What kind of accuracy should I expect from JDAMs?

[Nealius]

(NOT a bug report)

Asking for details on expected CEP to manage my expectations because, in the past on different platforms in DCS, JDAMs have been reliably accurate enough to hit vehicles with boring regularity (particularly in the A-10C). In the Viper I'm having a substantial number of GBU-38s missing the TGP markpoints by about 8m or so, with others being smack-on. Is this typical accuracy with Viper+GB38+Litening+Markpoint employment?

Edited October 2, 2022 by Nealius

[Tom Kazansky]

JDAMs are specified 43 ft (13m).

I read of realised 5m to 7m with GPS/INS and 30m with INS only.

To check accuracy in DCS use unlimited weapons and active pause. Then drop as many bombs as you need on the same spot and check circle of hits on the ground. It helps if you place an indestructible target with known size.

 

Edited October 2, 2022 by Tom Kazansky
typo

[Gunrun_KS]

13 hours ago, Nealius said:

(NOT a bug report)

Asking for details on expected CEP to manage my expectations because, in the past on different platforms in DCS, JDAMs have been reliably accurate enough to hit vehicles with boring regularity (particularly in the A-10C). In the Viper I'm having a substantial number of GBU-38s missing the TGP markpoints by about 8m or so, with others being smack-on. Is this typical accuracy with Viper+GB38+Litening+Markpoint employment?

 

I've wondered this as well.  I did watch a video that says to target the base of the target rather than the top of it.  It has helped some but still not a sure fix.  I've also noticed my gbu-83 will pass the target and come back to it.  Is this normal behavior?

[Raviar]

"In its most accurate mode, the JDAM system will provide a weapon circular error probable of 5 meters or less during free flight when GPS data is available"

https://www.minot.af.mil/About-Us/Fact-Sheets/Display/Article/264282/joint-direct-attack-munitions-gbu-313238/

https://www.minot.af.mil/About-Us/Fact-Sheets/Display/Article/264282/joint-direct-attack-munitions-gbu-313238/

Edited October 3, 2022 by Raviar

[Skyracer]

 

[Fromthedeep]

Only the devs can answer this, speculating on the intended behaviour is not going to give you an accurate answer because JDAMs are very complicated and the integration will change from jet to jet. Since JDAMs are so complicated beasts, copy pasting a CEP value from a public source will leave you with a very incomplete picture. Remember that CEP is basically a statistical value, not the exact dispersion that your bombs will necessarily have in practice. It's a very long and complex discussion, but just to illustrate the point, let's get into the weeds of JDAM a little bit.

 

What factors contribute to JDAMs missing their targets? The potential inaccuracies are the GPS solution, imperfect control inputs by the autopilot (guidance error) and target location error. Obviously the error due to the autopilot is something that's inherent to the weapon itself and you cannot do anything about it as an aircrew.

 

GPS related errors are multifaceted issues, it can be due to reduced accuracy in the navigation solution (improper geometric alignment of the satellites, terrain masking, increased solar activity etc.) or it can be the result of the User Equivalent Range Error, which is essentially due to inaccuries tied to the GPS receiver itself. [This topic in general has plenty of fully open source literature so if someone is interested they can really dig into GPS as a whole.] 

 

TLE is the result of inaccuracies when deriving coordinates for an arbitrary point in 3D space. Simply put, you cannot be sure that using your on board sensors, the coordinates that they will generate will be the actual coordinates of what you want to target partially due to the small statistical uncertainty of the GPS solution. This uncertainty is understood by asking where the center of the GPS solution is on the aircraft. The center of the jet itself? The cockpit? The bomb? The GPS antenna? The targeting pod? All of these are a small, but measurable distance away from one another. This issue is exacerbated if the GPS solution is reduced in accuracy due to any of the previously established factors.

 

The other part that can contribute to TLEs are essentially errors. These errors are mainly influenced by slant range. Pointing the targeting pod at something from far away may not actually point at what you want it to point at, parts of the picture can be foreshortened if the graze angle is too shallow, the visual fidelity may not be good enough to accurately ascertain what it really points at and using passive ranging it can give erroneous values. Using the laser rangefinder in a situation like this to figure out slant range (and therefore height above target using trig) can make the problem worse, because you can end up getting into the first-last return problem due to the increased spot size (spot size increases with increased slant range and decreased graze angle) and having the slant range being calculated from a point that's not the intended target.

