Earth Magnetic Field Part II: MagVar Reference Table

Last chapter of this quick-dive into the effects of the Magnetic Variation on the daily routines of the F-14.

Part II pulls off some numbers to give a better idea of the impact of the MagVar. I tested in every map available in DCS besides the new Channel Map and shown how the MagVar changes though different locations and years. The goal, eventually, is creating a reference table that can be used by the Radar Intercept Officers to fix their INS reliably no matter what the mission designed sets as date.

MagVar Through the Years

As we know, the MagVar changes depending on geographical position and current date, so I collected some data changing these two variables.

Period

I considered from the 1961 to 2036, measuring the MagVar in two airfields per map every 5 years. Day and Month are always 21/06; hour is 0800.
I started so early because, hopefully, at some point we’ll see the F-4 and perhaps (totally random guess here) it will have some INS drift correction methods as the F-14 (TACAN INS Fix in this case).
Years from late 70s to 80s and the new millennium instead are plausible settings for the Tomcat since it is still in service in Iran.

Maps

Four maps are currently available in DCS World, a fifth is in EA and three more will be released at some point (Mariana Islands, Syria, Falklands).
Caucasus is free and the default map, the veteran of the game even before it was called DCS.

  • Normandy
    I backed DCSWWII on kickstarter in 2013, and the map was included. Despite being less relevant than other maps, I did experiment with the magnetic variation for the sake of testing.
  • Caucasus
    The default, free map in DCS (soon™ joined by the Mariana Islands for some ArmA3 vibes). This map has been restyled with the advent of the new engine some years ago but, besides that, it is pretty much the same since forever.
    TACAN stations equip only some of southern airfields.
  • Persian Gulf
    This map has TACAN stations all over the map, thanks to the fact that Iran used and still uses US equipment (such as F-4 and F-14).
    ED has changed some airfields and channels through the development (when I have some spare time I will update the TACAN INS Fix included in my Kneeboard pack).
  • Nevada
    I have the map, definitely taken over by PG in terms of popularity. It was a nice change when Caucasus was the only available map but PG offers much more in terms of variety.
    For this article’s sake I tested two AF.

Results

magnetic-variation-part-2-sample-results

The sample above shows how the MagVar changes through the years (data are from the game itself). The trend is different depending on the location and the delta between each survey is different as well. Nevertheless, it is clear how different the outcome of an INS correction via TACAN can be if the date is not correct.

For example, if a mission is set in the Cold War (let’s say early ’70s) and the map in use is the Caucasus, the difference in MagVar is a couple of degrees: this is a flat error caused “for free” simply because we are using the wrong reference table.

magnetic-variation-part-2-magvar-sample-chart

(There is no practical reason for plotting the chart above, but charts are always cool, right? 😛 )

Tests on the Field

The following are a couple of simple tests that aim to show the error induced by using a wrong Magnetic Variation.

Example I: VC/VM INS Fix differences

In this scenario I re-aligned the INS using different settings of Magnetic Variation: I spawned my F-14 in the air, around 50nm W of Kutaisi (44X) and set one WP over the TACAN station of Kutaisi. I then re-aligned my INS and noted the Deltas.

The goal is quantifying the difference between the different MagVars by comparing the Deltas. As the aircraft has just spawned, the INS should be quite correct so the Delta displayed as the TCN Fix is activated should be very close to zero. The column DIFF [nm] is the result of Δ[0°] minus Δ[TCN Fix].

Date is default (21/06/2016), Heading 088°T. Manual MagVar=6.6°.

DIST [nm] VM VC Used FIX ΔLAT FIX ΔLONG ΔMV DIFF [nm]
50.3 6.6° 5.4° VC N00°01.2 E000°00.0 1.2° 1.0
48.4 4.4° 5.4° VM N00°02.3 E000°00.1 2.2° 2.3
47.4 6.6° 5.4° VM N00°00.3 E000°00.2 0.0° 0.36
46.8 0.0° 5.4° VM N00°05.6 E000°00.1 6.6° 5.6

These tests show again how an accurate MagVar results is a very, very precise INS Fix.

magnetic-variation-part-2-test
Correct MagVar results in a very accurate INS TCN FIX

Example II: “DCS” position vs INS position

This second test is similar to the first one, but this time I re-aligned the INS using VC and VM and compared the coordinates from the TID to the aircraft position provided by the F-10 map.

VC = 5.1°, VM = 6.6°. Curious how, although I used the same scenario, the VC is different.

magnetic-variation-part-2-vc-vm-test-VC magnetic-variation-part-2-vc-vm-test-VM
MagVar F-14 LAT F-14 LONG F10 LAT F10 LONG DIST [nm]
VC N42°13.4 E041°22.7 N42°14.841 E041°22.707 1.44
VM N42°14.7 E041°26.0 N42°14.655 E041°25.881 0.1

(0.1nm = 185m and 1.44nm=2666m; for us on the other side of the Channel)
The example proves again how the correct MagVar provides very precise results.

Conclusions

After these tests and experiments the question is: do we really need such precision?

The answer to that question, the vast majority of times, is no, we do not. It is no coincidence that, for example, bombs delivery always require line of sight in the F-14A and B: the INS is simply not precise enough to drop on a set of coordinates. Therefore, even a spot-on INS, although nice to have, it is not fundamental. On the other hand, if the INS is degraded and requires a correction, then better do it properly. Since the INS naturally drifts other time, a precise Fix means that the correction will last longer.

A good compromise, valid for the vast majority of the situations, is checking the VC and compare it to the MagVar reference table. If such difference is grater than 1.5° or 2°, then using the manual MagVar may be worth it, especially if the distance from the station is greater than 30nm-40nm.

On top of such considerations, I’m putting together additional material for us, virtual RIOs:

New Reference Sheets: Caucasus and Persian Gulf

I collected the Magnetic Variation values following the same criteria used for the data sample (only Caucasus and Persian Gulf so far, as I don’t use other maps at the moment). The results are included in a PDF linked in the Download page.
This should help to improve the precision of the TACAN INS Fix.

Mission Datacard Generator v2.1

The new version of my Mission Datacard Generator (not released yet, see the article about the MDCG v2.0 here) will feature a new optional Kneeboard Page with data displayed depending on the date set in the mission. By doing so, the updated and corrected MagVar values for the scenario will be part of the documentation of the mission automatically, eliminating the need of having a reference page flying around.

At some point I will do the same for other navigational systems, such as the VOR.

One comment

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.