Some things that *should* be in V1.11+

[GGTharos]

Heh, I got a 25nm kill against a receding target. Ludicrous ;)

 

I don't know about PN, I didn't really botehr to check, but you may be correct ... in which case, the modelling of this wasn't correct.

[SwingKid]

- the 120C has an updated guidance section (i.e. hardware), afterall.

 

Ok, now that you've repeated this a couple of times, I would like to know your sources of info about AIM-120C.

 

AFAIK, any software updates to the C can also theoretically be be programmed into the B - that was the whole point of the reprogrammable B, to prevent needing new hardware when new software came along.

 

The C is only supposed to introduce clipped fins, but I can understand how this would require the flight control software to be reprogrammed with teh new lift coefficients of the reshaped control surfaces. That would be specific to the "C", and the new coefficients would certainly give the C "improved" performance over a C that was programmed with original B coefficients, but I don't know if I would call this a guidance section "upgrade" in capability over the B.

 

The C-1, 2, .. 6 AFAIK are intended to introduce non-essentials like an improved fuze, and a new motor section that is designed to burn from both directions if accidentally ignited on board a ship, etc. Maybe some "patches" were made to the software along with these packages to fix some small bugs, but I don't remember reading about any major changes that would make the B in need of replacement. It's pretty easy to see whether the B is still in use with US forces, simply by looking for clipped or unclipped fins.

 

The way I see it, the AIM-7M, for whatever reason, is tremendously more resistent to chaff.

 

If you wish to test this hypothesis, I would propose to conduct look-up attacks against a chaffing target with both missiles. This is the only way to remove the ground clutter, which plays a major role in Lock On, from the equation.

 

Sure, the AIM-120C may be in a look down situation, but the angle of descent relative to the horizon would produce some pretty long slant ranges to the ground (and back up). On a 5-7 nm ranged radar, ground clutter shouldn't factor in that much at 5000m, certainly not enough to cause a such a huge difference in Pk between the two missiles.

 

AFAIK, you cannot get rid of ground clutter by range gating when using MPRF pulse-Doppler. MPRF and HPRF require integration of thousands of pulses in order to perform frequency filtering, and that will inevitably cause inter-mixing of reflections from distant and near reflectors, that can only be separated by frequency. I.e. if the target has a large enough RCS, the AIM-120 can see it at practically infinite range - like a heat-seeker locked onto the sun, that is billions of miles away. The ground clutter has the RCS of a planet and can only be filtered out by frequency, at virtually any look-down slant angle.

 

Maybe this is what we're disagreeing on. I'm seeing guidance as resistance to chaff and ECM

 

Resistance to chaff is based entirely on Doppler shift, and resistance to ECM is based almost entirely on a monopulse antenna. As long as you have enough processing power and software to permit using these two ECCMs, then I don't see where programming can make any further significant contribution. The best I can imagine is to fix bugs to ensure basic functionality - although I wouldn't put it past a missile maker to name such bug-fixes, "improvements for better performance." ;)

 

Sorry, I don't understand this question. Don't look-down targets actually have to be relatively close to the ground in order to blend into its clutter?

 

Generally, no. An aircraft reflects a smaller fraction of your radar energy the farther away it is. So, it has a constant RCS, and the reflected signal follows 1/R^4 rule. The ground's RCS actually increases the farther away you are, because it's larger than your radar beam - and so you simply illuminate a larger chunk of it the farther away you are, because your radar beam has spread wider at that distance. So, it doesn't obey the same R^4 rule and can be a serious problem at any range.

 

...AFAIK.

 

-SK

[D-Scythe]

Ok, now that you've repeated this a couple of times, I would like to know your sources of info about AIM-120C.

 

Just search up WGU-44/B on google. Here's a link anyway:

http://designation-systems.net/usmilav/asetds/u-w.html#_WGU

 

The AIM-120C uses the WGU-44. The B uses the WGU-41/B. I actually, however, don't know the difference between the two.

 

AFAIK, any software updates to the C can also theoretically be be programmed into the B - that was the whole point of the reprogrammable B, to prevent needing new hardware when new software came along.

 

Theoretically, it can. However, AFAIK, only the U.S. has the AIM-120C, which is the main focus of the constant updates. I'm not sure if the same updates have been installed on the -120B.

