Missile Dynamics - A discussion

[Druid_]

The idea is that the lock is transfered to the higher RCS of the chaff as it leaves the aircraft. By the way, as its released its travelling the same speed as the aircraft, it doesn't come to a complete stop.

[Weta43]

Yes, but it's got almost no mass, so decelerates very quickly.

If at the outer limits of seeker guidance, a seeker ilocks onto chaff deployed by a target aircraft, though the attacking planes radar continues to track the aircraft itself, within some fraction of a second wouldn't one of two things happen:

For the SARH missile, the chaff is no longer illuminated, stops reflecting, and the missile starts searching for a target - which at a distance it should still have time to re-acquire and persue.

For an ARH missile, the chaff is still illuminated by the seeker, but is presumably rejected once its speed drops to zero, and then the ARH missile also searches and re-aquires.

So if that's the case, chaff should only be effective if the missile locks it while close enough that once the chaff stops, or moves out of the illuminating beam, the target has already left the seekers search area (though re-acquiring and hitting from a sub optimal trajectory might be different things).

Anyone know what the max distance chaff is effective in the game at the moment is ?

[Kuky]

No it doesn't come to complete stop but high air drag makes it slow down very fast... drive a car and open the window and drop something out of the window... even at very slow speed the car does compared to aircraft, what ever you chucked out the window will slow down very very fast... unless it's something heavier (because it will have more inertia in it). Higher mass will have more inertia and with regards to chaff, I don't know what material is used but I am think it's like thin metal foil/pieces which will have low mass and high surface area, hence very high drag

[Weta43]

Be interesting to know what residual speed it end up with - within a second or so I'd say it'll be doing the same speed as the air around it, but is that still forwards at a relatively high speed because it's caught up in the wash, or backwards because it's entrained in the exhaust gasses ?

[lunaticfringe]

That's not right because chaff doesn't "hang around" your aircraft, you release it and it's far behind you in instant.

 

If the dispersion is met within the length of one of the radar's resolution cells, depending on PRF and signal processing capability the change in RCS is a factor; same reason that multiple fighters in close formation cannot be separated but by the most advanced radar systems- you're making the fighter look larger when the signal is processed. On the subsequent pass, if the intended target has maneuvered effectively, that massive (albeit decelerating) cloud of chaff is still on the original course, and may now be looked at as the return.

 

Higher mass will have more inertia and with regards to chaff, I don't know what material is used but I am think it's like thin metal foil/pieces which will have low mass and high surface area, hence very high drag

 

Dielectric fiber, metal coated. Length and diameter varies by intended bandwidth denial based on a specific ratio; given that you're talking hairs, you can stick hundreds of thousands, if not millions, of varying lengths in a single chaff packet, making a single usage effective against many (if not most) different opposing radar systems operating bands.

 

Be interesting to know what residual speed it end up with - within a second or so I'd say it'll be doing the same speed as the air around it, but is that still forwards at a relatively high speed because it's caught up in the wash, or backwards because it's entrained in the exhaust gasses ?

 

What matters is not dispersion and deceleration within a "second or so", but in *thousandths* of a second, depending on the opposing radar PRF. Microwave-shiny fiber, floating in the wind, ejected from a fighter doing 400 knots can decelerate 2-300 knots in that first second alone; all that matters is that it is well dispersed within a few pulse repetitions of the opponent's radar, and that within that timeframe it's still moving at a faster relative speed than that for rejection, it's in play and moves into the realm of a signal processing challenge.

 

And if the radar's processing logic can't connotate the existence of a "new" return (the deploying aircraft) as being the *same* object as the original, albeit "expanded" target, or, perhaps- can't even SEE the fighter because *he* maneuvered to cause apparent relative deceleration, the defender wins.

[Kuky]

By the time you start your engagement you will be well within distinguishable resolution to separate aircracft in close formation so you can ignore your theory of making targeted aircraft "larger" due to use of chaff... if that were true they would stop making use of chaff ;)

[lunaticfringe]

Ain't worth the time. :yawn:

Edited December 5, 2012 by lunaticfringe
See post.

[Eddie]

If the dispersion is met within the length of one of the radar's resolution cells, depending on PRF and signal processing capability the change in RCS is a factor; same reason that multiple fighters in close formation cannot be separated but by the most advanced radar systems- you're making the fighter look larger when the signal is processed. On the subsequent pass, if the intended target has maneuvered effectively, that massive (albeit decelerating) cloud of chaff is still on the original course, and may now be looked at as the return.

