F-15C avionics wishlist

[SwingKid]

I think other methods of defeating ECM should be considered as well, like the ability to time-code pulses for example.

 

Can you provide a reference that says time-coded pulses are useful against DECM? I still think they are only good against white noise, or for improving range resolution.

 

-SK

[D-Scythe]

Just a quick question, a bit OT for this thread. We've been talking about ECM and stuff on fighter radars...but what affect would they have on the radars on the R-77 and AIM-120? Furthermore, what about AAMs/SAMs with active radar fuses? Most missiles have a tiny, cheap radar to set off the fuse once the target is close enough, but what it also be possible to model the SPJs on some fighter aircraft to detonate incoming missiles prematurely?

 

And theoretically, if the small active radar on an AIM-120 and R-77 is accurate enough to successfully guide the missile within the kill zone of its warhead (i.e. less than 50 ft), shouldn't the bigger, more expensive and much more complex radars like the APG-63(v1), Slot Back, etc. use the same methods as the guidance section of the missiles, and also much more advanced and accurate techniques, to provide a better counter to ECM, and much better target precision?

[SwingKid]

Just a quick question, a bit OT for this thread. We've been talking about ECM and stuff on fighter radars...but what affect would they have on the radars on the R-77 and AIM-120?

 

It depends on the ECM. Generally speaking though, it would turn such missiles into a "big Sidewinder" - they don't know the range to the target, they just follow the target emissions until they hit. So, the Rmax is reduced because there's no loft, and the missile flight trajectory will have more drag.

 

Furthermore, what about AAMs/SAMs with active radar fuses? Most missiles have a tiny, cheap radar to set off the fuse once the target is close enough, but what it also be possible to model the SPJs on some fighter aircraft to detonate incoming missiles prematurely?

 

Radio fuzes are pretty sophisticated, short-range and I think potentially difficult to jam outside their lethal radius - the R^4 range equation benefits the radar, not the jammer, at short ranges - one of the cases where burn-through works. Such missiles have been known to detonate when they fly through chaff clouds, however, leading to the recent laser fuzes.

 

And theoretically, if the small active radar on an AIM-120 and R-77 is accurate enough to successfully guide the missile within the kill zone of its warhead (i.e. less than 50 ft), shouldn't the bigger, more expensive and much more complex radars like the APG-63(v1), Slot Back, etc. use the same methods as the guidance section of the missiles, and also much more advanced and accurate techniques, to provide a better counter to ECM, and much better target precision?

 

Sure - the APG-63 can guide the F-15 to collide with the jammer, no problem. Just follow the strobe. Why would you want to?

 

-SK

[SwingKid]

In addition, a missile could still loft against its target, and in fact help itself to a range determination by vertical displacement.

 

If this were true, then the Sidewinder would be doing the same thing. I've never read of any missile lofting while homing. Loft is a ballistic flight profile, with the seeker not on the target.

 

Besides, the KR technique is preferable for estimating target range to within a dozen nm to make a shoot/no-shoot decision when nothing else is available, but it is nowhere near accurate enough to compute a missile flight trajectory against a maneuvering target. If a target at 2.5 nm distance turns away from the IRL together with the fighter or missile, it might seem an infinite distance away instead of 2.5 nm away. The truest guidance method for a passive missile is homing.

 

-SK

[D-Scythe]

Sure - the APG-63 can guide the F-15 to collide with the jammer, no problem. Just follow the strobe. Why would you want to?

 

Haha, no that's not what I meant. You said they act like Sidewinders. The key point is...doesn't a Sidewinder *know* the range to the target? I was a big fan of Janes F-15/F/A-18 study sims, where the Sidewinders do fly as you say (i.e. following target inefficiently). But then Lock On comes along, and all of a sudden the AIM-9M and the R-73 have proportional nav? I inquired about this since Lock On first came out, and the response I got was "they're supposed to," and further went on to explain something about the seeker tracking angles or something.

 

So this assumes that the Sidewinder would *know* the range, and accurately enough to use proportional nav to place its tiny warhead within its 25 ft kill radius. I'm AM NOT saying this is the case IRL, but some Lock On apologists have stated this case, and I couldn't object. So, is this wrong? Should heat seekers be modelled like they were back in Janes F/A-18? Or if they can, then why can't the AMRAAM do the same when homing in on a jamming target?

