w.i.p. F-16 Pre-setting JDAM Accuracy (Steerpoint) in free server after april Patch

[opps]

On 5/2/2024 at 7:27 PM, Lord Vader said:

Hi all.

After the last INS refactoring, the GPS coordinate logic for all IDMs needs to be reviewed by the users, especially in older missions.

Now, weapons like the JDAM, JSOW or WCMD will get the GPS coordinates and the INS drift from the MMC. This means that pure coordinates given to the weapon without a designation (via TPOD, FCR, etc) may introduce and undesired drift (up to 30m in the worse case scenario), adding to the weapon CEP itself (5m).

So, as a rule of thumb, all targets sent to IDM weapons should be designated by one of the aircraft sensors for accuracy.

In the future, some details may be changed, for example the ability for the weapon to have its own guidance. But, for now, this is working as intended.

Just curious, if MMC has drift (up to 30m), how does sensor designation offer better accuracy? Doesn't aircraft sensor also depend on MMC's accuracy?

Edited May 4, 2024 by opps

[KlarSnow]

It sounds like there is no correction for relative targeting, and the 30 second figure is a "maximum GPS acquisition time", the bomb attempts to acquire GPS 3 seconds after launch.

Please reference these 3 papers and they should be fairly illuminating on the errors that are being discussed here, and the things that the JDAM does to resolve these errors. 

The third paper also references a 5 meter CEP for JDAM when target location error is removed (if you input perfect coordinates) the 13 meter CEP often referenced includes an arbitrary target location error that is not related to the bombs ability to guide. If the bomb has perfect coordinates and acquires GPS, it should guide to within 5 meters, not 13.

All three of these papers are publicly available and approved for public release.

https://trace.tennessee.edu/cgi/viewcontent.cgi?article=6123&context=utk_gradthes

https://trace.tennessee.edu/cgi/viewcontent.cgi?article=3487&context=utk_gradthes

https://apps.dtic.mil/sti/tr/pdf/ADA389516.pdf

[Corrigan]

Maybe we should ping  @Lord Vader too. The above documents seem valuable. 

[Sinclair_76]

Fast takeaway. The second document provided pretty much all the info we need on page 28&37.

3 hours ago, KlarSnow said:

https://trace.tennessee.edu/cgi/viewcontent.cgi?article=3487&context=utk_gradthes

 

There are four elements that contribute to JDAM system accuracy:

1) the JDAMs ystem components, including guidance hardware and software;

2) the delivery aircraft transfer alignment time hand-off accuracy, including location and velocities;

3) the GPS satellite error; and

4) the target location error (TLE) and the associated coordinate format.

 

And...

 

Once GPS-aiding is accomplished, the aircraft hand-off error is removed and is not a factor in system accuracy. 

 

Furthermore (page 37)...

 

The handoff of high quality GPS information to the bomb, allows the JDAM to achieve full position and velocity acquisition within a maximum of 27 seconds and full GPS navigation within 28 seconds after release.

 

This document states that after a gps handoff and roughly 28 seconds after release the aircraft error is not a factor anymore.

Thank you @KlarSnow!

Edited May 4, 2024 by Sinclair_76

[VGlusica]

I will be entirely candid here, for me DCS is a tertiary hobby, so I am not all that invested in this, but since I am now semi-retired I have time to ruminate on various esoteric subjects. @Lord Vader The 13m CEP came from F-16.net.  However, this is actually somewhat irrelevant.  First, I want to be clear that I am not directing any of my comments at you but rather to the "team," this is what I mean when I say "you." 

In large part the point that I am trying to make is that if you are relaying on publicly available information the CEP modeled should not be 13m or 30m it should be 5m.  All publicly available open-source literature references this as JDAM accuracy.  Both @Sinclair_76 and @KlarSnow provided you some very good reference materials to this fact, I would suggest that you take what they are saying into consideration.  One thing that seems to happen any time a discussion like this comes up, is that "realism" is brought up in defense of the underlaying decision.  When, however, that definition of realism is brought into question, the second thing invoked is "official sources."  What ends up frustrating number of us is that you don't have access to "official sources" either, as these sources carry various levels of classification and are not in public domain.  So, making a statement like "we cannot use websites without official sources" feels very disingenuous to some of us.  I would like to make my point one more time, the "team" does not have access to "official sources."  The best you have is access to SMEs whose qualifications are known only to the "team."  I am somewhat skeptical of some of those qualifications, because a few years ago we had an SME who did not know how an ASL properly functions.  Regardless, even the best of the SMEs in this subject field are bound by oath, duty and law not to disclose classified information.  This leaves us all back at the beginning and my original point from my previous post.  All of the publicly available open-source information points to 5m CEP not anything else.  No open-source documents point to error stacking between launch platform and the weapon, and this is exactly what is currently modeled.  If the "team" wants to be accurate it should model things and behaviors that can be verified.  What is being modeled is the conjecture based on unverified information provided by someone who is only peripherally familiar with architecture of GPS/INS guidance systems and not reflective of real-world behavior and performance.