 

 

 

Dielectric fiber, metal coated. Length and diameter varies by intended bandwidth denial based on a specific ratio; given that you're talking hairs, you can stick hundreds of thousands, if not millions, of varying lengths in a single chaff packet, making a single usage effective against many (if not most) different opposing radar systems operating bands.

 

 

 

What matters is not dispersion and deceleration within a "second or so", but in *thousandths* of a second, depending on the opposing radar PRF. Microwave-shiny fiber, floating in the wind, ejected from a fighter doing 400 knots can decelerate 2-300 knots in that first second alone; all that matters is that it is well dispersed within a few pulse repetitions of the opponent's radar, and that within that timeframe it's still moving at a faster relative speed than that for rejection, it's in play and moves into the realm of a signal processing challenge.

 

And if the radar's processing logic can't connotate the existence of a "new" return (the deploying aircraft) as being the *same* object as the original, albeit "expanded" target, or, perhaps- can't even SEE the fighter because *he* maneuvered to cause apparent relative deceleration, the defender wins.

 

Nice to see some sense around here. +1 sir.

[Kuky]

Ain't worth the time. :yawn:

 

That's cause you don't get it, To say dropping chaff makes you somehow more visible is just stupid

[Kuky]

If the dispersion is met within the length of one of the radar's resolution cells, depending on PRF and signal processing capability the change in RCS is a factor; same reason that multiple fighters in close formation cannot be separated but by the most advanced radar systems- you're making the fighter look larger when the signal is processed. On the subsequent pass, if the intended target has maneuvered effectively, that massive (albeit decelerating) cloud of chaff is still on the original course, and may now be looked at as the return.

 

I've highlighted the key words... you don't fly in close formation with chaff :music_whistling:

 

The whole idea of using chaff (which does work) is to create more "noise" and false contacts around and away from your aircraft (once more... you are not flying formation with it :D) to present and bait the missile into going for it.

 

And not being able to separate the contacts in this situation is because you are not in close enough range, once you are in firing range your radar will have enough power and resolution to distinguish two or more contacts flying in formation

 

the rest is bla bla bla

Edited December 5, 2012 by Kuky

[falcon_120]

That's cause you don't get it, To say dropping chaff makes you somehow more visible is just stupid

 

Off course if you oversimplify like that,but I think he made a quite good explanation without getting into

some more technical radar theory or DSP algorithms that have to deal with chaffs or ECM.

[Kuky]

I know what he's talking about, same applies to optics, it's all about resolution which determines how far out you can see and in what detail.

 

What he doesn't take into account is that distances under which you do fire your weapons are small enough for the radar to distinguish this because this cell of resolution is small enough to separate one from the other.

 

Only if you are flying dead ahead into the missile and chaff is directly behind out and lined up with radar beam, only then you'd actually look to the radar as one big blob of reflection... but this doesn't happen

Edited December 5, 2012 by Kuky

[falcon_120]

I've highlighted the key words... you don't fly in close formation with chaff :music_whistling:

And not being able to separate the contacts in this situation is because you are not in close enough range, once you are in firing range your radar will have enough power and resolution to distinguish two or more contacts flying in formation

 

the rest is bla bla bla

 

A modern radar for example operates in the I-X band range,that might be as much as 7-12 Ghz frecuency,

7-12 *10e9 pulses per second, don't you think that for about 100 pulses the chaffs might perfectly be

in the resolution cell of a radar? I don't recall how many pulses integration did use radar of this kind to better the false alarm probability but 100 pulses seems pausible enough to give a false target.

[Kuky]

A modern radar for example operates in the I-X band range,that might be as much as 7-12 Ghz frecuency,

7-12 *10e9 pulses per second, don't you think that for about 100 pulses the chaffs might perfectly be

in the resolution cell of a radar? I don't recall how many pulses integration did use radar of this kind to better the false alarm probability but 100 pulses seems pausible enough to give a false target.

 

so what? what is 100 pulses at that frequency? How do you know how many pulses it takes to give contact reading? That would be something only radar manufacturer would know and people that code the software which would be secret and I don't think even pilots would be aware of that info (as they don't need it)

 

I would rather think its about how much signal difference is there between contact(s) and surrounding noise (SnR)

Edited December 5, 2012 by Kuky

[falcon_120]

 

I would rather think its about how much signal difference is there between contact(s) and surrounding noise (SnR)

 

I'm by no means an expert in the matter,but the SNR is not so important in this context,the

SNR takes a big part in the deteccion range ,but in this context you are already well inside deteccion range.