 

This is why in reality, the aircrew is given a set of parameters where the designation is considered valid and that is established through testing. These errors regarding the sensors themselves can be mitigated by using the proper sensors for the specific aircraft (some can use SAR mapping to generate valid coordinates while other may not, some can use passive TGT pod ranging accurately, some need the laser, some are not allowed to use DTED derived elevation and some may be etc.) at the appropriate range and altitude to drop the designation.

 

This is not a separate issue but having accurate altitude of the target is incredibly important, it must be emphasized. Remember that the bomb is seeking a 3D point in space, so even if the 2D position is correct, if the altitude is not, it will likely land long or short. This is sometimes called "6-12 error" (imagine a clockface, target in the middle and the bomb either lands towards the 12 o' clock position or the 6 o' clock position) and it can be heavily mitigated by using steeper impact angles and having very accurate elevation for the target. This is the reason why using on board DTEDs is likely not going to be good enough because their accuracy can be limited.

 

But remember, not just the pointing errors can contribute to TLE but the aircraft's own position error itself. The absolute position of the target can be expressed in Earth based coordinates. These Earth based coordinates in a preplanned strike can be mensurated and uploaded to the bomb. If dropped, the bomb will directly guide to the these coordinates on its own. In this instance you mitigate all the potential issues, because even though the bomb's INS is aligned based on where the aircraft thinks it is through the transfer alignment (so the aircraft can hand off an erroneous position), it will acquire the satellites after release and have the ability to correct both for handoff error and the drift of its own INS. The coordinates are absolute (Earth centered) and accurate and with the satellite data it will also have an accurate position and at this point, the main contributing factors that can cause a miss are the autopilot guidance errors and a potentially degraded GPS signal.

 

 

The issue here, of course is that you want the JDAMs to be flexible and the ability to employ on non preplanned targets. But the aircraft's own position may not be perfectly accurate in relation to the Earth even using the most modern EGI and if that's the case, it may generate erroneous coordinates. There may be a small discrepancy between where the aircraft thinks it is (and the coordinates in the TGT pod will be derived based on its present position and where the pod is looking) and where it actually is. In this instance the coordinate is generated based on where the aircraft thinks it is, the position of the target is determined relative to the aircraft. 

 

 

To mitigate this issue, there's a special logic that the bomb can utilize called relative targeting. After acquiring the satellites, it can compare its own GPS accuracy to the value it was handed off by the aircraft. And in relative targeting logic if there's discrepancy between the two it can quantify it and give it a vector and using this vector, it can offset the target coordinates themselves. 

 

This is also called bomb on target (BOT) vs bomb on coordinate (BOC), with BOT being relative targeting and BOC being absolute targeting. To put it in simple terms, in absolute targeting, the bomb knows where it needs to go and during guidance it can correct its position compared to the position it was given by the aircraft. In relative targeting, the destination itself will also get corrected if needed. The exact mechanics of this are very complex and arcane and knowing that it can do it is more than enough for a general understanding of JDAM. (Hell, even real aircrew don't typically get into the weeds of this outside of like Weapons School.)

 

The last point that we have to understand is that the the bomb's internal coordinate system may not be the same as the aircraft's. The altitude used by the aircraft for targeting is in MSL and the bomb requires height above ellipsoid or HAE. Converting to HAE is done automatically but errors can happen which is why its normal TTP or "read back from the bomb" or to look at the appropriate JDAM page and see the elevation that is fed into the bomb, not the elevation of the designation that the aircraft is using. 

 

So, knowing all this, how is the F-16's JDAM integration? It's not a particularly sophisticated or well integrated system. It cannot directly address the individual bombs nor can it display what's being uploaded in the bomb. It's a simple hack, at pickle the bomb without introducing any slew, the bomb will get the coordinates from the steerpoint/markpoint and invoke BOC logic, if designating through the pod, it invokes BOT logic. It cannot do proper multi targeting where you store different self designated targets while still utilizing BOT logic nor can you read back from the bomb. So IRL the employment method is lasing and then pickle for a self designated target or dropping on the steer/markpoint and incrementing if needed. 

