phase structure is linked and the character of this linkage is known.

Source:
  • David K. Barton & Sergey A. Leonov, eds, Radar Technology Encyclopedia, Artech House, 1998
« Last Edit: January 10, 2006, 02:39:06 am by overscan »
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Offline Dilbert

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Re: Mikoyan MiG-23 Avionics
« Reply #25 on: January 10, 2006, 02:21:50 pm »
Given that the "MV" mode has a fixed, known scan pattern, the average phase of the ground clutter return will vary with the speed of the host aircraft.

I think you might be confusing "phase" with "Doppler".

The Doppler frequency profile of the ground clutter will shift according to the speed of the host aircraft, yes.

"Phase" refers to something else.  A 10 GHz radar signal like that of a fighter radar will pass through a full 360 degrees of phase in a space of only 3 centimeters - much, much smaller than the swath of earth being illuminated.  The ground clutter signal will be the sum of reflected signals from many different points on the ground, and thus the phase of this combined reflected signal will be something totally random and unrelated to anything.  To apply the word "coherent" to this signal makes about as much sense as saying white noise is "coherent" - you can say it, but it's meaningless, because then there doesn't anymore exist any signal in the universe that could be called NOT coherent.

And therein lies my objection.  By the western definition, Stimson is able to provide concrete illustrative examples of two signals - one coherent, and one non-coherent, and all is understood by the comparison.

This is impossible for the Russian definition, because it says that all waves are coherent.  Without an example of a non-coherent signal to compare against, the word "coherent" becomes devoid of meaning.  It's like trying to draw a map of the continents, after you have redefined "land" to also include "water" - or, trying to see stars during the daytime, or a snowman in a blizzard... there's no contrast to separate one from the other.

Quote
In radar, coherence is considered in a broader sense, and typically the signals are considered to be coherent if their phase structure is linked and the character of this linkage is known.

This is not a definition, it is a plea for acceptance, at best... or simply nonsense at worst.  ("Character of the linkage of the phase structure"   ???)

A plea that I must objectively refuse, until Leonov (or anyone who agrees with him) can give a counter-example: what is a non-coherent signal?

Any example he gives, I will be able to use his own definition against him, to prove that it's coherent, exactly as they have done with ground clutter - one of the noisiest, most random signals that could be imagined!

Of course you are completely innocent in this overscan, and I don't know what to say...  As a self-professed electrical engineer, I feel shamed by this turn of events - that someone should open an Artech House textbook and find gibberish inside!  Who are you supposed to believe now, an anonymous, faceless forum poster named after a cartoon character, to correct this mess??  It's a travesty.  Bankers, lawyers, doctors, politicians, pilots, economists... these people can tell you hand-waving meaningless feel-good nonsense and I don't mind, that's life.  But a degreed engineering professor writing a textbook?!?  Champion of the one cold, hard, true mathematics that never lies?!?  Is nothing sacred?!?  I can't express the personal shame and humiliation I feel to be in any way associated with this - after all the times I encourage people to research, to read books, and now this.  On behalf of my colleagues - I'm sorry.  Really I am.   :-[

Offline PaulMM (Overscan)

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Re: Mikoyan MiG-23 Avionics
« Reply #26 on: January 10, 2006, 11:31:48 pm »
I realised after reading back what I wrote that I was confusing frequency and phase.

However, how do you account for the Western use of "TACCAR" (Time-Averaged Clutter-Coherent Airborne Radar) to describe a clutter referenced MTI technique used on the E-2? In what way is it coherent?

« Last Edit: January 10, 2006, 11:43:05 pm by overscan »
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Offline Dilbert

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Re: Mikoyan MiG-23 Avionics
« Reply #27 on: January 11, 2006, 08:29:24 am »
TACCAR relies on a coherent local oscillator (COHO) as the reference signal.  The signal from this oscillator must fully satisfy the western definition of "coherent" - maxima are separated by an integer number of wavelengths, with extremely low phase noise.

Does Sapfir-E have one of these as well?  I thought that it was using the ground clutter reflection itself as the reference.

Offline PaulMM (Overscan)

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Re: Mikoyan MiG-23 Avionics
« Reply #28 on: January 26, 2006, 12:52:58 pm »
Sapfir-23 (Izdeliye 323)

From Phazotron museum, missiles.ru.

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Offline crossiathh

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Re: Mikoyan MiG-23 Avionics
« Reply #29 on: January 26, 2006, 02:01:48 pm »
Nope, this is not a 323, this should be a Sapfir-23MLx Radar. The Sapfir-23DIII (item 323) or Sapfir-23E (323E) has the transmitter/receiver centered.
BTW, the yellow wave guide on top is part of the, so called, "compensation channel". Introduced with the RP-22 radar (MiG-21 variants and MiG-23MS) it was one step to reduced the minimal operational altitude from 1500m to 1000m (because of ground clutter). It was also used with the Sapfir-23. Normally extend above the fixed part of the radar antenna, except the Sapfir-23DIII and Sapfir-23E. There it is directed to the moveable part of the antenna. You can see it on the first picture of mrdetonator's description about the Sapfir-23E.
This channel was activated only in "SMV" radar mode (scan lines above horizont Hs < 1.5km).