UPDATED • INS drift.. Bug?

UPDATE

Heatblur reply. Looks like that Active Pause messes up the INS quite badly. Worse, the issue is still present even after disabling it. On the other hand, they are now aware of the issue and will track it.
Active Pause is not a priority of course, but it’s still a good learning and testing tool.



I am currently working on some articles about INS drift, INS Updates and AG (on top of a couple dozen of other things, I wish I had more time!). I was collecting the last screenshots when I realized something odd: the INS latitude-wise degrades fast. Really fast. I noticed it already, especially in the last few weeks, but I thought it was only the result of long sessions of tests. I never went into the details of this “issue” so I wanted to find out. Is that a bug? Normal feature? What am I missing?

According to this post from Heatblur, the INS:

A fully aligned AN/ASN-92 INS, in accordance with the requirements of the navy specification, for the latitude of 45 degrees North, should provide the following performance:

  • 3 arc minutes for heading,
  • 2.5 arc minutes for pitch and roll,
  • Position error rate of 0.75nm per hour (CEP),
  • Velocity error of 3 feet per second.

All values stand for standard deviation and assume a normal distribution of the error.

My impression was very different so I decided to test it practically. I created a new mission, spawned a single F-14 near Kutaisi at 10,000ft. I launched the mission, turned on the Active Pause, created a WP on Kutaisi TACAN Station latlongs and activated the TACAN FIX.

Here are the results. I used this LatLong / Distance calculator to measure the distance in [nm]. First value was collected 1’40” after spawning. I have disabled the Active Pause before taking the last measurement and let the F-14 fly for a few minutes. The Δ increment didn’t slow down.
The MagVar between the spawning point and Kutaisi is the same, 6.2°.

TIMESTAMP TCN LAT TCN LONG ΔTIME DIST [NM]
08:01:40 0°00’5 0°00’3 0 0.547
08:03:40 0°01’5 0°00’5 2′ 1.545
08:06:40 0°04’2 0°01’2 5′ 4.29
08:11:40 0°11’2 0°02’9 10′ 11.401
08:16:41 0°18’2 0°04’5 15’1″ 18.498

This is even more interesting when you scroll the pictures and see the position changing in the TID. Remember, I was in active pause so my F-14 wasn’t moving at all, except for the last screenshot.

This slideshow requires JavaScript.

This is the result of pressing FIX ENABLE after the last screenshot.

bug-INS-latitude-TCN-fix
Effect of FIX ENABLE

The latitude delta is remarkable and only a few seconds have passed. The F-14 is not represented in the correct position longitude-wise.

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