So DSP theory about how the radar choose what is his target and what not is what mostly matter.

 

As you'll know the Rmax of a radar can be calculated as:

 

Rmax= (Ptx*G^2*lamda^2*RCS*N*E(N))/((4*PI)^3*k*To*Bn*Fn*L*SNR)^(1/4)

 

where lamda is c/Freq,E(N) is the integration eficency and N the number of pulses integrated.

 

I put this to show that SNR is in fact important,but no so much once we are well inside the deteccion range

of the target.Again,maybe someone more knowledgeable on the matter can give you a better view,because I had a

radar theory subject in my degree but that was a bit ago,and there are lots of things I should look again to remember.

Edited December 5, 2012 by falcon_120

[Kuky]

I'm not familiar with this formula so you probably know more than me here, but that SnR being on the bottom part of the fraction seems wrong as higher SnR value should give higher Rmax. Where did you get this formula?

 

quick google search and I find this:

 

The power Pr returning to the receiving antenna is given by the equation:

where

 

• Pt = transmitter power
  
• Gt = gain of the transmitting antenna
  
• Ar = effective aperture (area) of the receiving antenna
  
• σ = radar cross section, or scattering coefficient, of the target
  
• F = pattern propagation factor
  
• Rt = distance from the transmitter to the target
  
• Rr = distance from the target to the receiver.
  

In the common case where the transmitter and the receiver are at the same location, Rt = Rr and the term Rt² Rr² can be replaced by R4, where R is the range. This yields:

This shows that the received power declines as the fourth power of the range, which means that the reflected power from distant targets is very small.

Anyway, this is more that I can comprehend right now so I'll just drop it
Edited December 5, 2012 by Kuky

[falcon_120]

Yes,my fault :). That is because that SNR in the fraction is defined as the SNR needed to obtain

a certain probability of detection.

The SNR I understand you are refering to would be the Ptx/(k*To*Bn*Fn*L)

Where the Ptx is the transmission power,k is the wolfmang constant,To is the equivalent

noise temperature,Bn the noise Bandwith,Fn is the noise your own system introduce

and L is the atenuation of your signal(both because of the distance as well as rain,etc)

 

Hope I didn't say nothing wrong this time(I've seen some old notes to remember)

So with a greater power of transmission or with a lesser noise,you improve the range at wich you detect your target,

the same if you increase the RCS or reduce the frequency(this is why long range deteccion system use lower frequencies,though

this bring a lot of troubles like a much worse resolution in both bearing and distance and some more things)

Edited December 5, 2012 by falcon_120

[falcon_120]

Yes,that formula is correct,from that formula you can obtain the Rmax and with some more equivalents you obtain the formula I told 2 post above.

Sorry for my english,talking about math there is a lot of words and expresions I unknow.

[Druid_]

Just for you Kuky, have a read of this

[Kuky]

Just for you Kuky, have a read of this

 

See this test shows that chaff is actually of very low terminal velocity (and has very high drag) and falls towards earth very slowly and is easily drifting due to wind. This can only mean once you release this chaff it'll almost instantly slow down to its terminal velocity (few cm/sec)

[GGTharos]

The stuff I'm telling you comes from a USAF countermeasures study. Now there's always the devil in the details: If the seeker is too close it might flat out kinematically filter out the chaff or flare (it just flies out of the FoV too fast), but that's what we call 'using your CM's too late'. The sim does take this into account in a somewhat implerfect way, too.

 

The figures I'm giving you are correct, though the study is old. :)

 

That's not right because chaff doesn't "hang around" your aircraft, you release it and it's far behind you in instant.

 

The only thing it can do is create a "cloud" of radar energy reflection and overwhelm reflection of the aircraft and missile can switch to this, loosing track of aircaft and become spoofed missile.

[lunaticfringe]

See this test shows that chaff is actually of very low terminal velocity (and has very high drag) and falls towards earth very slowly and is easily drifting due to wind. This can only mean once you release this chaff it'll almost instantly slow down to its terminal velocity (few cm/sec)

 

I like how you keep purposely throwing out the pulse over time factor, acting as though the back-end processing takes place "almost instantly"; it almost gives your argument validity. Kind of like the equation

 

"Almost Instantly" =/= Instantly

 

Doesn't matter how fast the chaff decelerates- the *instant* it is ejected, it's a potential change in signal return. For an illuminated target hundreds, if not thousands of pulses will hit that *cumulative* return- airplane noise + chaff noise, inside the width, height, or depth of a resolution cell- your choice. Doesn't matter what your beam width is; the change will happen within the dimensions of a resolution cell, because the expansion of that chaff packet is occurring in the same thousands of a second the pulses are arriving at.