 

Besides, the KR technique is preferable for estimating target range to within a dozen nm to make a shoot/no-shoot decision when nothing else is available, but it is nowhere near accurate enough to compute a missile flight trajectory against a maneuvering target. If a target at 2.5 nm distance turns away from the IRL together with the fighter or missile, it might seem an infinite distance away instead of 2.5 nm away. The truest guidance method for a passive missile is homing.

 

But isn't that the point of an active radar missile? Provided that KR can narrow down the target's location to within a few miles (within a dozen miles, as you said, is hardly accurate enough to make a shoot/no-shoot decision, considering that the majority of the AIM-120's envelope is below 20 miles, so let's assume an error of around 5 nm on a level, non-manouvering target), why not have the AIM-120 loft, but in a less efficient, smaller loft trajectory and use both the AOJ information collected passively by its radar and the range/firing solution estimated by KR (since the fighter should still be sending data to the missile through d/link) to put itself in a valid position to complete the intercept? As long as the FCR/missile guidance computers always assumes the worst case scenario (i.e. the closest range the jamming target is to the fighter, so that the lofting AIM-120 doesn't run the risk of overflying its target), and the target isn't significantly closer than estimated, the AIM-120 is guaranteed to at least lock up its target (providing that its recieving information up to the point of exiting out of the inefficient loft and into the HOJ intercept profile), even if KR was (initially) off by as much as 5 miles, because not only is the fighter constantly feeding the missile its estimated target KR/AOJ information relative to the missile and itself, but also because its own seeker is also tracking passively. So long as the loft profile keeps the target within its radar gimbal limits, this should work. Right?

 

Ideally, (i.e. if the target continues straight and level before the AIM-120 lights up its radar), such a loft profile will be much more efficient than just simply passive-homing bee-line pursuit, but because range data is denied and thus also proportional navigation (to an extent - again, estimated target range can be used, but remember, the computer must assume the target at its closest range to prevent the target from flying under the AMRAAM during loft), it would be much less efficient than the normal loft against non-ECM targets. It would be considerably less effective against a manuevering target at BVR, since range data is estimated and the missile might run out of energy, but the idea is to get the missile into the general area of the target, and if the target is manouvering, than so much the better, because it would not deviate as much (relatively) from the general area, and so long as it continues to jam, the fighter can continually feed KR/AOJ information to the missile to within a good distance of the optimum intercept point (where the seeker would normally light up in ECM-less conditions). At best, the AIM-120 will kill; at worst, it forces the bandit defensive, because it will not only always lock onto the target, but do it passively, with absolutely no warning.

 

Even if we take the worst case scenario, as you mentioned: a high-speed, manouvering target suddenly makes a radical change in direction and position as the missile's in flight. Sure, the AIM-120 on HOJ would be trashed, but ANY missile, one using the bee-line pursuit HOJ and even one that can magically 'see-through' the ECM, would likewise be trashed unless it was a Phoenix or Amos, because in all likelihood the target is going to outrun it. If the target makes a radical position as KR is calculating the range before a missile shot...well, that's a chance you have to take. And that's what AWACs are for :)

 

Nonetheless, the AIM-120 will still end up where the target *would've* been a threat, and continue to pursue it until it runs out of energy, thus forcing the defender to lose initiative and giving it (and control of the engagment) to the attacking fighter. So, still, I would rather have my AIM-120 use the estimated KR range to use loft (even a conservative 7-8 degree pitch) and arrive on target with a bit more extra energy, than not use lofting at all. Of course, the presupposition here is that the launching radar does have KR or some other ECCM capable of determining range to a certain extent.

[GGTharos]

If this were true, then the Sidewinder would be doing the same thing. I've never read of any missile lofting while homing. Loft is a ballistic flight profile, with the seeker not on the target.

 

Besides, the KR technique is preferable for estimating target range to within a dozen nm to make a shoot/no-shoot decision when nothing else is available, but it is nowhere near accurate enough to compute a missile flight trajectory against a maneuvering target. If a target at 2.5 nm distance turns away from the IRL together with the fighter or missile, it might seem an infinite distance away instead of 2.5 nm away. The truest guidance method for a passive missile is homing.