 

The C is only supposed to introduce clipped fins, but I can understand how this would require the flight control software to be reprogrammed with teh new lift coefficients of the reshaped control surfaces. That would be specific to the "C", and the new coefficients would certainly give the C "improved" performance over a C that was programmed with original B coefficients, but I don't know if I would call this a guidance section "upgrade" in capability over the B.

 

Normally, I wouldn't either. But the -120C is constantly being upgraded through the P3I program, while the -120B is not (maybe because Meteor is on the horizon?).

 

If you wish to test this hypothesis, I would propose to conduct look-up attacks against a chaffing target with both missiles. This is the only way to remove the ground clutter, which plays a major role in Lock On, from the equation.

 

I can't exactly. I'm not at home anymore (in res) :(

 

AFAIK, you cannot get rid of ground clutter by range gating when using MPRF pulse-Doppler. MPRF and HPRF require integration of thousands of pulses in order to perform frequency filtering, and that will inevitably cause inter-mixing of reflections from distant and near reflectors, that can only be separated by frequency. I.e. if the target has a large enough RCS, the AIM-120 can see it at practically infinite range - like a heat-seeker locked onto the sun, that is billions of miles away. The ground clutter has the RCS of a planet and can only be filtered out by frequency, at virtually any look-down slant angle.

 

Hmm, interesting. Awesome info.

 

Resistance to chaff is based entirely on Doppler shift, and resistance to ECM is based almost entirely on a monopulse antenna. As long as you have enough processing power and software to permit using these two ECCMs, then I don't see where programming can make any further significant contribution. The best I can imagine is to fix bugs to ensure basic functionality - although I wouldn't put it past a missile maker to name such bug-fixes, "improvements for better performance." ;)

 

Again, that's interesting. But just one question....can't they have more elaborate forms of these ECCMs/processing algos to make them more bullet-proof?

 

Besides, the AIM-7M does not exactly benefit from computer technology. The AIM-120 should have FAR more processing power, and if not used to make the missile more ECM resistent, than at least to add more features.

 

Generally, no. An aircraft reflects a smaller fraction of your radar energy the farther away it is. So, it has a constant RCS, and the reflected signal follows 1/R^4 rule. The ground's RCS actually increases the farther away you are, because it's larger than your radar beam - and so you simply illuminate a larger chunk of it the farther away you are, because your radar beam has spread wider at that distance. So, it doesn't obey the same R^4 rule and can be a serious problem at any range.

 

...AFAIK.

 

-SK

 

I see. I still don't think any radar can perform that badly from so high up though. Not even the F-4's radar, and that was 30 years before AMRAAM.

 

EDIT: SK, how bout some problems associated with SARH guidance? Like "angle off", and target orientation? Isn't it possible for the smaller guidance radar on the AMRAAM to put more RF energy on target than the RF energy recieved by the AIM-7M's seeker at certain parts of the end-game?

 

The way I see it, SARH seekers should suffer ALL the faults active radar missiles have, plus their own, as their seeker is basically an AMRAAM seeker that can't emit radar energy.

[GGTharos]

I think you can do quite a few things with processing as ECCM - you can, for example, attempt kinematic ranging against a jamming target (which restores your lofted trajectory to some degree, but may increase the intercept footprint so that actual Rmax is decreased).

 

You can try encoding data in the pulses to prevent repeaters from fooling the jammer (okay well, that's what i'd do, if you know a reason why it can't be done, by all means, I want to know)

 

You can hop frequencies, you can do histogram analyses, you can do motion pattern matching, and all sorts of other things which can help both with the initial launch /and/ the terminal homing.

 

This is all speculation of course, but I still don't get the whole 'well, processor speed doesn't matter' thing ... it -always- matters.

 

Faster computation means smaller time steps, smaller time steps probably mean greater accuracy for example.

[SwingKid]

Just search up WGU-44/B on google. Here's a link anyway:

http://designation-systems.net/usmilav/asetds/u-w.html#_WGU

 

Oh yeah.. I could be mistaken but isn't that the one with the smaller dimensions? I remember reading that one of the AIM-120 versions freed up more space in the missile tube for fuel and warhead by using smaller (not necessarily better) electronics, but that - at least initially - this extra space was not actually used for anything. Hence the new version C-1,2,...6 each of which was making different improvements in stages. This site:

 

http://www.designation-systems.net/dusrm/m-120.html

 

does refer to "improved ECCM with jamming detection," but I'm not sure what this means since even the AA-10A has "improved ECCM with jamming detection" - there are many different kinds of jamming. My best guess is that this would be an attempt to deal with terrain-bounce glint (one of the few ECMs that might still work against monopulse, once in a blue moon), that might provide no new capability at all against non-terrain-bounce ECM - i.e. it might not improve the (already superb) resistance to chaff at all.