 

(And FWIW, cell *depth* never changes over range, which means an appropriate change in aspect against the antenna shifts the chaff employment dynamic to your favor.)

 

Until the signal processor has had the number of pulses (and it ain't two, three, five, or ten) required plus the time required to properly acknowledge and reject what it believes to be noise, OR acknowledge the apparent shift in Doppler between the aircraft and the decelerating chaff, the radar does not know the difference. Should the defender, over the expanse of seconds the radar/CM battle occurs employ maneuver, it may reach an angle where it's Doppler return is rejected by the processor, which is nice, but not necessarily required to defeat the weapon.

 

ALL of these aspects take place, and they ALL matter. Your choice to only focus on return vs. noise (before the combined return is in fact recognized as noise by the processor) amounts to erroneous target fixation, and obfuscates the depth of what is going on. And the best part?

 

For those few thousandths of a second that it takes before either the aircraft passes beyond the full dispersion of it's last chaff packet, OR the radar has recognized (through multiple pulses to the apparent cumulative return) there are two distinct Doppler shifts, the fighter *and* dispersing chaff appear the same as an apparent in-formation cumulative return, because the signal processor hasn't yet figured out they're separate objects.

 

These interactions are not instantaneous, no matter how badly you wish it were. *Time* is the vital factor- to disperse the CM, for pulses to be sent and return, and for the signal return to be processed.

 

If the above facts weren't the case, to use your argument from last night, they'd have stopped cutting billions of metal coated fibers into varying lengths and shoving them by the gross into chaff packets decades ago; there's got to be a cost savings in there, somewhere. ;)

Edited December 5, 2012 by lunaticfringe

[Kuky]

I think something completely got lost in translation as at times reading your last post (and GG's) I get the feeling we are no longer talking about the same thing so I'll try one more time to say what I think:

 

The chaff is of very small diametter and weight and when released it doesn't stay near the aircraft very long (within 2 seconds it's reach it's terminal velocity and efectively become stationary object in sky while aircraft continues on its flight path so they get - within those 2 seconds it's decelerate from 250-280 m/sec to few cm/sec so even after 1/4sec its speen will drop to less than 1/2)

 

Now this pieces of fibre, as I read in that PDF few posts ago, when radiated by the radar, creates current in them and radar reciever sees them as signal and if this signal is higher than aircraft return it might go for it.

 

I think you think I am saying chaff doesn't work (from that last paragraph) but that's not what I am saying... chaff should and does work (of course not in all situations) so what I was saying was stupid thing to say was that chaff deployment is bad idea because as you put it, makes targeted aircraft more visible to the radar... which actually does happen for that split of a second but the point is... radar sees a return and locks onto it and follows it... suddenly this return get bigger... next thing it should hopefully see small part of it separate and decide to remain tracking the bigger return that comes from chaff.

[GGTharos]

There is a little more to the missile going after chaff (depending on the generation of the missile/guidance radar) than the chaff just having a higher RCS than the plane though :)

 

For example, for just about every player-deployed radar guided missile in-game, chaff should be soundly rejected in all aspects by the beam. After that, there should be a chance for biting off on the chaff on the beam - less chance for more modern missiles, almost to the point of immunity.

 

With look-down there is more of an opportunity to ditch a radar missile as the aircraft can dissapear in the notch.

 

The reason for this is that by putting the missile on the beam, you make the closure of the chaff to missile same as plane to missile more or less, so it becomes more difficult to discriminate the aircraft from the chaff with the doppler filter.

 

This is sort of implemented in the sim, but not as well as it could be.

[blkspade]

I've mentioned this in another thread, but there is a method that can be faithfully reproduced where you can ditch an R-77 fired from as close as 9nm nearly every time. This was in FC2 against AI. Seeing as the AI tends to very predictably, I could get them to do the same things repeatedly in response to my flying. It was basically an experiment of employing AIM-7s against R-77s, to evade the 77 without giving up position. The R-77 seeker seems to be better at looking down as opposed looking up, the reverse seems to be the case with the AIM-120.