Aim-54 CCM

[Galinette]

17 hours ago, GGTharos said:

RGPO is complex and chaff may help with it a little, but the whole thing is also irrelevant.  RAZBAM decided to display the break-lock as 'RGPO' but it's merely one technique among many and the only thing that matters is the track-break, especially in DCS.  Most DECM will blind angles and range simultaneously AFAIK, so there's nothing or more like 'everything' to lock onto so that the track breaks.   DECM don't do 'noise' at all AFAIK, they're just not powerful enough.

 

On the M-2000C the displayed jumping contact range is not actually RGPO itself but the result of lost range tracking that can't be disambiguated anymore by STT. Jamming models VGPO and RGPO together by adding returns whose velocity and range quickly sweep out of the jamming aircraft values. What you see on the radar display is the STT which is lost in range ambiguity once the tracker picked the wrong returns, which can happen in a few other cases.

Chaff do a kind of  RGPO and VGPO too modelled on the M-2000C but they decelerate so hard that the tracker has no problem following the actual aircraft doppler velocity. Except maybe in some corner cases (target flanking when notch rejector disabled in look up, in that case chaff range and velocity stay very close from those of the target and the radar has no other tool than monopulse and angular tracking to make the difference, which is harder)

[IronMike]

1 hour ago, Kercheiz said:

On the M-2000C the displayed jumping contact range is not actually RGPO itself but the result of lost range tracking that can't be disambiguated anymore by STT. Jamming models VGPO and RGPO together by adding returns whose velocity and range quickly sweep out of the jamming aircraft values. What you see on the radar display is the STT which is lost in range ambiguity once the tracker picked the wrong returns, which can happen in a few other cases.

Chaff do a kind of  RGPO and VGPO too modelled on the M-2000C but they decelerate so hard that the tracker has no problem following the actual aircraft doppler velocity. Except maybe in some corner cases (target flanking when notch rejector disabled in look up, in that case chaff range and velocity stay very close from those of the target and the radar has no other tool than monopulse and angular tracking to make the difference, which is harder)

Thank you, this makes much more sense.

[okopanja]

On 12/12/2021 at 3:04 PM, GGTharos said:

There exist research papers, you can google for them.  There are figures on chaff speed and bloom.

If you have a direct link that would be perfect. So far I found the naval variant, which is not exactly what I aim to: e.g. high altitude.

IMHO: modeling of chaff deserves more attention, and although the book "Electronic warfare fundamentals" is mostly informative, it simply does not provide enough information that can be used for modeling. Meaning: current implementation is based on assumptions.

Quote

Chaff slows to near zero relative velocity almost immediately after dispensing.

E.g. time interval and dynamics of speed decrease? I would expect that target RCS as perceived by PD will instantly grow briefly for the duration of couple of ms up to several seconds.

Quote

For self-protection chaff to be effective, the aircraft velocity relative to the radar site must also be near zero
This occurs when the aircraft's aspect to the radar is 90°, or on the beam. By
maneuvering to a beam aspect against a Doppler radar, the pilot is exploiting the
“notch” where radar cannot discriminate targets based on Doppler frequency
shift (Figure 13-17).

I am truly puzzled by this one: if the above is correct, why throwing chaff in the first place when you are in notch (PD assumed). It's as if radar switches to a different mode when crossing the Doppler gate?

On 12/12/2021 at 3:04 PM, GGTharos said:

They are used as briefed vs. specific SAMs.  Stealth makes it so much more effective (you might add enough noise in the air to drop your signature below the noise floor, but that's just a guess on my part).   And there are situations where you could find yourself next to a SAM and at that point stealth counts for very little if anything, if you recall the F117 shoot-down.

I would expect the stealth would mean you do benefit from dispersing additional RCS in your vicinity,  unless the aircraft is not stealth from certain angles.

Clearly despite the following article being lots of speculations, something is going on in this area since there are references to advanced chaff. So far it looks like endurance and size optimizations are in place.

https://www.thedrive.com/the-war-zone/29734/the-f-35a-will-finally-get-chaff-countermeasures-to-confuse-enemy-radars-next-year

[dundun92]

2 hours ago, okopanja said:

I am truly puzzled by this one: if the above is correct, why throwing chaff in the first place when you are in notch

because when you are in/near the notch, the speed of the chaff and your closure is roughly the same, making it the hardest for the radar to distinguish the two. Specifically, this may cause the radar to lock onto chaff and be forced to go back search mode, giving the aircraft time to perhaps maneuver away out of the scan cone at close ranges, or at the very least delay re-acquisition.

