Earth Magnetic Field Part I: MagVar & INS

I mentioned in other articles the effects of such phenomenon and the impact of a misaligned INS on the navigation. This is a more in-depth look at the topics.

Maps in DCS have a number of interesting features, some of which affect how we fly or navigate. One of those is the Geomagnetic Field: it impacts the heading of our aircraft and how to we fix certain issues, namely a drifted INS.

True North vs False Magnetic North

Besides the GOT vibes (how to ruin a series, btw), the True North (or South) represent a geographical point whereas the Magnetic North is where the compass points. As Earth’s magnetic field is not the same everywhere, the difference between True and Magnetic, called Magnetic Declination or Variation, changes depending on the location of the aircraft.
The F-14 operates in Magnetic, make sure your controller knows that because in some parts of the world, the MagVar can be easily be greater than 10°-15°.

Excursus: Error induced by inaccurate INS

As we know already, the INS of the F-14 is quite old, therefore more prone to drifting and sensible to hard manoeuvring. The lack of a system that automatically corrects it means that at some point the INS will not represent fairly correctly the position of our aircraft. This is not always a problem but, in case the position drifts up to a point where it is not acceptable any more, then the RIO must be able to correct the issue.

One of the simplest methods to correct a drifted INS is by means of the TACAN. I already explained how this method works and in my kneeboard pack there is reference page with TACAN stations, their latlongs and other useful details. The Magnetic Variation plays and very important role in the success of the operation. According to the manual:

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.

The error induced by a wrong MagVar seems trivial, but it becomes more relevant as the INS continues to drift. At the end of the day, an error of the magnitude of 1-3 nm is usually not a problem but what does it mean, when you are actually in the cockpit?

Since seeing a situation is better than just imagining it, consider the following examples (it is worth noting that on a standard flight the INS will not degrade so much, but better be prepared!):

3nm on the Ground

Nav guidance is pretty much always available on every airfield. The most commonly used is the TACAN, but there are others. If TACAN is not available or INS is used to get closer to the airfield, then 2-3 nm can be a non-ignorable difference.
The following image showing a trigger with radius 3nm (so ⌀ 6nm).

magnetic-variation-3nm-radius
Trigger ⌀6nm

Another immediate and more tangible example, now that we have visualized how much 3nm are on a map, is the case of conducting a Strike or being tasked to CAS friendly units.
In a Strike scenario, a WP may be used to have a general idea of the location of the target. A distance of 3nm may compromise timings and the efficiency of the attack especially when weather or other conditions may prevent the identification of the target at longer range.
Even worse, in a CAS scenario, a Keyhole or Target not matching the intended location can make the use of the LANTIRN very difficult since it is usually slewed to a waypoint and then adjusted from there.

3nm in the Air

DL is one of the most important tools to build and maintain SA. What does a 3nm drift means? We can have a general idea of the tangible effects of the drift by calculating the induced bearing angle delta at different ranges (α).

Distance Error Slant Range Angle (α)
5nm 3nm 5.830951895nm 30.96375653°
10nm 3nm 10.44030651nm 16.69924423°
20nm 3nm 20.22374842nm 8.53076561°
50nm 3nm 50.08991915nm 3.433630362°

For simplicity’s sake I considered a rectangle triangle so SR is simple Pythagoras and the angle is the arcsin of Error / SR.

magnetic-variation-3nm-to-range

In other words, if the DL says you have a target on your nose, it may be instead (in this scenario) on your left or right and, the shorter the range, the bigger the error (up to 30° at 5nm!).

Again, this is an oversimplification of the effects of the INS drift but it’s an effective mean to pass the concept that having a properly aligned INS is very important. Long story short, check your INS now and then, RIOs!

The unexpected guest: Time

It’s all bells and whistles.. until the mission designer changes the in-game date..
Karon the Wise – 21/06/2020

Back to Earth and MagVar, there is another criterium that has not been considered yet: time.

My reference sheet, part of the kneeboard pack I linked above, shows MagVar from a specific year (2011). The reason why I specified the year, is that the geomagnetic field is not a static phenomenon: it changes and evolves though the years. This is not a scientific website of course, so I won’t go into the details of why this happens but I strongly suggest you to go deeper into this fascinating topic.

From the perspective of us, virtual Radar Intercept Officers, the evolution of the magnetic fields implies that we can’t rely on a single reference set of values for any time setting.


Part II of this brief series will try to quantify the MagVar changes in the four maps now available to DCS.

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