 

-SK

 

SK: No, I disagree; the Sidewinder is a short-ranged missiles for which a lofting trajectory would not be very useful (unless you datalinked, and that's not the point). On the other hand, the AMRAAM can be datalinked and updated with new range estimates, giving you a longer shot. Given that your shot is not liley to exceed 20nm realistically (from what I've gathered so far) it may not be very realistic to say that there's enough time to do good KER and then take the shot as well - you may well end up just doing a visual attack or range estimation yourself. But my point is that you -can- get the range and knowing the range you -can- loft the missile to have a better energy advantage than if you had launched straight. This isn't a 5nm launch dealie here when the loft would barely make any difference at all, it's 15nm or more.

 

D-Scythe: No, a sidewinder doesn't how range to target. How would it? It's completely passive. Proportional navigation gets AROUND the issue of not knowing the range, however. Completely. Your only contraint here is that you must know the range before launch. By comparison, even a SARH missile can tell the distance (because it receaives the radar pulse from the tail end, and then again, reflected to the front end) so it can time a coded pulse on its own - it also gets the closure in this manner. Neither piece of data is available tot he sidewinder. Despite this SARH and ARH missiles as well also use proportional navigation, but this is a 'homing pahse' issue like SK pointed out. Lead pursiot is more likely to be used via datalink until homing takes over. Proportional navigation is pretty efficient, which is why it's so widely used.

 

Now to answer yoru questions as to why a missile wouldn't do proportional navigation against a jamming target: Actually, it would, or it would attempt to, from what I've read (links presented repviously, I don't wanna go digging'em up now - but some of the PDFs that I sent to SK mention this as well I think) but there are jammers that can abuse your proportional navigation by sending erroneous closure data (shifting the frequency). There are also other tricks, such as 'walking the gate' off the aircraft, which can also confuse the missile (or any radar tracking it, for that matter)

This is the reason why in LOMAC the HoJ shot is modelled as a pure pursuit deal - because the ECM environment is too complex to represent as a whole within the game.

[D-Scythe]

Sure, it's complicated, but ED can make try to come up with a scripted version, that wouldn't be very hard. Most of the mechanics of active radar missiles vs. jamming targets would be invisible to the pilot (us) anyway. It still doesn't mean that the AIM-120 can't perform a small loft profile using estimated Kranging data continuously fed to it by the launching aircraft, then as a result probably exit a bit its loft profile early into a bee-line pursuit to complete the intercept.

 

I know it's impossible for ED to completely model the intricacies of the entire ECM vs. ECCM vs. EC3M etc. thing, but we're only aiming for a scaled down, more general model of what *really* goes on so that we can use and experience some of the real world tactics and stuff we know about in Lock On. A KR mode that, as SK said, that gives a rough approximation on the F-15 VSD of range to target, and then an AIM-120 perform a smaller and less efficient loft before moving into its normal bee-line pursuit of the target. If no ECCM ranging data is available, either before launch or after launch, the missile automatically goes to pure pursuit. If the target is not jamming, normal proportional navigation with the expanded, more efficient loft profile would be used. This is 'for example.' But yeah, sure, a simplified simulation of the whole thing can't hurt - nobody's asking for an AFM for the whole ECM vs. ECCM thing here.

 

BTW, if the AIM-9 and R-73 don't know range to target, than it isn't entirely possible for the missiles to use proportional navigation as they do right now in Lock On, right? That's what I'm trying to get at.

[GGTharos]

But the 9 and the 73 DO use proportional navigation. You don't -need- range to do proportional navigation.

[D-Scythe]

Sorry then, I misunderstood you. However, if no range data is required, and let's say ECM techniques like the walking gate off aircraft thing were discounted (or HOJ in missiles can defeat them), then are you saying that the AIM-120 and any other missile capable of HOJ should do proportional navigation? Because range data is not needed, of course.

 

That's sorta contradictory to what we're saying about inefficiency and all...

[SwingKid]

Haha, no that's not what I meant. You said they act like Sidewinders. The key point is...doesn't a Sidewinder *know* the range to the target?

