reported earlier Aim-120C does not always track target in TWS if lock is broken while lofting

[J-man]

I have tested the Aim-120C with the F-15C, and sometimes the missile keeps going up in a straight line if it is launched in TWS mode if the lock is broken shortly after launch.

This behaviour seems highly erratic, sometimes multiple missiles follow this behaviour, sometimes they track fine and sometimes only one of the missiles does not track properly.

 

Images and Trackfiles attached. As you can see, sometimes the missile just keeps going up, instead of tracking the target, even if all 3 missiles were fired the exact same way and in direct succession.

 

120C_TWS_InfiniteLoft_01.trk 120C_TWS_InfiniteLoft_02.trk

Edited March 22, 2021 by J-man

[dundun92]

Known bug, and has been reported countless times. Its due to a lncomplete missile INS simulation. So at this point all we can do is wait tbh, and simply support shots till active.

[Csgo GE oh yeah]

This also happens very regularly if lock is NOT broken. 
Honestly i don't even mind it that much if it happens when the lock IS broken. But to have a perfect lock on a hot target aspect the whole way, and then seeing in tacview your missile just went stupid right off the rail is a bit sad  
 

Edited March 22, 2021 by Csgo GE oh yeah

[GGTharos]

14 hours ago, Csgo GE oh yeah said:

Honestly i don't even mind it that much if it happens when the lock IS broken.

 

You should, the INU is there for a reason.

[Csgo GE oh yeah]

yeah i know but at this point i'll take anything 

[Bear21]

There is a description on how the AIM-120 behaves when e.g. TWS track is lost or STT lock broken and the one-way datalink updates via the radar goes missing in the otherwise excellent Hoggit F-18 resource. Here the quotes with link: https://wiki.hoggitworld.com/view/F/A-18C#AIM-120_AMRAAM

 

"Inherently, the AMRAAM is a partially "fire and forget" missile in that once it acquires the target with its own Radar (termed the "Active" phase), maintaining the target Radar trackfile is no longer necessary; " 

 

This is incorrect, you don't HAVE to support the missile once it leaves the rail, the aircraft MC has stored a 3D target space vector in missile memory before launch that tells the missile computer how to calculate the predicted intercept point (given target doesn't maneuver) and its INS tells the computer the missile 3D space vector at all times. The missile autopilot then flies the missile based on this information to X nm (Pitbull distance) of predicted intercept point, where the missile radar searches for the target. If the target has changed course during time of flight the probability of finding the target is reduced.  But the missile flight path shall not be affected. A datalink update of the target 3D space vector will affect the missile flight path, however, if the resulting calculated intercept point by the missile computer is changed.

 

The missile will at all times fly to the latest predicted target intercept point, based on the target 3D space vector info it has stored.

 

This is not what the Hoggit text says: 

" Post-launch Datalink continues until the missile enters the "Active phase" at an optimal point automatically. A Time to Active (TTA) cue is provided to the pilot as explained below. If the target trackfile is lost prior to this point, the missile goes Active prematurely."

 

This is incorrect, the missile does not open the radar prematurely when the datalink update goes missing. It continues flying to the latest intercept point it has (this is verified information). If this is indeed the DCS implementation it's wrong.

Edited April 3, 2021 by Bear21

[Cmptohocah]

On 4/3/2021 at 9:54 AM, Bear21 said:

... It continues flying to the latest intercept point it has (this is verified information). If this is indeed the DCS implementation it's wrong.

 

I don't see this being an issue in practical sense. Unless the target is on some sort of autopilot and it does not change course/speed/altitude, the 120 that lost its DL will just fly into empty space, as far as the target is concerned. I mean it will fly to the intercept point sure, but the target won't be there.

[Bear21]

4 hours ago, Cmptohocah said:

I don't see this being an issue in practical sense. Unless the target is on some sort of autopilot and it does not change course/speed/altitude, the 120 that lost its DL will just fly into empty space, as far as the target is concerned. I mean it will fly to the intercept point sure, but the target won't be there.

Which is fine, this is a predictable behavior and a missile that doesn't get DL updates on the targets position can't do much else.

 

It's also the only reasonable programming of the missile as everything else (like missile turning on seeker at DL loss to look for targets) would create an unacceptable risk of friendly fire. In effect, in a CAP package, no member can advance towards any targets as any missile fired behind their 9-15 line could suddenly turn on the seeker and kill them from behind.  

