SARH v ARH Seeker Perofmances ETC

[GGTharos]

Okay, a continuation from the F-15 Avionics thread ...

 

D-Scythe, yes, the ARH is 'virtually' immune, but not completely so. For example, the closer it gets, the greater the chance that a chaff bloom will 'intervene' and the missile will lock on to it, or be blinded by it (chaff RCS is huge) and by the time is dissipates the target is out of seeker constraints.

 

Chizh already implied that chaff's a problem when he stated that the RVV-AE uses special trajectories to avoid potting chaff between itself and the target, for example.

 

But zzzspace is right baout the target switching: It has happened a number of times that I'd pump out chaff and the missile would them go for my wingie instead of me, or in a couple times, much more to my amusement, I'd be fired on by my dancing partners wingie, dump chaff and the missile would then kill the guy i was circling with, allowing me to escape.

[F l a n k e r]

Sorry Tharos, where I can find info about the seekers? The thing interest me so much, thanks in advance.

[GGTharos]

I don't think you can find any info -directly- :/

 

You have to sift through so much stuff on the net and then infer it's not funny.

 

For example, I got my hands on a number of PDFs describing seeker guidance laws (but not specifics about seekers!)

 

Somewhere else still I read up on things like monopulse radar and jamming and chaff and flares etc.

 

When i find my sources I'll post them here ... but they are also posted somewhere towards the end (but not right at the end) of the F-15 avionics thread I think.

[GGTharos]

Interesting ...

 

http://radarproblems.com/

[GGTharos]

Eureka, i think!

 

http://www.knovel.com/knovel2/Toc.jsp?BookID=701

 

Chapter 19 even explains TVM!!!!

 

Patriot and S-300! And it seems to shed some light, albeit only a little, as to how that F-16 might have known ... I still think that in LOMAC those systems should not produce lock/launch warnings.

[GGTharos]

It's a little OT but ...

 

Chapter 9, section 4, equation 9.1 ... this very interesting reading!

[F l a n k e r]

Boys, you are fantastic!

[Chizh]

Re: SARH v ARH Seeker Perofmances ETC

 

...

Chizh already implied that chaff's a problem when he stated that the RVV-AE uses special trajectories to avoid potting chaff between itself and the target, for example.

...

I think that at AIM-120 there are similar trajectories but the information about it is completely closed.

 

Actually chaff bloom are dangerous not only for missile’s seeker but also to a proximity radar-fuse which can burst from a dense chaff bloom.

[GGTharos]

I agree Chizh, the information on anything concerning the AMRAAM save for 'brochure' information like 'top speed' and 'range' seems to be classified and impossible to find - but I also would agree with you that it is correct to assume that it employs similar pursuit methods, it only makes sense.

 

I have heard a number of things, such as 'the range of AIM-120B has inscreased over AIm-120A' and 'Range of AIM-120C has significanly increased over range of AIM-120A' but it is difficult to know in what context or cases.

 

Also, I have from some of the sources I posted, inferred that to defeat a jamming target it is not at all necessary to 'burn through', there are techniques that can be used to achieve relatively accurate data well before burn-through, especially with digital radars.

 

Other things I learned are that digital radars can maintain same range as radars with more raw power because the way that noise is filtered allows them to achieve a better SNR, thus detecting the target at longer range than an analog radar of the same transmission power. (This is similar to the reason why digital cellphone transmit at much less power that analog ones)

 

There are specific methods of jamming to deal with everything, of course, but especially monopulse radars for example, in particula digital ones, apparently have a good chance of seeing through the jammer by coding their pulse and filtering everything that does not match the pulse. While the radar may not receive or process reliable information 100% of the time, and may not be able to hold a lock, it can statistically accumulate the target's position and guide a weapon to it using inertial navigation and MCU's. This is what I inferred from reading various sources.