 

The AIM-120C uses the WGU-44. The B uses the WGU-41/B. I actually, however, don't know the difference between the two.

 

My guess would be size/weight. The R-77/RVV-AE also have different guidance sections of different size/weight, depending on the manufacturer of some components, but the ECCM capability (AFAIK) remains the same.

 

Again, that's interesting. But just one question....can't they have more elaborate forms of these ECCMs/processing algos to make them more bullet-proof?

 

See "terrain-bounce glint" above.

 

You can tailor your ECCM to whatever specific threat you want to imagine, but in the words of the engineers who make ECCM - you're fighting a "paper enemy". i.e., the specific ECM your ECCM is designed to defeat will quickly be discovered by the enemy not to work anymore, and thus not be used against you, at best... or might not even exist in the enemy arsenal in the first place, at worst. It can thus be a waste of time and resources to develop in wartime, but a sweet military pork-barrel job in peacetime because you can keep selling "improvements" to pre-emptively defeat different kinds of purely imagined ECM, regardless of the probability the enemy actually using it. The best practical ECCM is "general" ECCM that is resistant to all kinds of ECM rather than a specifically tailored one-for-one countermeasure. And this is something that monopulse does really well - no matter if you're using range gating, blinking, almost whatever ECM, monopulse can still usually get a bearing lock and track you HOJ. Any further ECCM is just icing on the cake.

 

I see. I still don't think any radar can perform that badly from so high up though. Not even the F-4's radar, and that was 30 years before AMRAAM.

 

The F-4 did not use PD or pulse integration, so it could theoretically filter out ground clutter by range, if the target was near enough and the ground far away. This may be why one Bulgarian MiG-23 pilot claimed that "in some situations" (i.e. anything but look-down), he preferred the MiG-23 radar to that of the MiG-29. The original APG-63 of the F-15A even retained an LPRF "Pulse" mode, "just in case."

 

Nevertheless, if you read "Clashes" you can see that even pulse-radar F-4s were still basically blind to anything flying below them, and depended on the "Combat Tree" enemy IFF interrogator to detect low-flying MiGs. The EC-131s would even fly at low altitude to bounce their radar beams off of the sea, in order to "look up" at enemy MiGs without ground clutter - even though that ground clutter would have been hundreds of miles away.

 

-SK

[D-Scythe]

 

 

I think you can do quite a few things with processing as ECCM - you can, for example, attempt kinematic ranging against a jamming target (which restores your lofted trajectory to some degree, but may increase the intercept footprint so that actual Rmax is decreased).

 

You can try encoding data in the pulses to prevent repeaters from fooling the jammer (okay well, that's what i'd do, if you know a reason why it can't be done, by all means, I want to know)

 

You can hop frequencies, you can do histogram analyses, you can do motion pattern matching, and all sorts of other things which can help both with the initial launch /and/ the terminal homing.

 

This is all speculation of course, but I still don't get the whole 'well, processor speed doesn't matter' thing ... it -always- matters.

 

Faster computation means smaller time steps, smaller time steps probably mean greater accuracy for example.

 

Yeah, that's what I would think too.

 

Oh yeah.. I could be mistaken but isn't that the one with the smaller dimensions? I remember reading that one of the AIM-120 versions freed up more space in the missile tube for fuel and warhead by using smaller (not necessarily better) electronics, but that - at least initially - this extra space was not actually used for anything. Hence the new version C-1,2,...6 each of which was making different improvements in stages. This site:

 

From my understanding, the free-space was only made available in the AIM-120C5 - the C5 incorporated the new smaller electronics, according to Airpower Review's issue on the F/A-22.

 

So if the WGU-44 was used in initial versions of the AIM-120C, prior to the C5, then I don't know....