[GGTharos]

4 minutes ago, Bear21 said:

It's also the only reasonable programming of the missile as everything else (like missile turning on seeker at DL loss to look for targets) would create an unacceptable risk of friendly fire. In effect, in a CAP package, no member can advance towards any targets as any missile fired behind their 9-15 line could suddenly turn on the seeker and kill them from behind.  

 

The missile still has range, angle and doppler gates to distinguish its desired target from anything else.  The reason for not powering the seeker early is that you're wasting battery power on something that isn't needed.  Yep, ECM might mess with some things, but that's not the reason that the seeker is being turned on later.  Sparrow has its seeker on shortly after coming off the rail and it could certainly p ick up a friendly in its FoV to lock onto as well.  But again, all those gates get in the way.

 

Overall, if you're worried about blue on blue then you shouldn't be shooting any kind of missile with friendlies ahead of the 3-9 line.

Edited April 8, 2021 by GGTharos

[Bear21]

1 hour ago, GGTharos said:

 

The missile still has range, angle and doppler gates to distinguish its desired target from anything else.  The reason for not powering the seeker early is that you're wasting battery power on something that isn't needed.  Yep, ECM might mess with some things, but that's not the reason that the seeker is being turned on later.  Sparrow has its seeker on shortly after coming off the rail and it could certainly p ick up a friendly in its FoV to lock onto as well.  But again, all those gates get in the way.

 

Overall, if you're worried about blue on blue then you shouldn't be shooting any kind of missile with friendlies ahead of the 3-9 line.

 

You can't compare an AIM-7 which needs the reflected DPI/CW energy from a locked-on shooter radar and the AIM-120 which is acting on it's own once launched. 

[nighthawk2174]

Well they actually are rather comparable fundamentally, the AIM-7 just like the AIM-120 homes in on reflected radar energy and there is a limitation to the resolution of that.  Their both not infinitely small beams that see only just the target your looking at.  The only difference here is where the parent radar source is located.  If anything the 120 would be safer to fire into a close fight (Especially if its getting datalink updates) due to its ability to get range information and due to it being monopulse; multitarget problem being able to know when there are multiple targets in the res cell.  And various other benefits of being a MPRF radar such as a smaller res cell.  It will also dump targets that aren't the primary one being sent via datalink updates.  

 

Edited April 8, 2021 by nighthawk2174

[dundun92]

8 hours ago, Cmptohocah said:

I don't see this being an issue in practical sense. Unless the target is on some sort of autopilot and it does not change course/speed/altitude, the 120 that lost its DL will just fly into empty space, as far as the target is concerned. I mean it will fly to the intercept point sure, but the target won't be there.

Not rly. From time playing "that other sim", which does simulate this, I can assure you that its is definitely not useless. Sure, a 15° FoV isnt super big, but its not that small either.

[nighthawk2174]

1 hour ago, dundun92 said:

Not rly. From time playing "that other sim", which does simulate this, I can assure you that its is definitely not useless. Sure, a 15° FoV isnt super big, but its not that small either.

Agreed the HPRF search beamwidth is not 'small' its often more than enough especially if you can support it even for a 1/4 of its flight time.  In that other sim its 12° and in MACE its 10° with MPRF being much smaller.

[GGTharos]

6 hours ago, Bear21 said:

You can't compare an AIM-7 which needs the reflected DPI/CW energy from a locked-on shooter radar and the AIM-120 which is acting on it's own once launched. 

 

Yes I can.  The AIM-7 is acting on its own, it just can't emit its own signal.

[Bear21]

An AIM-7 is never acting on it's own, it can only home on the target that the shooter aircraft illuminates (except for HOJ, which we don't discuss here), i.e. the shooter aircraft decides at all times what target it homes onto. In PDI mode (CW is no longer used really), you have a beam width for the selection of target and illumination of 3°, four times more discriminating than the AIM-120 seeker, and this selection of the target is decided by the shooting aircraft until hit.

Edited April 9, 2021 by Bear21

[nighthawk2174]

And same with the 120 if you support it to pitbull and even more so than the sparrow if supported to impact.  Again as I said earlier if you have two targets really close to each other i'd take an amraam over the sparrow due to the range differences from the radar source.  

[Bear21]

1 minute ago, nighthawk2174 said:

And same with the 120 if you support it to pitbull and even more so than the sparrow if supported to impact.  Again as I said earlier if you have two targets really close to each other i'd take an amraam over the sparrow due to the range differences from the radar source.  

Yes, I agree we we have interrupted datalink all the way to Pitbull, but what we discuss here is the case where the datalink contact is broken in an early phase.