 

No, range information is not required to fly a proportional-navigation (PN) homing trajectory. Rather, the missile detects drift in the target line-of-sight (LOS), and steers into the direction of the drift until that drift reaches zero. If the LOS drift rate is zero, then the missile is on a collision intercept course, regardless how long that course may be. There is a good description of this on page 37 of Shaw's "Fighter Combat". Consider that the A-10A manages to employ the Sidewinder with no range data.

 

Civil pilots are similarly trained that if they see another aircraft in the canopy, no matter which direction, and it does not seem to be moving left or right at all, then they are probably near a collision course, and should change direction. Same for how combat pilots are instructed to tell if a SAM is tracking on them.

 

Or if they can, then why can't the AMRAAM do the same when homing in on a jamming target?

 

Not sure if this is still a question - that's what I wrote earlier, that an AMRAAM approaching a jamming target should fly just like a big Sidewinder.

 

But isn't that the point of an active radar missile? Provided that KR can narrow down the target's location to within a few miles (within a dozen miles, as you said, is hardly accurate enough to make a shoot/no-shoot decision, considering that the majority of the AIM-120's envelope is below 20 miles, so let's assume an error of around 5 nm on a level, non-manouvering target),

 

Active radar has little to do with HOJ - the earliest version of AIM-7 employed HOJ without any active radar.

 

Even a non-maneuvering ECM target is still moving at an unknown speed. This makes KR a very coarse approximation, and only useful at all if there is no other method of estimating range available.

 

why not have the AIM-120 loft, but in a less efficient, smaller loft trajectory

 

It's important to understand that a lofted missile actually follows a longer trajectory than a missile flying in a straight line, which is the shortest distance between two points. The only way the loft is useful is if it puts the missile into a much higher altitude of lower air density for lower friction, and the missile doesn't try to turn at all while it's up there - less air resistance also means less efficiency for the control surfaces. So, if you are only lofting the missile for it to compute range, and then have it suddenly turn around at high altitude and dive on the target instead of falling naturally, that can actually give the missile a shorter effective range than if it had not been lofted at all. Try to think of it as a ballistic missile leaving and then re-entering the atmosphere. It cannot efficiently change direction once it leaves the air. For this reason, the amount of loft is always calculated before the missile is launched, and varies according to the target's (already known) range - too high or low a trajectory, and the missile will "re-enter" the denser, maneuver-friendly air at a point that over- or -undershoots the target.

 

Another issue is that a missile seeker's tiny radar antenna is much less directive than a fighter's radar antenna - getting a mere 2 degrees angle-off will not be enough for the missile to compute triangulation, even if it's enough for the fighter radar. A "shallow" loft angle, i.e. a quick peek at the target from a few thousand feet higher altitude, is not going to be enough for KR - the missile will have to head much higher, and then have to come all the way back down again.

 

The trade-off is that loft always extends Rmin more than it does Rmax. At the same time as it pushes the missile envelope farther outwards, it also makes it smaller. When the target range is unknown, that's a disadvantage.

 

and use both the AOJ information collected passively by its radar and the range/firing solution estimated by KR (since the fighter should still be sending data to the missile through d/link) to put itself in a valid position to complete the intercept?

 

This is where we might end up with an unprovable difference of opinion, because it depends on how accurate you insist that the range data must be, and the truth is surely a guarded secret. Considering that I can make KR think I am thousands of miles away when I am only 2.5 nm away by simply flying a parallel course, and how critical I believe accurate range data is for a lofted shot, I don't think any missile engineer would trust KR range data to compute a lofted trajectory before launch. AFAIK the AMRAAM couldn't loft at all, even against non-maneuvering, non-ECM targets until recently, ten years after the missile's service entry, because the lofted trajectory was so difficult to optimize.

 

As long as the FCR/missile guidance computers always assumes the worst case scenario (i.e. the closest range the jamming target is to the fighter, so that the lofting AIM-120 doesn't run the risk of overflying its target), and the target isn't significantly closer than estimated,

 

A KR-range-measured target is always at risk of being closer than it appears. In fact, about the only thing KR can tell you with precision is where the target is not, since almost any target motion is guaranteed to produce errors. Assuming the worst case scenario to avoid overshoots would mean not lofting - exactly as the F-16MLU manual describes the AIM-120 not lofting, when launched against near targets.