On the other hand, there are other methods of jamming specifically designed to cause digital radars trouble, but I have found nothing that can reveal the method of their operation or effectiveness. My guess is that they are based on defeating the SNR averaging/extraction algorithms of the radar, which are themselves a closely guarded secret, but more than likey almost exclusively based on Fourier Analysis ;)

 

Also, it seems that range-based jamming (the type that seems to be simulated in LOMAC) does deny range to the -radar- but not to the operator, apparently, because the false targets are concentrated on the target itself, and their density decreases as they fan out. This implies that a digital radar can also determine an approximate position of the aircraft and guide a weapon to it, but with less accuracy. An operator could also manually set up some range settings (i have heard that this is possible) for the missile and guide it in and it will work if it has passive or active homing.

 

It seems that digital radar (As in one which processes the signal using a digital computer) appears to have a number of advantages in this manner.

 

Simialrely, apparently HoJ uses proportional navigation, but it is 'worse' than active or semi-active according to what I have heard. SK suggests that it could be because the missile will not loft for without range information, but hopefully I have to some small extent demonstrated that the operator can determine and set the range for the missile - therefore, I think the problem here is that the missile cannot determine closure, while a SARH/ARH can, the ARH because it sports its own radar, the SARH because it receives the launching ship's waveform via rear antenna and can thus filter out anything but the doppler-shifted reflection and at the same time the doppler shift allows it to approximately compute closure.

 

The ARH gets far more accurate results for closure computation so in turn it is a more accurate weapon, I think.

[SwingKid]

GGTharos,

 

The link you provided is a good one, but is not a free site for those who are not connecting from university or research center addresses. Can you quote a few bits of the relevant text? (I had access to the site from another PC earlier today, but not at the moment...)

 

(1) I'm interested what you read about TVM. I could only find it described there as "basically similar to semi-active homing", together with "normal command guidance". Indeed it's descriptions like that that made me wonder why TVM would be anything special from the RWR's perspective, and I needed to be convinced by mirv's opinion. What were your thoughts regarding the F-16 event, and the text that prompted them?

 

There are specific methods of jamming to deal with everything, of course, but especially monopulse radars for example, in particula digital ones, apparently have a good chance of seeing through the jammer by coding their pulse and filtering everything that does not match the pulse.

 

(2) From this description and others, it sounds like you are confusing CDMA in cell phones with the purer radar concepts of monopulse, Barker codes and range gating (which are three completely different things). The N019 is an analog radar but has a monopulse antenna - it allows angle-on-jam tracking, but can't provide range data and is independent of whether the radar is digital or not. Barker coding improves resolution but cannot AFAIK used for filtering without losing lots of targets, due to eclipsing and other effects... High-resolution "noise" is still "noise". Range gating improves SNR and detection range (allowing lower power levels) at the expense of needing more hardware, but it also affords little protection against repeater-type ECM. The Achilles heel of all of these concepts is that they are still used together with the pulse integration of medium or high PRF, against which ECM can deny accurate range data with ease.

 

On the other hand, there are other methods of jamming specifically designed to cause digital radars trouble, but I have found nothing that can reveal the method of their operation or effectiveness.

 

If you're referring to cross-pole (which is a technique against monopulse, as opposed to digital, radars), a description can be found in Stimson's "Introduction to Airborne Radar", also available online from some university libraries.

 

Also, it seems that range-based jamming (the type that seems to be simulated in LOMAC) does deny range to the -radar- but not to the operator, apparently, because the false targets are concentrated on the target itself, and their density decreases as they fan out.

 

I can't think of how ECM could create such an effect... Do you have a source text describing this?

 

Simialrely, apparently HoJ uses proportional navigation, but it is 'worse' than active or semi-active according to what I have heard. SK suggests that it could be because the missile will not loft for without range information, but hopefully I have to some small extent demonstrated that the operator can determine and set the range for the missile

 

There is a way to do this approximately with "kinematic ranging" - basically a bearing-only emitter triangulation "on the move". But, this method doesn't involve the advantages of digital radar - the MiG-29 can also do it.

 

therefore, I think the problem here is that the missile cannot determine closure, while a SARH/ARH can, the ARH because it sports its own radar, the SARH because it receives the launching ship's waveform via rear antenna and can thus filter out anything but the doppler-shifted reflection and at the same time the doppler shift allows it to approximately compute closure.

