F-14 INS: TACAN Update

The INS is a fundamental component of many operations aboard the F-14 and it degrades naturally over time. High-G and aggressive manoeuvres can accelerate the process, as well as other external factor more related to DCS.
Being able to check the precision of the INS alignment and restore its status is fundamental for navigation, Air-to-Air engagements and Air-to-Ground bombs employment.

I am working on a series of articles about Air to Ground techniques and the INS alignment must be precise in to make such procedures work. Air to Air operations are also affected by a poor INS as well.

Restoring the INS

I tend to avoid updating while airborne because it is an intrinsically imprecise operation and I prefer to do tabula rasa of the INS once landed and whilst R&R. The ground INS alignment is usually not much longer than those operations.

The F-14 manual has a lengthy chapter about the topic.
In this article we will go into the details of how to perform an INS TACAN Update.

A step at the time

In primis we must remember these two important points:

To perform a nav fix by TACAN, requires that a pre-stored waypoint shares identical LAT and LONG values with the TACAN station that will be used for the fix. [..]
During a TACAN FIX, the MAG VAR must be the same as the TACAN station magnetic variation, or the update will be in error. Given a TACAN station with a range of 100 NM from ownship, a 1°MAG VAR error introduces a 1.74nm error into the ownship’s TACAN update.

Therefore, the following information are required in order to perform an accurate INS TACAN Update:

  • TACAN Station Magnetic variation;
  • TACAN Station latlong coordinates.

I have gathered the latlong coordinates of the TACAN stations in the Caucasus, Persian Gulf and Nevada maps, initially from the game files (thanks Pikes for the suggestion!) but later directly from the F-14 Kneeboard.
The Magnetic Variation changes over time. These are the tests I made in Kobuleti and Kutaisi by setting a different year in the map:

Year Kobuleti Kutaisi
1990 5.0° 5.1°
2000 5.1° 5.2°
2010 6.0° 6.1°
2011 6.1° 6.2°

As you can see DCS models the Magnetic Variation. On one hand this is very interesting and kudos to ED; on the other this means that writing a table that covers any timespan takes very long (any many pages).
For that reason, the 132nd VW do not change the mission year and it is set to the default value, 2011.

The following is the table I use to update the INS of my F-14 in Caucasus and PG. I didn’t include the Nevada because the list of the TACAN stations there is quite long.
This table will soon be part of a “Kneeboard pack” I intend to release, along with the AWG-9 Antenna Elevation Model and other tables currently WIP.

rio9-INS-fix-TACAN-Caucasus-PG

With such data, we can proceed and perform a TACAN Update (from the manual):

  1. Select a TACAN channel whose latitude and longitude correspond to an update point.
  2. Hook desired update point (WAYPT 1, FIX PT, HOME BASE, etc.).
  3. CATEGORY switch – NAV.
  4. TACAN FIX button – Depress.
  5. Observe present position delta readout.
  6. If delta is unsatisfactory, deselect TACAN FIX and repeat steps 2 through 7.
  7. FIX ENABLE button – Depress.

The delta readout is hard to estimate sometimes due to a number of reasons (see the last paragraph) but if every step has been followed correctly, the station is reasonably close and the magnetic variation difference is not terrible, the outcome should be fairly precise.
Remember to set the Elevation of the TACAN station WP before doing the fix. The Aircraft-TCN Station range is slant but the altitude of the aircraft and the elevation of the station are both taken into account. Therefore, if the station elevation is not set, the Δ will be higher and will cause a less precise fix.

Practical test

I thought about recording a brief video but it wouldn’t be longer than a minute so I’d rather stick to paper.

This slideshow requires JavaScript.

  1. Initial situation. TID and HSD distances to the TCN station match. The INS is therefore properly aligned and accurate;
  2. By pressing TACAN FIX, the Δ is shown on top of the TID;
  3. For testing purposes, I changed the coordinates of the F-14 to match WP1. TID and HSD distances to the TCN station do not match anymore.
    This simulates a heavily degraded INS (drifted to unrealistic levels);
  4. By hooking the TCN station on the TID and pressing FIX ENABLE in the CAP, the F-14 coordinates are updated. The fix is not as precise as a full re-alignment would be but it is definitely better than Point 3.

NOTE: the F-14 was active and flying while I was conducting these tests so the range difference between Image 1 and 4 is expected.

TACAN limitations and tolerances

From the discussion on ED’s forum about the bug I run into recently (it’s an odd interaction between the Active Pause and the INS rather than a problem of the F-14 module) I learnt new details about the TACAN. I knew that its tolerances are modelled, quoting Grover:

the bearing accuracy of digital readout of the AN/ARN-84(V) – the TACAN – is between 0.5° (for signals stronger than -82 dBm) and 2.0° (below -90 dBm). And we model that in our TACAN receiver.

What you observed and recorded in the video is that TACAN inaccuracy together with possible mag var drift.

Although I didn’t expect the quality of the delta readout to degrade so much in conditions such as the medium-to-long range (>40m, or >50nm depending on other factors) and even simple manoeuvres (such as literally banking a few degrees). I also noticed that the TACAN FIX Δ tends degrade over time whilst manoeuvring so either disable TACAN FIX in this phase or wait until the aircraft is stable before starting the TACAN INS Update procedure.

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