The reason why this whole wall of text was necessary because this stuff is complex, depends on how the individual jet is integrated (like remember that the reason why the University of Teneessee whitepaper exists is because the Harrier back in the day didn't properly invoke BOT logic when using the TGT pod to self designate) and how much of these confounding factors are implemented by ED. Most of these aspects are not random errors, the expected accuracy can be better or worse depending on the conditions and aircrew actions. It's also a possibility for one jet being a more realistic implementation of a specific concept than an older jet in DCS, so only the devs can say what the intention here is.

Edited October 4, 2022 by Fromthedeep

[TheBigTatanka]

Excellent write up. Anytime someone is talking about height above ellipsoid, or TLE, we should probably listen. Thanks for the words!

Sent from my Pixel 3 XL using Tapatalk

[Nealius]

14 hours ago, Fromthedeep said:

This is not a separate issue but having accurate altitude of the target is incredibly important, it must be emphasized. Remember that the bomb is seeking a 3D point in space, so even if the 2D position is correct, if the altitude is not, it will likely land long or short.

Perhaps it's partly my target elevation (dropping on mountainous areas) as well as the Viper-specific integration that you mentioned later in regards to BOT logic and dev coding. 

 

[Fulgrim]

5 hours ago, Nealius said:

Perhaps it's partly my target elevation (dropping on mountainous areas) as well as the Viper-specific integration that you mentioned later in regards to BOT logic and dev coding. 

 

Are you lasing the target while you TMS-Up? 

[Hobel]

Am 2.10.2022 um 06:22 schrieb Nealius:

(NOT a bug report)

Asking for details on expected CEP to manage my expectations because, in the past on different platforms in DCS, JDAMs have been reliably accurate enough to hit vehicles with boring regularity (particularly in the A-10C). In the Viper I'm having a substantial number of GBU-38s missing the TGP markpoints by about 8m or so, with others being smack-on. Is this typical accuracy with Viper+GB38+Litening+Markpoint employment?

 

The Lua says Cep 5m

But I know what you mean, from time to time, even though all the parameters are correct, the bombs just drift a lot, it looks like something is wrong with the autopilot.

 

Like this https://streamable.com/pu5gkc

Edited October 4, 2022 by Hobel

[Fromthedeep]

13 hours ago, Nealius said:

Perhaps it's partly my target elevation (dropping on mountainous areas) as well as the Viper-specific integration that you mentioned later in regards to BOT logic and dev coding. 

 

Its also entirely possible that these nuances aren't implemented in DCS and they simply hardcoded a given CEP and the bomb will sometimes randomly miss.

[Nealius]

On 10/4/2022 at 4:42 PM, Fulgrim said:

Are you lasing the target while you TMS-Up? 

No, but I am definitely within range to do so. Would that make things more accurate? 

[Moonshine]

I think there is more to this; according to wags Viper mini updates, one of the listed points is „GPS target coordinates shifting“. Maybe that has to do with those weird „near miss“ situations even when the solution looked perfect.

the other thing is still the damage modelling. A near miss with a 500lbs bomb should not leave any sort of vehicle unharmed..

[Exorcet]

4 hours ago, Moonshine said:

I think there is more to this; according to wags Viper mini updates, one of the listed points is „GPS target coordinates shifting“. Maybe that has to do with those weird „near miss“ situations even when the solution looked perfect.

I hope so. I can't use VIS mode at all. Even if I release right after designating the target, the bomb just flies off on its merry way to anywhere except where I told it to go.

[Fulgrim]

5 hours ago, Nealius said:

No, but I am definitely within range to do so. Would that make things more accurate? 

Yes, laser will give your coordinates more accuracy, specially with elevation. It is recommended to lase with guns, rockets, CCIP, and Tpod designation.

[ARM505]

All I heard was.........'the missile knows where it is....