 

You only need to detect closure to filter out ground clutter, and a HOJ missile has no problem with ground clutter, because the ground is not emitting, so ECm denying it closure data will have little effect. The problem created by ECM against a monopulse seeker is denial of range data.

 

The ARH gets far more accurate results for closure computation so in turn it is a more accurate weapon, I think.

 

Unconvinced. The online book you've referenced doesn't propose any accuracy advantage at all to ARH over SARH, yet it specifically describes the advantages of SARH/ARH/TVM over command and beam-riding guidance. Everything I read there suggests that properly-implemented SARH, ARH and TVM can enjoy theoretically identical accuracy.

 

-SK

[GGTharos]

D'oh! I didn't realize that, and I was posting from the university.

 

My source on ranging a jamming target is one of those things I haven't been able to find again ... I've looked through my stuff but there's just so much of it I can't find the specific passages.

 

Specifically the spoke of the target always being in the 'center' of the false returns - I don't know anything about how the jamming signal is generated, but it could be some sort of timing issue related to both receiving and sending out an approrpiately modified pulse. The specifically implied that this would actually allow the operator, but not the radar itself, to know the range and possibly track the target.

 

No, i'm not confused about CDMA and the radar, at least I don't think so - I read in one of the links I've previously posted that coding pulses in some manners can be used to defeat jamming - don't get me wrong, I doubt it means 'you get the same range as on a non-jamming contact) but I'm pretty certain these techniques can be used to deal with the problem better the closer the target gets. In some way, it doesn't matter you're outshouting the radar and providing it with false information, because it'll only consider the pulse it coded, and the next one will be coded differently - the reflection will arrive before your repeater can do much about it and you can ignore repeats - at least that seems to make some sense to me though I see some problems with it as well.

 

TVM Guidance: Specifically the mode of the radar - it indeed doesn't 'lock on' to the target, it just paints it for a moment and goes off to paint another. This might not be picked up as a 'lock' by an RWR until you program it to recognize a specific 'paint' pattern as a 'lock'. At the same time, this may have not been somethign that earlier RWRs could deal with due to either processing power, sensitivity, or both.

Keep in mind that the HARM equipment is actually, AFAIK, quite a bit more edvanced than an RWR when it comes to collecting and processing signals, which may explain why the F-16 'got it' and other aircraft didn't.

 

On Accuracy: From what i understood from these texts, the ARH missile gets far better closure information and can compute an intercept path with less uncertainy, but again, I can't say I really know.

 

 

I'll try to come back this ASAP, probably when I get sick and tired of solving Rings and Fields problems (but I must :P )

[SwingKid]

No, i'm not confused about CDMA and the radar, at least I don't think so - I read in one of the links I've previously posted that coding pulses in some manners can be used to defeat jamming - don't get me wrong, I doubt it means 'you get the same range as on a non-jamming contact) but I'm pretty certain these techniques can be used to deal with the problem better the closer the target gets. In some way, it doesn't matter you're outshouting the radar and providing it with false information, because it'll only consider the pulse it coded, and the next one will be coded differently - the reflection will arrive before your repeater can do much about it and you can ignore repeats - at least that seems to make some sense to me though I see some problems with it as well.

 

Well, the first problem I see is eclipsing. Let's say a pulse is 3 us long and is modulated with 3-bit encoding "1, 1, -1". The radar transmits this pulse, and it gets reflected from the target (which we assume to be a perfect mirror that introduces no distortions), and returns to the radar antenna during the "listening pause" between pulses.

 

If the received signal arrives at exactly the beginning of the "pause", the receiver can hear the full "1, 1, -1" reflection. But if the target was a little bit farther away and the refelcted signal arrives too late, the radar will hear "0, 1, 1" and then start transmitting the next pulse. It won't hear the full code. This problem is called "eclipsing" - because of the time-sharing functions of the antenna, it is always possible that some or even all of the reflected pulse cannot be received, so the radar must be able to accept only partial pulses.