[Swift.]

On 5/4/2024 at 4:46 PM, KlarSnow said:

It sounds like there is no correction for relative targeting, and the 30 second figure is a "maximum GPS acquisition time", the bomb attempts to acquire GPS 3 seconds after launch.

Please reference these 3 papers and they should be fairly illuminating on the errors that are being discussed here, and the things that the JDAM does to resolve these errors. 

The third paper also references a 5 meter CEP for JDAM when target location error is removed (if you input perfect coordinates) the 13 meter CEP often referenced includes an arbitrary target location error that is not related to the bombs ability to guide. If the bomb has perfect coordinates and acquires GPS, it should guide to within 5 meters, not 13.

All three of these papers are publicly available and approved for public release.

https://trace.tennessee.edu/cgi/viewcontent.cgi?article=6123&context=utk_gradthes

https://trace.tennessee.edu/cgi/viewcontent.cgi?article=3487&context=utk_gradthes

https://apps.dtic.mil/sti/tr/pdf/ADA389516.pdf

The thing that I've always been missing when trying to make a report for this is how the decision between relative and absolute modes is supposed to work in viper.

It's explained very clearly for strike eagle, that a Sequence Point or SIT designation will trigger absolute mode and a TPOD/Radar/HUD designation cause relative.

But without explicit documentation to show that viper uses absolute for SPI with no slew applied and relative for SPI with a slew applied, then we can't really report anything?

[Sinclair_76]

Good point.

• For BOC (bomb on coordinate)/ PP (preplanned) the JDAM employs absolute mode
• For BOT (bomb on target) / TOO, the JDAM uses relative targeting mode

Basically in absolute mode, aircraft error is removed by JDAM GPS after 28 seconds and for relative targeting mode the error (or bias) is maintained.
This document (p 7-8) perfectly describes the JDAM relative targeting mode (https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=&httpsredir=1&article=6123&context=utk_gradthes)

When using the preplanned data transfer method, in-flight data transfer method, or entering coordinates through cockpit data entry devices, JDAM will be released using the target coordinates in an earth-based reference frame (absolute mode).

Furthermore...

The relative mode of the weapon is used when onboard the aircraft sensors are used to generate the target coordinates. In the relative mode, once the JDAM acquires a GPS signal after release from the aircraft, the weapon compares its GPS position to the JDAM INS position (previously aligned to the aircraft). Any difference is assumed to be an aircraft induced position error. The error is then applied as a correction factor (bias) to the coordinates (Figure 1-3).

Now with a quote from @Lord Vader

On 5/2/2024 at 12:27 PM, Lord Vader said:

Hi all.

After the last INS refactoring, the GPS coordinate logic for all IDMs needs to be reviewed by the users, especially in older missions.

Now, weapons like the JDAM, JSOW or WCMD will get the GPS coordinates and the INS drift from the MMC. This means that pure coordinates given to the weapon without a designation (via TPOD, FCR, etc) may introduce and undesired drift (up to 30m in the worse case scenario), adding to the weapon CEP itself (5m).

So, as a rule of thumb, all targets sent to IDM weapons should be designated by one of the aircraft sensors for accuracy.

In the future, some details may be changed, for example the ability for the weapon to have its own guidance. But, for now, this is working as intended.

Now I think I understand where ED is going. They have relative targeting mode modeled but not yet absolute mode. And that is what is probably (assumption/conjecture) is meant by 

Quote

In the future, some details may be changed, for example the ability for the weapon to have its own guidance. But, for now, this is working as intended.

And to overcome the fact that absolute mode is not modeled.