 

does refer to "improved ECCM with jamming detection," but I'm not sure what this means since even the AA-10A has "improved ECCM with jamming detection" - there are many different kinds of jamming. My best guess is that this would be an attempt to deal with terrain-bounce glint (one of the few ECMs that might still work against monopulse, once in a blue moon), that might provide no new capability at all against non-terrain-bounce ECM - i.e. it might not improve the (already superb) resistance to chaff at all.

 

How do you know that the resistence to chaff is "superb"? I mean, if it's supposed to be good, then we know for sure the AMRAAM got porked in that respect in Lock On.

 

And shouldn't whatever the AMRAAM suffer, the AIM-7M suffer even more? They both basically have a passive inverse monopulse seeker, except that the AIM-120's can be active too. In principle, the AIM-7 should have a lower PK (for example, two ways to defeating it as opposed to one in the AMRAAM), right?

[SwingKid]

How do you know that the resistence to chaff is "superb"?

 

Chaff = airborne ground clutter.

 

Any radar that is LD/SD is equally resistant to chaff as it is to ground clutter.

 

I mean, if it's supposed to be good, then we know for sure the AMRAAM got porked in that respect in Lock On.

 

What makes you think Lock On's AIM-120 is also susceptible to chaff? I thought we agreed the AMRAAM was being defeated in tests by ground clutter and beaming.

 

And shouldn't whatever the AMRAAM suffer, the AIM-7M suffer even more? They both basically have a passive inverse monopulse seeker, except that the AIM-120's can be active too. In principle, the AIM-7 should have a lower PK (for example, two ways to defeating it as opposed to one in the AMRAAM), right?

 

If what you're saying is, "any TARH missile must surely also be capable of SARH," then I hesitate to agree that this is a valid argument. The problem is not that the diving AMRAAM can't see the illuminated target, but rather that it CAN see the illuminated ground, because its own active transmissions, and this creates an SNR problem. These will be present regardless is the target being illuminated "look-up" by the launching fighter or not, because the diving AIM-120 is still illuminating the ground - whereas the diving AIM-7 is not. Even in the "passive" HOJ mode, the AMRAAM still transmits MPRF pulses.

 

I don't recall reading that that AIM-120 has an "inverse processed" seeker that would allow it to home on third-party illumination - inverse processing is something distinct from monopulse and is associated only with SARH. Do you have a source?

 

-SK

[D-Scythe]

Chaff = airborne ground clutter.

What makes you think Lock On's AIM-120 is also susceptible to chaff? I thought we agreed the AMRAAM was being defeated in tests by ground clutter and beaming.

 

No...?

 

You can test it out yourself, but from my experience, the AMRAAM's were mostly being defeated right at when target initiates evasive maneuvers (about 6-7 miles, not look down, target breaks left and chaffs) or right before impact - the AI "missile forcefield" affect (missile is, from what I can tell, beam, looking down, but stops guiding only seconds before impact).

 

The former can only be due to chaff, and at 50%, it was responsible for most of the misses. Can't recall how many though.

 

The only time the Sparrow missed (at 50%), it was due to the forcefield affect - the missile went stupid seconds from impact.

 

If what you're saying is, "any TARH missile must surely also be capable of SARH," then I hesitate to agree that this is a valid argument.

 

Why would a TARH missile be capable of SARH?

 

To clarify before, I said that an TARH missile should be more capable than an SARH missile, all things being equal? Because of the two ways vs. one, etc. stuff.

 

The problem is not that the diving AMRAAM can't see the illuminated target, but rather that it CAN see the illuminated ground, because its own active transmissions, and this creates an SNR problem. These will be present regardless is the target being illuminated "look-up" by the launching fighter or not, because the diving AIM-120 is still illuminating the ground - whereas the diving AIM-7 is not. Even in the "passive" HOJ mode, the AMRAAM still transmits MPRF pulses.

 

Wait, just to clarify...you said this before?

 

Maybe this is the path to a solution - it should be possible to beam the SARH missile if EITHER the illuminating fighter radar is in a look-down situation, OR the missile itself is in a look down situation.

 

Since the missile is looking down (even though illuminating radar is looking up), and chaff is being employed....shouldn't there be clutter to the AIM-7M regardless of look-up or look down?

 

I don't recall reading that that AIM-120 has an "inverse processed" seeker that would allow it to home on third-party illumination

 

It absolutely does not, AFAIK. My mistake.