My thesis, corroborated with active AIM-120 pilots, is the AIM-120 flies to the intercept point calculated from the latest datalink target position estimation. It doesn't turn on the seeker at loss of datalink updates and wonder off on an own hunt for targets, it will fly to the estimated intercept point based on latest data and there activate the seeker at Pitbull distance.

[nighthawk2174]

Yes that is my understanding of its operation as well.  Which is what i've based my statements on, which if DCS does not operate this way well that's incorrect.

[GGTharos]

5 hours ago, Bear21 said:

An AIM-7 is never acting on it's own, it can only home on the target that the shooter aircraft illuminates (except for HOJ, which we don't discuss here), i.e. the shooter aircraft decides at all times what target it homes onto. In PDI mode (CW is no longer used really), you have a beam width for the selection of target and illumination of 3°, four times more discriminating than the AIM-120 seeker, and this selection of the target is decided by the shooting aircraft until hit.

 

The AIM-7 is acting on its own (this is what homing really means), and it can home in on anything that's either in or even a bit outside of the beam (the beam definition is mathematical according to power fall-off from the center) - there are other options too, HoJ being one of them and EMI another although in both cases, Pk should be greatly reduced.  The point is that there's still plenty of 'choice' for the AIM-7 to go hit something that is not the designated target as long as it's somehow 'in the way'.   The aircraft doesn't make that choice - the missile does, the aircraft only supplies the gates at launch to bias the missile's target selection.

 

At the distances you're shooting the 7, the aircraft's beam will be illuminating plenty of stuff around it and high RCS targets will be well illuminated outside of that beam.

 

And of course, none of this is modeled in DCS.

Edited April 9, 2021 by GGTharos

[Harker]

My thesis, corroborated with active AIM-120 pilots, is the AIM-120 flies to the intercept point calculated from the latest datalink target position estimation. It doesn't turn on the seeker at loss of datalink updates and wonder off on an own hunt for targets, it will fly to the estimated intercept point based on latest data and there activate the seeker at Pitbull distance.

This is correct, according to everything I've read about the AMRAAM and according to a discussion I've had with an actively serving pilot.

What's more, the fighter-missile datalink does not get severed at any point (by design, it can still happen if the fighter loses the target or the datalink is severed unexpectedly), and the missile can use it to help refine the expected position of the target, alongside its gates. Which means that a notch becomes extremely difficult, since you'll have to notch both the launch platform and the missile, in order for it to completely lose you, potentially increasing Pk. In DCS, as soon as the missile goes active, the datalink connection is severed.

[falcon_120]

Is this fixed in 2.7?

[nighthawk2174]

24 minutes ago, falcon_120 said:

Is this fixed in 2.7?

No

[Cmptohocah]

On 4/9/2021 at 5:08 PM, Harker said:

 

... Which means that a notch becomes extremely difficult, since you'll have to notch both the launch platform and the missile, in order for it to completely lose you, potentially increasing Pk. In DCS, as soon as the missile goes active, the datalink connection is severed.

Why does it matter if you are notching the launching platform's radar or not? If you are blind to the missiles seeker it's not gonna track you, regardless of the DL updates.

Airplane: "Hey AMRAAM the target is there."
120: "Where? I don't see it"
Airplane: "Move a bit to the left, it's right there."
120: "Ok, I moved but I can't find it."
Airplane: "I still see it, go a bit to the right."
120: "Cool, I did that, but where's the target?"

Edited April 23, 2021 by Cmptohocah

[TAW_Blaze]

1 hour ago, Cmptohocah said:

Why does it matter if you are notching the launching platform's radar or not? If you are blind to the missiles seeker it's not gonna track you, regardless of the DL updates.

Airplane: "Hey AMRAAM the target is there."
120: "Where? I don't see it"
Airplane: "Move a bit to the left, it's right there."
120: "Ok, I moved but I can't find it."
Airplane: "I still see it, go a bit to the right."
120: "Cool, I did that, but where's the target?"

 

 

Because once you're closing inside 2-3 miles to the missile it'll be next to impossible to stay within the notch gate. Currently in DCS you're more often defeating the missile much further away and it never bothers to track again, or if it does it doesn't matter anyway because it reset it's flight path to a trajectory that will make it impossible to hit you in the event that it manages to reacquire. By getting DL updates up to this range you are patching that gap and driving the missile reliably in the correct direction and once inside that range it's not realistic to reliably stay in that notch anymore.