Heading - Direct To - OBS - Magnetic vs True and Magvars

Here is an interesting Rabbit Hole to go down.

We all fly Vor Radials, we all sometime fly Direct-To, and some even fly an GPS OBS radial.

Mostly, these are over relatively short distances ( less than 200 miles), so we do not notice what is really doing on, and how local & destination magvars play into this.
Most also, do not have a detailed understanding of Projection types, and use in MSFS, or as used in real world Aviation.

Over longer distance, Direct-to is NOT the same as flying a magnetic radial to a VOR, that is far away with a different magvar to that where the aircraft is.

In MSFS, due to the Projection it uses, Direct-To over great circle routes is very different to flying an OBS heading to the same distant destination.

Try sitting at NY (KJFK) setting up to fly DIRECT-TO the UK.
Then try setting up to fly GPS OBS to that same point,

Look at the Magenta line on the GPS in both case.

Remember, that you, in the plane are using your local Magvar, while the VOR or GPS OBS, has a magnetic bearing using its local magvar !@!

Things get even more messed up / confusing, if you so this using the MSFS GPS systems, and say the Garmin Trainer (Rea world), each of which use a different projection system …

I never got this deep into these details , before MSFS – have to do something when MSFS starts CTD’ing all the time when I actually want to fly with it !!

Anyone interested in this stuff ?? Any “Positive” thoughts (apart from what does it matter in a sim !! )

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In your example, surely the GPS would be following a geodesic?

according to Matt (WT) –

“OBS is not a great circle path but a rhumb line. The Garmin trainer uses a stereographic projection and MSFS uses a Mercator. We cannot change that”

Note: at this point, I have few answers, but lots of questions !!

MSFS

DIRECT-TO the UK from USA (Great circle route)
direct-uk

GPS OBS to UK form USA (Bearing route - due East)

obs-uk

Note The GPS OBS magenta line is rotated referenced to a magnetic bearing, using the magvar at the Destination OBS rotation point, (not where the aircraft is ) ???

Also,
The Garmin training, displays a different Projection to the garmins in MSFS
(no pics of that – yet)

An interesting topic! I think that sometimes this becomes a self limiting issue however.

GPS calculation in aircraft IIRC, is really done in True. The GPS doesn’t need Mag…it already has its own model of the world upon which to navigate.

Mag is just calculated in for pilot reference, as pilots still are required to use Mag (vectors, runway headings, etc). It will be interesting to see if this remains the case indefinitely.

So from my memory, I’d say that it would work something like:

  1. Direct to a point (GPS), will generally use great circle nav in True with a changing Mag course calculated for pilot reference. It may use present position MagVar or your Direct to target’s MagVar or some combination, depending on how the software is written. This may seem odd, but since the plane doesn’t care (and you have no easy way of checking) if the great circle route is magnetically accurate at any point along it, there is a strong argument to just use the target’s MagVar for display purposes.

  2. GPS OBS obviously constrains the course to a single OBS line extending to a target (actually on an Airbus, I believe it constrains it to a single line FROM a Present Position [PPOS] waypoint TO a target THROUGH a single line). Because this functionality would be used to emulate intercepting a radial to a VOR, I’d guess that this would actually need to reverse calculate a True course from the Mag course so that the GPS (I’m mixing a lot of FMS, GPS, DME/IRU here so I’m really speaking about the box, however a particular one is designed) could calculate its required course.

  3. GPS Airway. I guess that this is really two things as you can use GPS to follow a hitherto VOR based airway or a newer GPS waypoint based airway. But, not knowing what the maximum permissible distance is allowed between GPS waypoints (VOR intersections would likely be limited by their established guaranteed service volumes), even if the GPS was allowed to navigate using pure great circle navigation, I suspect that the relatively short distances involved mean that any deviation between “OBS” course and “great circle” course would still be less than the airway width (4NM either side of the centerline?) and from practical experience still far more accurate than in the radio days with aircraft stacked almost EXACTLY over the top of another.

So, in short, DTO doesn’t care about MagVar, putting in an “intercept” radial to a point is rarely done beyond any great distance (for several practical reasons), and Airways are likely anchored by smaller segments where the difference between the two courses is likely not a factor.

Just as an aside, MagVar tables and Navaid MagVars are an entirely different rabbit hole…:grin:

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Just to pick up on one of the many good points you made.

In MSFS, the GSP OBS lines are almost infinite … can be 1000’s of miles, which, as you said, would not typically be used.

In the Gramin SImulator, they are drawn only to a few 100 miles, although I have been told, that even if they are not Drawn that far, that the non displaying extended length is still relevant, and used to intercept and track that OBS bearing – (not that anyone would really do that, !! )

While non of the above is sticky relevant to Real World navigation, and is often never looked at in any detail, from the MSFS point of view, I find it interesting to discover how the MSFS “Simulation” varies from the Real world , even when pushing things to a non practical limit.

Understanding how and why the GPS displays what it does, rather than just blindly following a manenta line, and no really understanding whay it is drwn the way it is.

ie My 1st two Graphics in this Post… One magenta line drawn as a Straight line, the other one, drawn as a curve

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