 

Ideally, (i.e. if the target continues straight and level before the AIM-120 lights up its radar), such a loft profile will be much more efficient than just simply passive-homing bee-line pursuit,

 

As written above, I disagree - unless the range is known with precision, bee-line is better, faster. This is based on my experience with the miniZAP simulator, trying to calculate optimal loft trajectories for maximum range. I quickly discovered that once you get a missile up there that high, it becomes a serious question just how you are going to get it down again. Gravity will pull it to a natural location where it wants to go, but any significant steering the missile tries to do at high altitude will make it "run out of steam" before it even comes back down again. I wouldn't even want to do course-corrections up there - wait until the missile descends half-way, then correct the course is how I'd do it. Unfortunately miniZAP doesn't model homing so I can't realy demonstrate this provably.

 

Just my opinion, though, in the end...

 

-SK

[GGTharos]

That much drag up there, SK?

 

Anyway, I think beeline is better at short ranges (the issue of time to target) but I see little reason not to loft against a far away target; a special trajectory to help with detecting range better (possibly triangulating with the help of the aircraft's own radar, but there's a battery life issue I'm sure) and you can measure angular change (part of PN) to know when to start your dive. Your trajectory may nt be the most optimal, but then, you don't really need it to be. You only need to make sure that it's optimal tolerance which -shouldn't- be too hard. Now, given that you've got minizap and the source, you -could- theoretically take one missile and model maneuvers that would mimic homing and see what happens no? But that's probably too much effort. Perhaps we'll never really agree on this issue ;)

 

AFAIK, D-scythe, there are methods of jamming which don't even force HoJ mode - like walking the gate off the aircraft. That's an 'intelligent' jammer. In fact it is specifically ebcause you're homing in on its emissions that it tricks you.

[SwingKid]

That much drag up there, SK?

 

It's not so much the drag, as the lack of turning force from the reduced air pressure. So to get any turning force, you need to deflect the control surfaces more than usual - but this increases the drag more quicklly than it increases the turning force, so you're worse off for the trouble of going high if you have to maneuver. I never heard of a "non-ballistic loft".

 

Anyway, I think beeline is better at short ranges (the issue of time to target) but I see little reason not to loft against a far away target; a special trajectory to help with detecting range better (possibly triangulating with the help of the aircraft's own radar, but there's a battery life issue I'm sure)

 

All anyone needs to do is provide referenced quotes for any of this stuff instead of speculating, and I'll jump right on board.

 

I recommend the book "Sidewinder" by Ron Westrum. It provides a very good insight into how simple missiles really need to be designed to be in order to get them to work reliably well and cost-effectively, and how quickly disaster strikes when engineers try to add too many features. Pure ballistic loft is a neat idea for extending Rmax in certain conditions, and there's plenty of evidence of how it works, but this "three-body datalinked kinematic-ranging maneuvering-at-high-altitude home-on-jam-versus-hovering-ECM-helicopter at somehow-we-know-the-range-to-sufficient-accuracy-before-launch..." sounds like a weapon that Saddam Hussein was hiding. :)

 

-SK

[GGTharos]

Eh, come on. Subs do it all the time, in worse conditions in some cases ;)

 

Although admittedly they don't really have to deal with jamming AFAIK.

[D-Scythe]

GG, submarines are different though. Not only do they move a lot slower, but the wire between the torpedo (like the Mk-48 ADCAP) and the sub (like a 688I, Seawolf) is a two way data link, IIRC. Information can be sent from the sub to the torpedo and vice versa. AFAIK, the d/link between the AIM-120 and its launching aircraft is one way - from fighter to missile. The AIM-120D (or AIM-120C7+, whatever the designation is) not in service yet, has a two way d/link, though.

 

Anyway, thanks for the info guys.

[tflash]

Pure ballistic loft is a neat idea for extending Rmax in certain conditions, and there's plenty of evidence of how it works, but this "three-body datalinked kinematic-ranging maneuvering-at-high-altitude home-on-jam-versus-hovering-ECM-helicopter at somehow-we-know-the-range-to-sufficient-accuracy-before-launch..." sounds like a weapon that Saddam Hussein was hiding. :)

 

-SK

 

I think indeed we should not overestimate the calculation options available if you have to do some mid-course corrections in a timeframe of a few seconds.

