F-4E-45MC INS: TACAN, GCI/Bullseye Fix Update (AN/ASN-63)

In the previous parts of the F-4E-45MC’s AN/ASN-63 INS discussion. We have seen how these devices suffer from an intrinsic problem: drift. In simple terms, the determined position tends to differ from the real position increasingly as the flight progresses. Moreover, hard manoeuvres tend to exacerbate the issue.

A solution, as long as the issue is confined to this positional error, is to perform a so-called “fix update”. Ergo, updating the position where the INS thinks the Phanom II is, thus annulling the existing error. For a time, that is.

Out of the several ways to perform such an operation, this video is focused on the TACAN and GCI methods. Both use bearing and distance to update the Phantom’s position.

TACAN Update

The primary tool for performing a TACAN Fix update is the BDHI, which stands for “Bearing, Distance, Heading, Indicator”.
When a TACAN station is selected, the crew obtains bearing and range from the aircraft to the emitter. The idea is to create a waypoint pointing towards the same location and adjust the aeroplane’s position counters so that both match.

Confused? Let’s break it down.
There are two main issues related to this operation. In primis, the perspective. To better explain this concept, let’s divide the BDHI into two, one whose input is the selected TACAN station, the second fed by the INS.

Let’s start with the TACAN reference. As the Phantom flies around, the BDHI constantly points towards the emitter’s position, with some accuracy caveats depending on range, intrinsic imprecision, et cetera. This situation is quite simple and straightforward, as this is how most waypoints, NAVAIDs, and so on work.
When the WSO manipulates the INS’ location instead, the perspective changes. As discussed previously, the INS does not see the world outside. Ergo, changing the position counters cause the world to move around accordingly. Note that in this example I am purposely disregarding the magnetic variation.

For simplicity’s sake, let’s freeze the whole planet and check the BDHI indication for the TACAN station: circa 090, 11 nm.
According to the INS instead, we have: 155, 5 nm. Our INS definitely needs a Fix update.

These considerations lead to the second point. What has been discussed so far implies that bearing and range are not changed directly: they result from the INS Fix’s position. In other terms, the WSO does not have a counter for the bearing and one for the range, but has to fiddle with latitude and longitude to achieve the desired result.
The problem of working with latitude and longitude coordinates to match the distance and the angle between two points is that we can end up with confusing results. For example, the behaviour of the range between position A and position B is an inverted Gaussian with, at the bottom, the correct range.
Ergo, as the WSO manipulates the counters, the range or bearing may decrease, reach the desired value, and then increase again. What makes the operation even trickier is that, once a parameter matches, setting the other may disrupt it.

GCI and Bullseye Update

INS Fix updates can be performed with the help of a Ground Controller. The procedure “by the book”, as envisioned 40 or more years ago, is very similar to the TACAN update: a reference point is created on the location of the GCI. The controller provides bearing and distance to such a point, and the WSO adjusts the counters to match the values.
It is as simple as that.

That said, I may have found a quicker way to perform such a Fix update, which is also more suited to the common DCS experience. It involves a controller, bullseye, and the “alpha check” request. Let’s call it the “bullseye update”.

Alpha Check
Request for confirmation of bearing and range from aircraft to described point.

Bullseye
An established reference point from which the position of an object can be referenced by bearing (magnetic) and range (nautical miles) from this point.

ATP 1-02.1/MCRP 3-30B.1/NTTP 6-02.1/AFTTP 3-2.5

Why is this idea potentially better than standard GCI update? In primis, the WSO may already have the bullseye set in the target counters. Therefore, this check can become routine and be performed now and then without taking too many mental resources or time. The TACAN method, instead, requires fiddling with the BDHI input.
If the bullseye is used, the crew probably has access to a spider card. This tool allows the crew to visualise the drift, immediately determine the correction required, and have a general idea of which axis needs it the most.
Moreover, this method works everywhere, as the bullseye is a point on a map, not necessarily corresponding to a geographical location or feature. The drawback of such flexibility is that the farther the range from the bullseye, the greater the potential imprecision.
In the case of DCS, AI AWACS are more ubiquitous than TACAN stations on several maps, especially on casual servers, allowing crews to perform the INS update with ease.

In this example, the Phantom II is travelling following its flight plan. The bullseye coordinates are already set in the usual position, Target 1. For simplicity’s sake, I used active pause whilst recording the video. In normal conditions, the WSO must ensure the operation is completed quickly and efficiently.
As discussed, the crew can request an Alpha Check on the AI AWACS frequency. Here, the AI will respond with the bullseye location from the fighter’s position. To verify the INS’ drift, the WSO can switch to TGT1 and compare bearing and range. Note that the provided bearing ignores the magnetic variation, which is circa 6.5° in the Phantom’s position at this date.

The current set of bearing and distance matches the BDHI, once accounted for the magnetic variation.

The video linked at the top of the page shows the process of updating the INS fix. It requires a certain amount of practice, especially considering that the Phantom II should be moving, not standing still, as in this case. Moreover, given this method’s intrinsic imprecision, it is better to approximate bearing and range rather than become fixated and try to achieve a degree of precision not expected when this method is used.

Conclusions

The drawback of the discussed methods is the delay between data gathering and update execution, which causes intrinsic imprecision.
Another issue is the accuracy of the avionics itself. Since the target and position counters have limited precision and the primary tool used is the BDHI, once again, a certain degree of imprecision is expected.
On the other hand, these methods allow the crew to perform an INS update with a great deal of flexibility. For example, they do not require flying over a specific reference to perform the update.