 

The second problem is - let's say there's one reflection from the nose of the target, and another from the tail, and the two reflections are equal in power but shifted in time, and superposed over each other at the receiver. Then your receiver hears something like "1,2,0,-1" - and the target is again invisible, or rejected as "ECM".

 

Finally, "you can ignore repeats" is only true for low PRF. At low PRFs, the IF stage no longer works to separate the Doppler shift of the target from the ground clutter. So, fighters and missiles use High and Med PRF, which depend on repeats to detect targets. So, the advantage at MPRF and HPRF accrues to the ECM: it can "listen" to the first few pulses and only jam the consecutive ones, and still have accomplished its task.

 

TVM Guidance: Specifically the mode of the radar - it indeed doesn't 'lock on' to the target, it just paints it for a moment and goes off to paint another.

 

I just didn't find this described in the chapter 19 that you referenced. I don't doubt it but please, if you get a chance, direct me to the exact text, I'm very interested.

 

Cheers,

 

-SK

[GGTharos]

But doens't monopulse radar deal with having to use a lot of pulses?

 

As for TVM, yes, it's there ... I'll look at it again and point you to the page and test. It specifically refers to how the beam is time shared between several targets (actually it has less to do with TVM but rather with the illumination of the target by the radar, TVM just mans the signals the missile receives - the reflections - are processed back at the CP) ... but it specifically refers to what we know is happenning - you don't have a 'beam' sitting on target, but it only paints it for a few moments, paints the others, and scans all in one go. This result is no 'lock tone', unless you can classify the radar as 'scanning you a little too fast for just a search'.

 

Like I said, I'll look at it gaian and point you at it. It's a reallys mall passage in the TVM guidance stuff.

[SwingKid]

But doens't monopulse radar deal with having to use a lot of pulses?

 

Hey, good question. Yes, technically monopulse does mean "single pulse," but it's a confusing semantic issue, because it's named that only to distinguish it from conical scan, which requires more than one pulse to computer bearing.

 

In conical scan, the antenna sends separate pulses slightly to the left, to the right, above and below the target, then checks to see which received signal is strongest to determine which direction is closest to the true bearing of the target. ECM that transmits a strong "spike" when a conical scan radar is looking in the wrong direction can confuse it.

 

In monopulse, the antenna receives the same reflected signals on two separate beams simultaneously, and compares these. The antenna doesn't need to look in any "wrong directions" to make comparisons, so spike and non-spike signals are received on both beams as if they were normal signals. The radar is thus not fooled about the bearing of the target, independent of signal amplitude.

 

None of the above has to do with determining range. That part is not addressed because radar is so complex, only want to talk about one part of the problem at a time. In terms of detecting target bearing, the system is either conical scan or monopulse. But, the PRF is typically tens of thousands of pulses per second. So even a conical scan radar is really sending many pulses above, below, left and right, not just one - and much faster than the antenna is physically steered around in the circle. The monopulse antenna works similarly - many individual pulses are transmitted in the time it takes for the first pulses to return to the antenna. These pulses are combined together (integrated) in the radar circuitry, where FM ranging resolves range ambiguities, the IF stage detects Doppler shifts and rejects clutter, and the combined pulses have a stronger signal strength.

 

So what they mean by "monopulse antenna" is just that there is no inherent bearing ambiguity to individual pulses as seen by the antenna - it does not mean the rest of the radar circuitry actually operates in a lower PRF. Monopulse radars use the same PRFs as conical scan, and the range ambiguities result from this choice of PRF. Only bearing (azimuth and elevation angle) ambiguities are solved by using a monopulse antenna.

 

I'd really refer you to the Stimson book for a better explanation of this, the diagrams are also better IMO to those in the reference you provided, and I find myself having a lot of trouble trying to describe it without them.

 

Hope this helps,

 

-SK

[GGTharos]

Ah, thanks SK!

 

Incidentally I actually found that book jsut before you mentioned it in your previous post, heh. I'm going to have to pick it up sometime ... but after i deal with this math stuff.

[GGTharos]

SK, chapter 19.3 'Sample Data Operation' on p20 of the pdf.