 

EDIT: Just out of curiosity, do you agree that an active radar missile can recieve more RF energy than an SARH missile as it gets closer and the target (because there is no angle off, plus there might be times when the target could align itself to reflect most of the energy away from the missile's passive reciever)?

[S77th-GOYA]

Gentlemen, any insight as to the characteristics as modelled for the F-15 as to bogeys travelling away from the 15's position? The radar seems to be fairly inept at seeing targets even with plenty of closure. I usually see this happen in a look-down situation, but closing bogeys in the same situations are much more easily seen. How close to the same speed as the 15 should the bogey be in order to take advantage of a beam effect?

[D-Scythe]

As for the guidance computers/processing debate:

 

The AIM-7M version has an inverse monopulse semi-active seeker which, with digital processing, greatly improves the missile´s performance under heavy ECM and weather conditions. It also has the advantage of an active radar fuze, which together with the built-in test system has provided a more reliable missile, capable of attacking low-flying aircraft targets.

 

The AIM-7P missile has improved guidance electronics, a new fuze and an onboard computer with twice the capacity and speed of the current models. It also has the capability to receive mid-course uplink information in the same formats used by the AIM-54 Phoenix, AIM-120 AMRAAM and AGM-78 Standard missiles.

 

Seems wierd just to have an upgrade on the fuze if improved guidance electronics and stuff don't really mean anything, no?

[SwingKid]

You can test it out yourself,

 

(sigh..) I was afraid you'd say that. True. ;)

 

but from my experience, the AMRAAM's were mostly being defeated right at when target initiates evasive maneuvers (about 6-7 miles, not look down, target breaks left and chaffs) or right before impact - the AI "missile forcefield" affect (missile is, from what I can tell, beam, looking down, but stops guiding only seconds before impact).

 

Ok. I may have misinterpreted message #93. I should probably run some tests, to convince myself.

 

To clarify before, I said that an TARH missile should be more capable than an SARH missile, all things being equal? Because of the two ways vs. one, etc. stuff.

 

What are the "two ways"? I thought you meant they were TARH and SARH.

 

Wait, just to clarify...you said this before?

 

This was drawn from our messages #90 and 93. Maybe miscommunication.

 

Since the missile is looking down (even though illuminating radar is looking up), and chaff is being employed....shouldn't there be clutter to the AIM-7M regardless of look-up or look down?

 

The clutter is not the ground itself, but rather radar reflections from the ground. As GGTharos correctly pointed out in message #97, when the fighter's own radar beam is "look-up", then there is no illumination of the ground to bother the AIM-7 seeker. But when the AIM-120 is "looking down", there is - and beaming (i.e. "hiding in the clutter") becomes possible.

 

EDIT: Just out of curiosity, do you agree that an active radar missile can recieve more RF energy than an SARH missile as it gets closer and the target (because there is no angle off, plus there might be times when the target could align itself to reflect most of the energy away from the missile's passive reciever)?

 

Monostatic radar follows a 1/R^4 equation, bistatic 1/R^2. So, active has a homing advantage in theory, but I don't think "angle-off" has much to do with it - the RCS in off-angle directions can be larger or smaller than backscatter, that's pretty random.

 

It also has the capability to receive mid-course uplink information in the same formats used by the AIM-54 Phoenix, AIM-120 AMRAAM and AGM-78 Standard missiles.

 

If the AIM-7P can receive AIM-120 datalink then it has multi-channel capability. This would probably require some new hardware and software.

 

-SK

[britgliderpilot]

I'm watching and trying to learn, but this is all complicated 'n stuff :p

 

 

Keep it up!

[GGTharos]

The 7P can allegedly be fired in TWS and the F-15's radar will go STT in the final seconds of intercept. Funky no?

[Kula66]

What makes you think Lock On's AIM-120 is also susceptible to chaff? I thought we agreed the AMRAAM was being defeated in tests by ground clutter and beaming.

-SK

 

SK, if you watch AMRAAMs miss, extrapolate along ithe length of the missile ... you can see the little chaff bundle it is now aiming for - nearly every time!