Quote

So, as a rule of thumb, all targets sent to IDM weapons should be designated by one of the aircraft sensors for accuracy.

If you have pre planned targets designate the with TGP to force the Relative Targeting Mode, because that is correctly modeled it seems.

 

 

Now I also understand the logic behind the strict steerpoint grouping. To answer to @Swift.'s conundrum, could this be a solution?

Steerpoint 1-20 are basically from the DTC and are preplanned or fatfingered/edited in the aicraft. They can be considered absolute.
Steerpoint 21-25 are MGRS are fatfingered and are considered absolute. JTAC BOC for example.
Steerpoint 26-30 are derivatives from AGR / TGP / JHMCS / HUD / Fly-Over and are considered relative at all times since the aircraft generated this coordinate.

The moment you use steerpoint 1-25 with no delta(slew) applied it can be considered absolute. In that case JDAM can use the absolute mode.
Once you introduce a delta(slew) with ie the TGP and update it (TMS up) the steerpoint with delta(slew) is considered relative since you're designating a target and JDAM should then use relative mode. Once you remove the delta(slew) with TMS down/CZ it is considered absolute again.
Using JDAM on a markpoint is always relative and relative targeting mode is applied.

 

Thanks @Swift. for pointing me in the right direction.

 

Edited May 6, 2024 by Sinclair_76
added slew to delta for clarification

[Swift.]

20 minutes ago, Sinclair_76 said:

 

If we apply the F-15E logic, then Absolute should be triggered by targeting of any steerpoint (including those pulled from datalink).

The 'nice' logic that could apply to F16 is that Absolute is triggered whenever a weapon is dropped with no cursor slew applied to the SPI, ie CZ has been selected. Relative would then be triggered whenever a cursor slew exists (ie whenever the SPI is driven by a sensor).

 

As a side note, Its very important to note in general however, that JDAMs even in Absolute will not be as precise as an LGB. I see a lot of people not understanding why their JDAMs are missing, and this is sometimes the cause. 

[Swift.]

On 5/3/2024 at 6:21 AM, Moonshine said:

now why would the bomb get degraded INS data if my jet has a perfect alignment due to having GPS and therefore no INS drift?

Important point on this, your jet doesnt have 'perfect' alignment due to GPS.

Remember that GPS is a measurement tool, and like all measurements there are errors. There's a complicated explanation of this that involves time of arrival and overlapping ellipses that you can google if you are interested. But the key takeaway is to never assume your GPS is 'perfect', this is why in F-15E at least, the best technique is to retarget using radar or TGP when delivering weapons, even if you have GPS.

[Hofftari]

The point still stands, that the way the F-16 has been changed concerning the JDAM deployment has been with this thesis:

Quote

The GPS guidance algorithm is only used to make corrections after about 30 seconds after release. -Lord Vader

So if I understand this correctly; currently in the game no GPS guidance can affect the JDAM until after it has had a 30 second TOF, while the .edu links above state that GPS guidance is activated 3 seconds after a drop, blending it in with the INS data, and instead after 28 seconds does it fully rely on GPS data with INS data being no factor anymore.

With the Litening pod shuffling around on the F-16 in its current state, not being able to stay on one fixed point anymore, the F-16 in its current state is broken when it comes to guided munitions.

Edited May 6, 2024 by Hofftari

[Sinclair_76]

1 hour ago, Hofftari said:

The point still stands, that the way the F-16 has been changed concerning the JDAM deployment has been with this thesis:

So if I understand this correctly; currently in the game no GPS guidance can affect the JDAM until after it has had a 30 second TOF, while the .edu links above state that GPS guidance is activated 3 seconds after a drop, blending it in with the INS data, and instead after 28 seconds does it fully rely on GPS data with INS data being no factor anymore.

With the Litening pod shuffling around on the F-16 in its current state, not being able to stay on one fixed point anymore, the F-16 in its current state is broken when it comes to guided munitions.

 

No not broken. 

Preplanned JDAM is less accurate at this moment. Using TGP to employ JDAM works perfectly fine for me, since it seems Relative Targeting Mode is modeled. 

Edited May 6, 2024 by Sinclair_76

[KlarSnow]

Just to be clear in case there is any confusion, with absolute targetting with perfect coordinates (no target location error) according to those papers, the JDAM should have a 5M CEP, not 13, which can in fact be better than an LGB depending on energy state and release parameters. With relative targeting the JDAM should have 5M CEP plus whatever the Target Location Error (TLE) of the designation source is.