 

This I belive has -direct- relevance to the way the S-300 and Patriot operates, with TVM itself being nothing more than an alternative to SARH. So it isn't specifically the TVM that causes (IMHO) the lack of a lock/launch warning, but the way the radar itself works to provide the illumination for the weapon.

 

It appears that the intermittance of the signal may be a singificant factor in detection avoidance here where a 'lock' is concerned, if we take older RWRs into account specifically.

 

You are correct in that there are ways to detect the lock - this I know because there was a thesis proving that, for example, the Raptor's LPI radar could be reliably detected with the proper hardware and software alteration to an RWR, which, IIRC, was not too expensive.

[crazyleggs]

You guys are pissing me off! Instead of doing my work (trust me, it's piling up rapidly) I find myself reading your posts and getting sucked in. Great discussion as always.

 

SK, you're totally right with Stimson's book. Introduction to Airborne Radar is a great ref for airborne EW and ECM techniques.

 

As for TVM, Skolnik's book reveals how TVM initially worked. TVM in newer air defense systems works differently.

 

False targets are just one of many ways to jam a radar. If the jammer is turned on after the fighter already has hits on you, false targets will increase his work load but he is still able to target you. On the other hand, if the jammer is on before the fighter paints you, he will see a bunch of targets (they can be random, line astern, line abreast, etc) and will need third party targetting (GCI, AWACS, Link) to sort you out of the targets.

 

One thing you have to remember is that it's always harder to fool a human than a computer. New digital radars which need processors to handle all the incoming data are succeptible to corrupt data. If you can find a way to screw with the processor then your chances of denying that fighter pilot his radar picture is great. On the other hand, if you have an A-Scope in front of you, you can see exactly what the jammer is trying to do to your poor radar. In other words, although software is always evolving, a digital radar will never be immune to jamming.

 

Barker coding is a form of pulse compression. It enables both long detection ranges and increased range resolution without needing extra power (range) and extremely narrow pulses (resolution). It is the received pulsed that is "compressed" into a small segment thus providing a higher peak amplitude. A 3 bit code will provide three times the amplitude and so on. Stimson's book explains this very well.

 

If you're interested in radar and ECM, Stimson's book is a must. Thanks for bringing that up SK.

 

Thanks.

[bflagg]

Guys, I have found this topic quite interesting and thank you both for bringing it to light...(tho I think this will never be realized in LockON)

 

but forgive me for being Naive...

 

I found myself asking this question:

If elispe is such a problem, why not have a seperate reciever in the nose?

 

I mean, I would figure that they could programmically remove these partial/incompleted returns and rescan or hold them and build a complete return from multiple partial returns... thus havin the need for a dedicated reciever a moot point..

 

.. but then.. I could be way off here.....

... just a thought...

[GGTharos]

A separate antenna adds weight and complexity - also AFAIK the radar antenna is a pretty good receiver of the specific radio signals you're looking for ... however, as explainined above, eclipsing isn't such a big deal, and easy enough to get around.

[SwingKid]

There are two additional concerns.

 

One is that antenna directivity is linked to antenna physical area. To have separate transmitting and receiving antennas in a given space means that each must be half the area, and thus be less able to focus the radar beams. The second concern is that if the antennas are close together and working simultaneously, the transmitted signals from the neighboring antenna would drown the weak reflections from the target.

 

There are some other applications where separate transmit and receive antennas are used, but for airborne fighter radars, the disadvantages outweigh the advantages.

 

-SK

[bflagg]

There are two additional concerns.

 

One is that antenna directivity is linked to antenna physical area. To have separate transmitting and receiving antennas in a given space means that each must be half the area, and thus be less able to focus the radar beams. The second concern is that if the antennas are close together and working simultaneously, the transmitted signals from the neighboring antenna would drown the weak reflections from the target.

 

There are some other applications where separate transmit and receive antennas are used, but for airborne fighter radars, the disadvantages outweigh the advantages.

 

-SK

 

As I was out with the wife for dinner, I began to realize the complexity of send/recieve at this point and "doh'ed" myself with out thinking first before posting.....

 

thank you both....