[MBot]

Altough I just understand about half what it is beeing said, it is a fascinating discussion. Keep it on guys :)

 

I realy like the point SwingKid made about the reliability of the missiles. Also from my uneducated point of view, I always was under the impression that pk was primary a function of missile reliability rather than of missle "effectivenes". If the missile worked as beeing designed ( and employed within designed parameters ), the weapon would almost certainly hit. The reason missiles miss the target was because some part of the system failed. Altough this point of view generaly describes the era of the vietnam war, when no evasive actions have been taken against Sparrow shots, I believe that it still plays an important role even today.

[GGTharos]

Actually, early sidewinders could be out-gimballed easily by just pulling a 5-6G turn.

 

Evasive maneuvers do matter.

Everything, EVERYTHING contributes to Pk. Period. ;)

[Cobra360]

Actually, early sidewinders could be out-gimballed easily by just pulling a 5-6G turn.

 

Evasive maneuvers do matter.

Everything, EVERYTHING contributes to Pk. Period. ;)

 

Or flying across the sun worked a treat aswell.

[Alfa]

Why would a TARH missile be capable of SARH?

 

Because using the more powerful pulse of the launching radar will help to extend the acquisition range of the missile seekerhead. The 9B-1388 seeker for the R-37 missile(MiG-31M) uses this combination of both SARH and ARH and AFAIK the AIM-54 does too.

 

Cheers,

- JJ.

[Weta43]

True, but it's a point worth re-itterating. Most of the discussion here turns around whether or not a perfectly functioning missile will have a particular likelyhood of tracking a target & what the PK should be based on that.

If missiles have a 10% failure rate then your PK should start at .9 & go down from there (1 in 5 fails - PK max .8 etc)

[MBot]

Actually, early sidewinders could be out-gimballed easily by just pulling a 5-6G turn.

 

Evasive maneuvers do matter.

Everything, EVERYTHING contributes to Pk. Period. ;)

 

 

True, thats why I wrote specifical about the Sparrow. As far as I know, Sparrow shots were soldom evaded by the NVPAF because these weapons were used on classic intercept situations mostly and came as a complete surprise for the MiG driver. So the weapon either hit or malfunctioned.

[D-Scythe]

Just one more question I still don't get about the active radar in a look-down situation, SK.

 

Firstly, are the chances of an AMRAAM losing its target in a look-down situation significantly worse than the Sparrow's?

 

I mean, the Sparrow doesn't have to deal with ground clutter...but it does have to deal with clutter caused by chaff and ECM.

 

And off angle...I mean it like this:

 

 

Not to say that the Sparrow wouldn't get any RF returns, but just that the majority would be deflected away from it. So it has to deal with recieving less RF energy whenever the target banks or something, plus the clutter from chaff.

 

Chaff, on the other hand, reflects RF energy as per normal, so it should be a bit more of a factor, no? Because to the seeker, the RCS of the target decreased while the RCS of chaff remained the same.

 

 

Does it exist? Or am I just making things up?

 

The AMRAAM, consequently, in addition to its 1/R^2 advantage (in theory), also doesn't have to deal with times the RCS of the target changes significantly, because there's no angle off: the seeker is basically always gonna get a return, even if it is mixed up in ground clutter and stuff.

 

 

The AIM-120 still has to deal with chaff, and, unlike the Sparrow, ground clutter too. Still, it does have that 1/R^2 (theoretical) advantage.

 

And that is basically the only way you can defeat an active radar missile, whereas for an SARH you can either break the lock of the missile (through chaff) or break the lock of the illuminating radar (through beaming).

 

Right?

 

I mean, I still can't really believe that ground clutter would be so big of a problem to cause an AMRAAM or R-77 trouble at 5000 m. Maybe at 1500m or something, but yeah. It's not like the efficiency of the RF waves travelling through the air, bouncing off the ground then back up is 100%. On an 8 nm radar, there should be a significant loss of RF energy, no?

 

Sorry, I don't really know much about range-gating and monopulse seekers and stuff. And I haven't read Clash.

 

So take it easy on me ;)

[GGTharos]

The reason why the range to the ground (theoretically) doesn't matter, is that (theoretically) you will have the same amount of energy bounced off from the ground as you have sent out - it may be larger in area, but it doesn't really matter because you still see all of it, just like a flashlight beam.

 

Of course, the terrain composition matters because you may have quite a bit of absorption/scattering/reflection, and therefore your mileage on the whole slant range will vary.

 

Naturally, at some ranges the target return will be much stronger (as you stipulated) and will be easily filtered out of the clutter (in theory).