 

From an AI point of view, the very hard part is the initial course you send your missile to. Taking into account the max manoevrability envelope of your target (a little bit easier to predict for a T-95 bomber then for a fast jet) you will want to have your medium range missile at the ideal mid-course position to engage the end-run.

 

The real endgame is up to the active seeker and what is left in energy in the then ballistic missile. I guess once the missile is within a certain sphere around the target aircraft you can actually speak, as the vendors do, of a no-escape zone. So the whole point is to get the missile into this sphere. This is a totally different situation than the one involving slow-moving targets and slow-moving weapons, like you have in topedo, JDAM and LGB's, cruise missiles and the like.

 

If my impressions are right, it is still often the case when you fire an AMRAAM in Lockon, that the missile has to take a very steep turn in the end-game to follow the target that is dodging away. I guess the real, classified, algorithms might take a slightly better guess at the mid-course, but really I don't know sh.. about it.

 

Always keep in mind that a very small difference in the mid-course position can make a BIG difference in the end-game.

[tflash]

I have another question: I would suppose that the APG-70 and APG-63(V)1 have a TWS AACQ mode. In fact, it would surprise me if not? Of course it will go into SST once lock has been achieved, but I would think it would build the cue automatically. Anyway, some (rather poor) F/A-18 sims do model the APG-65 that way, with AACQ in TWS.

[SwingKid]

I have another question: I would suppose that the APG-70 and APG-63(V)1 have a TWS AACQ mode. In fact, it would surprise me if not? Of course it will go into SST once lock has been achieved,

 

I don't understand. Isn't the whole point of the F-15's TWS, to prevent going to STT if it can be at all avoided?

 

-SK

[GGTharos]

That's what I'm thinking too...why go to TWS from AACQ? You're only looking for one target after all, may as well STT it and drop the guesswork inherent in TWS.

 

TWS is nice because it allows you to:

 

• Designate and attack targets while continuing to search
  
• Avoid automatic triggering of SPJ's via STT lock until the missile goes active at least
  
• Possibly do away with just about any warning whatsoever to the enemy until the missile is active
  

For these reasons TWS should never enter STT unless the pilot commands it.

[tflash]

Oops, forget about the SST part: of course it doesn't have to enter SDT but can stay in TWS. But what do you think about AACQ in TWS?

 

If NCTR/IFF is deemed reliable enough, the TWS could build up a shooting cue automatically (so without the pilot marking the targets of interest with the TDC). Anyway, this is the way it is supposed to be in F/A-22, but I would guess the F-15 also has this mode.

[GGTharos]

AACQ to me replies automatic acquisition at short range. That in turn implies STT. Are you trying to say something else?

[tflash]

Well, suppose you're in TWS mode. You see some contacts on the radar screen. Some are hostile contacts. You can then slew the TDC over them and add them to the TWS cue. When in range, the first target will be locked and can be engaged.

 

All i'm saying is: do you need to manually mark the contact with the TDC? I can imagine this goes automatically, where the software chooses the most imminent threat? In fact, this is exactly the way it is modelled for the Su-27 now, or not?

[GGTharos]

The modelling for the 27 is simplistic, and it's absed around interception of incoming aircraft, not a mixed up fight.

 

You definitely do NOT want your computer designating stuf fon its own, especially when you've got friendlies up ahead and an ECM environemnt has the possibility of making IFF dubious.

[Ice]

F15C avionics wishlist?

 

This thread useful as it is, has morfed into a "Wishlist" only. Would it be wise to re-title it just that? "Wishlist"

 

Might minimise confusion and stop a lot of new threads that could be added here.

 

just an idea :icon_idea

[SwingKid]

F15C avionics wishlist?

 

This thread useful as it is, has morfed into a "Wishlist" only.

 

It has? :confused:

 

-SK

[D-Scythe]

F15C avionics wishlist?

 

This thread useful as it is, has morfed into a "Wishlist" only. Would it be wise to re-title it just that? "Wishlist"

 

Might minimise confusion and stop a lot of new threads that could be added here.

 

just an idea

 

What are you smoking? :P We've spent the entire thread talking about the F-15's avionics.