[FIXED][1676] INS drift doesn't appear to contribute to actual CEP

[caelunshun]

The new GPS jamming feature is very cool, but I'm seeing some confusing results. In the attached scenario an F-35 fires 8 SDB weapons (INS + GPS-guided) at some trucks. GPS is globally denied via side enablers. The message log reports INS drift calculations as expected. However, the miss distances seem much smaller than expected. For a CEP increase on the order of 50-100m I would expect the weapons to miss by much more than 20-30ft (7-10m) on average. In fact, it appears the miss distances in this scenario are close to the same on average whether GPS is enabled or disabled.

Here is the message log from one run of this scenario:

7/5/2025 7:50:59 AM - Weapon: GBU-39/B SDB #13 has been without a GNSS update for 6 min 25 sec. Weapon has INS: 1990S+ tactical weapon INS. Max drift: 99m. Actual drift (CEP increase): 55m

7/5/2025 7:50:59 AM - GBU-39/B SDB #13 MISS: Missed UAV Launch Truck by 27ft.

7/5/2025 7:51:00 AM - Weapon: GBU-39/B SDB #17 has been without a GNSS update for 6 min 22 sec. Weapon has INS: 1990S+ tactical weapon INS. Max drift: 98m. Actual drift (CEP increase): 15m

7/5/2025 7:51:00 AM - GBU-39/B SDB #17 MISS: Missed UAV Launch Truck by 13ft.

7/5/2025 7:51:01 AM - Weapon: GBU-39/B SDB #12 has been without a GNSS update for 6 min 28 sec. Weapon has INS: 1990S+ tactical weapon INS. Max drift: 100m. Actual drift (CEP increase): 48m

7/5/2025 7:51:01 AM - GBU-39/B SDB #12 MISS: Missed UAV Launch Truck by 14ft.

7/5/2025 7:51:02 AM - Weapon: GBU-39/B SDB #16 has been without a GNSS update for 6 min 25 sec. Weapon has INS: 1990S+ tactical weapon INS. Max drift: 99m. Actual drift (CEP increase): 3m

7/5/2025 7:51:02 AM - GBU-39/B SDB #16 MISS: Missed UAV Launch Truck by 16ft.

7/5/2025 7:51:04 AM - Weapon: GBU-39/B SDB #14 has been without a GNSS update for 6 min 29 sec. Weapon has INS: 1990S+ tactical weapon INS. Max drift: 100m. Actual drift (CEP increase): 92m

7/5/2025 7:51:04 AM - 100% penetration achieved

7/5/2025 7:51:04 AM - UAV Launch Truck has suffered weapon damage: 71.3 DPs

7/5/2025 7:51:04 AM - UAV Launch Truck damage report: UAV Catapult (1x UAS [NATO Class I, Small] (20-150kg)) has been moderately damaged.

7/5/2025 7:51:04 AM - GBU-39/B SDB #14 HIT: Has impacted UAV Launch Truck.

7/5/2025 7:51:04 AM - UAV Launch Truck has been destroyed!


7/5/2025 7:51:06 AM - Weapon: GBU-39/B SDB #15 has been without a GNSS update for 6 min 30 sec. Weapon has INS: 1990S+ tactical weapon INS. Max drift: 100m. Actual drift (CEP increase): 41m

7/5/2025 7:51:06 AM - GBU-39/B SDB #15 MISS: Missed UAV Launch Truck by 26ft.

7/5/2025 7:51:09 AM - Weapon: GBU-39/B SDB #19 has been without a GNSS update for 6 min 29 sec. Weapon has INS: 1990S+ tactical weapon INS. Max drift: 100m. Actual drift (CEP increase): 100m

7/5/2025 7:51:09 AM - GBU-39/B SDB #19 MISS: Missed UAV Launch Truck by 32ft.

[blu3s]

Thanks for the report

Logged 16844

[c3k]

What sort of INS drift are you looking for?

Usually INS drift is a measured by "distance/time" and is a contractual maximum allowed. For example, with old 1970/1980's gyros, 2.5nm/hour. Actual drift could be ~.6nm/hr., far lower than specced. It would rarely be greater.

Ring-laser gyros have much lower drift.

For an SDB, I have no idea what type of INS is used or what type of drift is specified. Flight time is just a few minutes.

Curious why short/small drift distances over a short flight time seem to be an error?

[caelunshun]

The message log is showing INS drift values that seem plausible to me in this case (50-100m typically for a flight time of 6 minutes, e.g. 0.6nmi/hr * 0.1hr = ~100m). So that is fine. But the issue is that the simulation is not actually applying the INS drift modifier to the CEP. The message log claims that the CEP is increased, but the miss distances do not reflect a higher CEP than normal. So INS drift has zero or close to zero effect, from what I can tell.

[Dimitris]

This has been fixed for the next update.