[D-Scythe]

The reason why the range to the ground (theoretically) doesn't matter, is that (theoretically) you will have the same amount of energy bounced off from the ground as you have sent out - it may be larger in area, but it doesn't really matter because you still see all of it, just like a flashlight beam.

 

But if you illuminate something with a flashlight, things get less and less bright the farther they are, until the disappear completely.

 

So what I mean to say is, if you have a huge flashlight (APG-70), it may be harder to gauge an object's (like a bat) distance because there is a small loss of brightness per unit distance, but in a smaller flashlight, there is not that much distance to find the bat in the first place (unless told where to look), and once you do find it, it shouldn't be that hard to lose it unless it is right up against the ground or some tree, or it exceeds the gimbal limits of your flashlight (wrist isn't fast enough to keep the light on the bat).

 

Naturally, at some ranges the target return will be much stronger (as you stipulated) and will be easily filtered out of the clutter (in theory).

 

Yeah, that's what I was trying to say :D

[SwingKid]

D-Scythe: 1

SwingKid: 0

 

Good grief. I just made some tests, in a look-up situation, and it confirms that I was wrong. The AIM-120 is very susceptible to chaff in Lock On - about 7% chance it will chace after each bundle, almost 50% cumulative probability it will get fooled. AIM-7 - I didn't see it chase after the chaff, not even once.

 

Firstly, are the chances of an AMRAAM losing its target in a look-down situation significantly worse than the Sparrow's?

 

I mean, the Sparrow doesn't have to deal with ground clutter...but it does have to deal with clutter caused by chaff and ECM.

 

Correct. If both the transmitter and the receiver are "looking down", then the AIM-120 can have some advantage because it "knows" the speed of the transmitter (i.e. itself), and can more accurately identify what is the Doppler shift of the ground clutter, so to filter it out. The AIM-7 has to make a "best guess" by "inverse-processing" all received signals, to determine which is the target and which is the ground clutter, and might have a wider "notch" for the target to hide in as a result.

 

If both the transmitter and the receiver are "look-up", then you have the same problem but only caused by chaff, not ground clutter. But, this doesn't seem to be the case in Lock On.

 

Not to say that the Sparrow wouldn't get any RF returns, but just that the majority would be deflected away from it.

 

Nice images. It isn't usually necessary to predict where the "majority" of energy will go though, because by the time the missile is in homing range, the 1/R^4 or 1/R^2 signal strength relation dominates the RCS anyway. RCS plays a greater role in the inital detection range.

 

Chaff, on the other hand, reflects RF energy as per normal, so it should be a bit more of a factor, no? Because to the seeker, the RCS of the target decreased while the RCS of chaff remained the same.

 

If it were a signal strength competition, the chaff would win every time, regardless of how much energy is being reflected from the target at a given aspect, because the chaff dipoles are cut to a length that is resonant with the radar wavelength, so their RCS is gigantic. (This is why the packets can be so small, and why F-4s were able to lay entire "corridors" of the stuff in advance of B-52 raids in Vietnam.) They can only be filtered out by Doppler. Imagine for example, shining your headlamp on a bicycle reflector, or on the bicycle itself. No matter which way you turn the bicycle, the reflector will always be more reflective, because that's what it's made to do.

 

I mean, I still can't really believe that ground clutter would be so big of a problem to cause an AMRAAM or R-77 trouble at 5000 m. Maybe at 1500m or something, but yeah. It's not like the efficiency of the RF waves travelling through the air, bouncing off the ground then back up is 100%. On an 8 nm radar, there should be a significant loss of RF energy, no?

 

Here is a good illustration of MPRF operation from Stimson's book:

 

 

On the one hand, it gives a pretty good indication that even at 24 nmi, ground clutter (the big black blob at the right of the scale) can easily swamp target returns. The planet just doesn't "go away" at any range, your radar just has to filter it out one way or another.

 

An LPRF radar can just filter out any signals that are received after a certain time, thus ignoring anything beyond a certain distance away, no matter how strong it is.

 

An MPRF or HPRF radar has no such luxury, because the second pulse is transmitted before the first pulse has had a chance to return. That is - by the time the signals from the second pulse are being returned to the radar, they are being mixed with longer-range returns from the first pulse. So, the returns get "collapsed" into the "range profile seen by radar". (For HPRF, you have the same effect but the collapsing interval is even smaller.) So, you have to filter out that ground clutter by its Doppler shift, because you will never beat it with target signal strength, until you're really really close.