[okopanja]

16 minutes ago, dundun92 said:

because when you are in/near the notch, the speed of the chaff and your closure is roughly the same, making it the hardest for the radar to distinguish the two. Specifically, this may cause the radar to lock onto chaff and be forced to go back search mode, giving the aircraft time to perhaps maneuver away out of the scan cone at close ranges, or at the very least delay re-acquisition.

Thanks, I gather that makes sense when the notching aircraft is not positioned against ground clutter?

[draconus]

4 hours ago, okopanja said:

Thanks, I gather that makes sense when the notching aircraft is not positioned against ground clutter?

It only makes sense against ground in a look down otherwise beaming against the sky won't fool the doppler radar. You're still flying toward the target so the Vc is equal to your speed but there is nothing around the target to clutter.

[IronMike]

1 hour ago, draconus said:

It only makes sense against ground in a look down otherwise beaming against the sky won't fool the doppler radar. You're still flying toward the target so the Vc is equal to your speed but there is nothing around the target to clutter.

Even then during STT highly unlikely. The target is moving to the side with velocity as well, and STT will have some recognition of that, which for the chaff will be different, and almost instantly zero. Even in pulse modes I would suggest. A target moving away, where the chaff has the same angular momentum (in pulse), would have a slightly higher chance. RGPO is really more a thing for jamming, than chaff.

[Lurker]

To me the the bigger issue in DCS is the "size" of the notch. Not sure how else to phrase it. In some ways it is very easy to spoof AIM54s or the AWG9 simply by flying perpendicular to the beaming radar source, and when I say perpendicular I mean a very "sloppy" perpendicular. For the most part this is true vs any aircraft radar in DCS but for some reason it is most apparent when facing Tomcats. Entering the notch just feels too easy, and it doesn't matter if it's TWS or STT, whether you are above or below the radar source etc. you can even do it against an active AIM54 with very high degrees of success. Not sure whether this is a "DCSism" or if it really was this way in real life. I'm not a real fighter pilot, what the hell do I know right? To me it just feels overdone. 

But if people more knowledgeable than me tell me, yep that's the way it was, I'm prepared to accept that and move on.

Edited December 14, 2021 by Lurker

[Noctrach]

Notch sizes are probably not very public domain kind of information. HB modeled the AWG-9 with ±133 knots based on their documentation, so I trust that is correct.

I doubt it's possible to get such objective data on how effective notching is for the modern radar systems on the F-16, F/A-18, JF-17 and AIM-54/120s. That said, in order to reliably filter out e.g. highway traffic with a mechanical array, you're probably always gonna be sitting at something like a ±60 knots filter for search modes. However, for STT modes I'd assume modern radars have a number of ways to filter and bias based on range, angular velocity, signature etc. to reduce this quite significantly.

On top of that I think the main DCSism is how permanent and instantaneous a notch tends to have effect on missile guidance or radar locks. Radar memory esque features just don't work very well in DCS multiplayer, leading to short beaming manoeuvres often completely breaking lock with no chance of reacquisition.

Add to that the fact that:

• chaff works as a chance-based radar flare for missiles for its entire particle lifetime
• resolving a radar contact for most modules is just a set of rules resulting in TRUE/FALSE for each given game object
• these rules themselves are very simplified, e.g. it doesn't matter if the target is at 40,000 or 40 feet, only "target below horizon TRUE/FALSE"

and you can be damn sure there's plenty DCSisms at play.

To an extent this is unavoidable without CPU-choking levels of simulation, but it's also just a large amount of LUA engine legacy. I'm sure they'll get around to creating a more rich simulation someday, but until then these are the game rules.

[Harlikwin]

On 12/14/2021 at 8:11 AM, Noctrach said:

Notch sizes are probably not very public domain kind of information. HB modeled the AWG-9 with ±133 knots based on their documentation, so I trust that is correct.

Most of the DCS "modern" fighters are like a bit less than half that value. But that seems fine for the AWG9