As to how the F-16 flips the absolute vs relative bit, without looking at pubs for the jet, I dont know, I would assume an unslewed steerpoint from what little I know about the F-16's navigation system.

Finally a relative targeted JDAM should be 5M plus whatever its TLE error is, what the relative target bit tells the JDAM is once it acquires GPS and figures out what the relative error is, it then shifts the target coordinates it received to correct for that positional error. So again it should be as accurate as the designation source plus 5 meters. If DCS has zero TLE in a targeting pod designation with laser rangefinding, then it should be that accurate.

[WHOGX5]

17 hours ago, KlarSnow said:

Just to be clear in case there is any confusion, with absolute targetting with perfect coordinates (no target location error) according to those papers, the JDAM should have a 5M CEP, not 13, which can in fact be better than an LGB depending on energy state and release parameters. With relative targeting the JDAM should have 5M CEP plus whatever the Target Location Error (TLE) of the designation source is.

As to how the F-16 flips the absolute vs relative bit, without looking at pubs for the jet, I dont know, I would assume an unslewed steerpoint from what little I know about the F-16's navigation system.

Finally a relative targeted JDAM should be 5M plus whatever its TLE error is, what the relative target bit tells the JDAM is once it acquires GPS and figures out what the relative error is, it then shifts the target coordinates it received to correct for that positional error. So again it should be as accurate as the designation source plus 5 meters. If DCS has zero TLE in a targeting pod designation with laser rangefinding, then it should be that accurate.

As far as I know, it is currently impossible to do absolute targetting in the DCS F-16C. I haven't bothered to actually report this yet or record a trackfile, but you will notice, especially at very low altitudes, that when doing a CCRP delivery with cursor zero and no offset, the release que will jump back and forward as you fly past terrain because the aircraft constantly updates the ranging to the target, which at low altitudes is often masked. Sometimes this will lead to the release cue blinking past the velocity vector, meaning that the weapon never releases, especially during lofting.

[Swift.]

5 hours ago, WHOGX5 said:

As far as I know, it is currently impossible to do absolute targetting in the DCS F-16C. I haven't bothered to actually report this yet or record a trackfile, but you will notice, especially at very low altitudes, that when doing a CCRP delivery with cursor zero and no offset, the release que will jump back and forward as you fly past terrain because the aircraft constantly updates the ranging to the target, which at low altitudes is often masked. Sometimes this will lead to the release cue blinking past the velocity vector, meaning that the weapon never releases, especially during lofting.

Do you have a reference for how F-16 should mechanize Absolute targeting? Without it, its kinda challenging to report anything.

[Sinclair_76]

6 hours ago, WHOGX5 said:

As far as I know, it is currently impossible to do absolute targetting in the DCS F-16C. I haven't bothered to actually report this yet or record a trackfile, but you will notice, especially at very low altitudes, that when doing a CCRP delivery with cursor zero and no offset, the release que will jump back and forward as you fly past terrain because the aircraft constantly updates the ranging to the target, which at low altitudes is often masked. Sometimes this will lead to the release cue blinking past the velocity vector, meaning that the weapon never releases, especially during lofting.

CCRP delivery isn't a delivery mode for JDAM, so absolute or relative doesn't apply.

[Furiz]

Tag was changed to W.I.P., guess they recognized the problem.

[llOPPOTATOll]

On 5/6/2024 at 3:41 AM, Swift. said:

The thing that I've always been missing when trying to make a report for this is how the decision between relative and absolute modes is supposed to work in viper.

It's explained very clearly for strike eagle, that a Sequence Point or SIT designation will trigger absolute mode and a TPOD/Radar/HUD designation cause relative.

But without explicit documentation to show that viper uses absolute for SPI with no slew applied and relative for SPI with a slew applied, then we can't really report anything?

The viper uses Absolute targeting when you don't slew the SPI, the bomb just loads the coords from the stpt into the bomb. If you slew then it goes into relative targeting

[Lord Vader]

22 hours ago, Furiz said:

Tag was changed to W.I.P., guess they recognized the problem.

The tag was changed because, like I said above, the JDAM own guidance logics are currently being worked on. So, again, this is work in progress for now. 