 

In case you're wondering what the huge spike is in Zone 1 - that's sidelobe clutter. It starts at a range of h (the fighter's altitude) and has a Doppler shift of zero as long as the fighter isn't climbing or descending. It needs to be filtered out by a "guard horn" or something similar. (This is why the Russian jets in Lock On have a "second notch" for pursuit targets that are "h" distance ahead, with a similarly low Doppler shift.)

 

-SK

[D-Scythe]

Good grief. I just made some tests, in a look-up situation, and it confirms that I was wrong. The AIM-120 is very susceptible to chaff in Lock On - about 7% chance it will chace after each bundle, almost 50% cumulative probability it will get fooled. AIM-7 - I didn't see it chase after the chaff, not even once.

 

Heh, I knew I wasn't going crazy :D

 

Nice images. It isn't usually necessary to predict where the "majority" of energy will go though, because by the time the missile is in homing range, the 1/R^4 or 1/R^2 signal strength relation dominates the RCS anyway. RCS plays a greater role in the inital detection range.

 

If it were a signal strength competition, the chaff would win every time, regardless of how much energy is being reflected from the target at a given aspect, because the chaff dipoles are cut to a length that is resonant with the radar wavelength, so their RCS is gigantic. (This is why the packets can be so small, and why F-4s were able to lay entire "corridors" of the stuff in advance of B-52 raids in Vietnam.) They can only be filtered out by Doppler. Imagine for example, shining your headlamp on a bicycle reflector, or on the bicycle itself. No matter which way you turn the bicycle, the reflector will always be more reflective, because that's what it's made to do.

 

Ah, I see.

 

Here is a good illustration of MPRF operation from Stimson's book:

 

 

On the one hand, it gives a pretty good indication that even at 24 nmi, ground clutter (the big black blob at the right of the scale) can easily swamp target returns. The planet just doesn't "go away" at any range, your radar just has to filter it out one way or another.

 

An LPRF radar can just filter out any signals that are received after a certain time, thus ignoring anything beyond a certain distance away, no matter how strong it is.

 

An MPRF or HPRF radar has no such luxury, because the second pulse is transmitted before the first pulse has had a chance to return. That is - by the time the signals from the second pulse are being returned to the radar, they are being mixed with longer-range returns from the first pulse. So, the returns get "collapsed" into the "range profile seen by radar". (For HPRF, you have the same effect but the collapsing interval is even smaller.) So, you have to filter out that ground clutter by its Doppler shift, because you will never beat it with target signal strength, until you're really really close.

 

In case you're wondering what the huge spike is in Zone 1 - that's sidelobe clutter. It starts at a range of h (the fighter's altitude) and has a Doppler shift of zero as long as the fighter isn't climbing or descending. It needs to be filtered out by a "guard horn" or something similar. (This is why the Russian jets in Lock On have a "second notch" for pursuit targets that are "h" distance ahead, with a similarly low Doppler shift.)

 

-SK

 

Good stuff. Thanks.

 

Just to make sure we're all on the same page here (stuff we agree on):

 

- Ways an TARH missile can be more effective than an SARH missile include the advantage of the 1/R^2 principle, and, in a look-down situation for both launching aircraft and radar, knowing its own own speed to better filter out ground returns, and finally, only having one lock to break - it's own. SARH missiles can have its own lock broken plus the lock of the illuminating radar.

- Ways an SARH missile can be more effective than an TARH missile is when the illuminating radar is below the target and the missile is above. This eliminates the ground clutter, whereas an TARH missile still has to deal with it.

- As seen in tests, the AIM-120's susceptibility to chaff is overmodelled, so if nothing else, it should be increased (I'm hoping to the level of the AIM-7 and R-27) until V1.2 and AFM/ASM(seeker) for weapons.

 

So, in conclusion, given that everything else is equal, the AMRAAM/R-77 should, in the majority of cases, be at least equal in ECM/chaff resistence to the AIM-7M/R-27ER, but with the advantage of allowing the launching aircraft to break and leave the fight.

 

Do we all agree on that so far? Anything I missed, please tell me.