Viper inertial-only nav drift rate (no GPS)

[Machalot]

Does anyone know the expected inertial drift rate of the Viper's nav system when GPS is disabled? The other day I flew a sortie from Incirlik to a target in Hama on the LK Flashpoint Levant server, which is an 80's historical setting. I used Stored Heading alignment and let it complete to 10.0.  For realism I disabled GPS. After 20 minutes my target waypoint had drifted from Hama all the way to near the coast. That seems excessive, but I have no data to say one way or the other.

Edited January 1, 2022 by Machalot

[Crptalk]

0.8 nm/hr circular error probability at full performance (status code 10) according to the old HAF blk50 -34.

INS drift without GPS seems inflated in DCS and affects both the F-16 and F-18. There's a long thread on ins drift and it excessively affecting CCIP in the F-18 forum.

Edited January 1, 2022 by Crptalk

[Machalot]

What I experienced was definitely more than 0.8 nm/hr (is that a 50% CEP?).

I'll have to do more testing.  After I realized my waypoint was miles from the target I relented on "realism" and turned on GPS.  Within a few minutes it snapped back to the correct position.

[QuiGon]

32 minutes ago, Machalot said:

I'll have to do more testing.  After I realized my waypoint was miles from the target I relented on "realism" and turned on GPS.  Within a few minutes it snapped back to the correct position.

Side question: How do you turn on/off the GPS while playing the mission?

[Kilo]

It's one of the switches to the right of the MIDS and NAV switches (the rightmost one, iirc).

[Frederf]

I don't know if CEP50 or CEP80 but full alignment should be more than usable over 20 minutes without GPS. Be aware we align to the nearest tenth of minute so unless we do a squat fix it's only useful to compare before and after drift. The points won't be exact unless we do a fix of some kind.

[QuiGon]

3 hours ago, Kilo said:

It's one of the switches to the right of the MIDS and NAV switches (the rightmost one, iirc).

Oh, he meant the GPS in the F-16? I thought he disabled/enabled GPS in the mission itself.

Edited January 3, 2022 by QuiGon

[Machalot]

14 minutes ago, QuiGon said:

Oh, he meant the GPS in the F-16? I thought he disabled/enabled GPS in the mission itself.

 

Yes, I was using the GPS switch in the F-16.

19 minutes ago, Frederf said:

Be aware we align to the nearest tenth of minute so unless we do a squat fix it's only useful to compare before and after drift. The points won't be exact unless we do a fix of some kind.

Can you explain a bit? Before and after what? And is there a way to do a nav fix in the DCS Viper? 

[Frederf]

Not properly but sort of if you're willing to use slew deltas. The fix stuff is placeholder. A squat fix is normal procedure to have a steerpoint at a known airbase location (parking spot, taxiway intersection, hold short, etc.). You do an alignment, set NAV, then drive over that spot to do a navigation fix while parked on the ground. This updates the 0.1' accurate alignment to 0.001'. You can also do an airborne fix does the same thing, but differently. Plus if you're stationary for a while a squat fix will calibrate the zero velocities (aka zero velocity update).

Thing is in DCS even though INS page only displays to the nearest 0.1', it's actually storing much more precision than that "off screen." So if you just hit ENTR to confirm L/L on alignment you're accepting position to 0.001' (or more) precision. If you manually type in a position (even if identical to current display) e.g. N-4-1-3-6-2-ENTR E-0-4-1-3-6-4-ENTR, it's not the same stored value. You can never type in the full precision the mission started with. But normally that's fine to just be close. The better your initial position the faster the alignment (8min is a maximum up to ~70 latitude). The fine position is adjusted with fixes.

What you can do is slew the GM FCR over where you know a steerpoint should be and then never hit CZ after that. That will adjust all your steerpoints by the systematic alignment error. Naturally with GPS you do get updates (a few times per second) which adjusts the INS to truth pretty fast, if available.

[QuiGon]

2 hours ago, Machalot said:

Yes, I was using the GPS switch in the F-16.

Alright, but if you're flying in an 80s setting this switch should essentially have no effect.

Edited January 3, 2022 by QuiGon

[Machalot]

8 minutes ago, QuiGon said:

Alright, but if you're flying in an 80s setting this switch should essentially have no effect.

 

Agreed, but the LK Flashpoint Levant server is an "80s setting" but the actual date of the mission is in the 90s, I presume to allow some anachronistic weapons that stand in for unavailable period weapons. 

[LazyBoot]

2 hours ago, Machalot said:

Agreed, but the LK Flashpoint Levant server is an "80s setting" but the actual date of the mission is in the 90s, I presume to allow some anachronistic weapons that stand in for unavailable period weapons. 

If I recall, it's also somewhat to work around the INS issues that seem to pop up for some planes when GPS is not available... (like for example what this thread is about)

[QuiGon]

20 hours ago, Machalot said:

Agreed, but the LK Flashpoint Levant server is an "80s setting" but the actual date of the mission is in the 90s, I presume to allow some anachronistic weapons that stand in for unavailable period weapons. 

Oh, alright then.