[Nedum]

On 5/6/2024 at 9:41 AM, Swift. said:

The thing that I've always been missing when trying to make a report for this is how the decision between relative and absolute modes is supposed to work in viper.

It's explained very clearly for strike eagle, that a Sequence Point or SIT designation will trigger absolute mode and a TPOD/Radar/HUD designation cause relative.

But without explicit documentation to show that viper uses absolute for SPI with no slew applied and relative for SPI with a slew applied, then we can't really report anything?

It would be nice if you could link me to the Strike Eagle papers for that SIT behavior.

But, if we don't know how do to do it right for the F16, we do it all the other way, in a way who is not even near of how it's done for the other modules? I can hardly understand that kind of logic. I would suggest doing it the way it's done for all the other modules and not to tell something about evidence if the way it's currently done has no evidence too.

[Swift.]

8 minutes ago, Nedum said:

It would be nice if you could link me to the Strike Eagle papers for that SIT behavior.

But, if we don't know how do to do it right for the F16, we do it all the other way, in a way who is not even near of how it's done for the other modules? I can hardly understand that kind of logic. I would suggest doing it the way it's done for all the other modules and not to tell something about evidence if the way it's currently done has no evidence too.

There aren't any modules currently that do it properly. F-16 is currently the most advance modelling of this, we are just missing a tiny little extra piece. They've changed the tag to WIP so hopefully that means the suggestions have been taken on board.

In the meantime remember to sweeten your designation with TGP or FCR before you drop.

[Rainmaker]

1 hour ago, Swift. said:

There aren't any modules currently that do it properly. F-16 is currently the most advance modelling of this, we are just missing a tiny little extra piece. They've changed the tag to WIP so hopefully that means the suggestions have been taken on board.

In the meantime remember to sweeten your designation with TGP or FCR before you drop.

That’s an incorrect statement. The one sensor lacking is SE is the TGP, and that’s because its still using ‘unadjusted’ coordinates as the pod always knows true coords. The other sensors are tied to the nav info provided by the jet. 

[Swift.]

9 minutes ago, Rainmaker said:

That’s an incorrect statement. The one sensor lacking is SE is the TGP, and that’s because its still using ‘unadjusted’ coordinates as the pod always knows true coords. The other sensors are tied to the nav info provided by the jet. 

Yeah so no jet (neither F-15E nor F-16C) currently implement an absolute vs relative logic in JDAM. In SE if you dropped on a targetpoint with the MN drifted, the weapon would miss the target because it would behave as if it were in relative mode.

Which bit did you think was incorrect in what I said?

[Rainmaker]

1 minute ago, Swift. said:

Yeah so no jet (neither F-15E nor F-16C) currently implement an absolute vs relative logic in JDAM. In SE if you dropped on a targetpoint with the MN drifted, the weapon would miss the target because it would behave as if it were in relative mode.

Which bit did you think was incorrect in what I said?

The SE very much has absolute vs rel targeting. How do you think you can make a JDAM target with a target point in the SE?  That should help you answer the question. 

[Swift.]

1 minute ago, Rainmaker said:

The SE very much has absolute vs rel targeting. How do you think you can make a JDAM target with a target point in the SE?  That should help you answer the question. 

So you are saying that if you have a drifted MN in the SE and drop on a targetpoint, the JDAM will acquire GPS during its flight and correct its trajectory to the GPS driven coordinates? Because that would require a modelling of that function on the weapon itself (which is EDs area), which I didnt think had been done yet.

Edited May 9, 2024 by Swift.

[Rainmaker]

1 minute ago, Swift. said:

So you are saying that if you have a drifted MN in the SE and drop on a targetpoint, the JDAM will acquire GPS during its flight and correct its trajectory to the GPS driven coordinates? Because that would require a modelling of that function on the weapon itself (which is EDs area), which I didnt think had been done yet.

 

You are trying to overlap two systems that are mech’d differently. I am not talking about the weapon self correcting after launch…I’m talking about your previous statement of absolute vs relative targeting not existing. It does. Example. BOC via CC mem vs sensor (radar in this case) targets. Nav coord transfer doesnt even exist in the SE in DCS, you are using sensor xfer in that case, so comparing that with how the -16 releases is apples and oranges here. 
 

But this is all getting well OT to the original report…that’s in a -16 forum so…