SimVar GPS GROUND TRUE TRACK confused with new GROUND PHYSICS at spawn

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1

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2 tags are required: One for platform, and one for aircraft.

Are you using Developer Mode or made changes in it?

NO

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YES

Which aircraft are you reporting an issue about? (Please also add the proper tag for it)

I think this his is a sim engine issue, but can be cofirmed with the Cessna 172.

Which aircraft version are you experiencing this issue on? (You can find this listed in the Content Manager under the Aircraft Name)

CESSNA 172 KYHAWK (G1000) v0.1.112 (but this isnā€™t particularly to do with the plane)

Brief description of the issue:

SimVar ā€œGPS GROUND TRUE TRACKā€ bounces about wildly when the aircraft spawns in. I suspect this is a SIDE EFFECT OF THE NEW GROUND PHYSICS confusing the prior sim ā€˜true trackā€™ calculation.
This causes the BING MAP (MapInstrument.js) in ā€œTRACK UPā€ mode to spin around for a few seconds during spawn in.

Provide Screenshot(s)/video(s) of the issue encountered:

(this requires a simvar watcher to watch the GPS GROUND TRUE TRACK simvar)

Detailed steps to reproduce the issue encountered:

use a SimVar watcher to view (A:GPS GROUND TRUE TRACK, degree). Spawn into the sim with the Cessna 172. You should see that simvar value bounce around wildly before it settles down.

PC specs and/or peripheral set up if relevant:

Build Version # when you first started experiencing this issue:

Noticed with 1.37.19.0

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Do you have the same issue if you follow the OPā€™s steps to reproduce it?

Provide extra information to complete the original description of the issue:

If relevant, provide additional screenshots/video:

2

This is not a bug. GPS track is based on the motion of the plane, not where the plane is pointed. Like a real GPS, it doesnā€™t take into account any plane heading, it just looks at the GPS and determines which direction the plane is going based on sampling the location.

When the plane is basically still, the track is essentially undefined. Any small motions will create a vector, which as you can see, will be a bit noisy.

To deal with this IRL, planes with a heading reference will use that even for track mode when the speed is slow. Planes without a heading reference will heavily filter the track and when the plane is slow enough will stop updating the track and remember it for the next time the plane is powered up.

I would not recommend using MapInstrument.js, as it is legacy and deprecated, and is not receiving updates. Instead, I would use MapSystem and MapSystemBuilder, which are available in the MSFS Avionics Framework. The mapping classes and systems in there can and do take these effects into account.

3

No, this is a bug. Try it in the sim.

milliseconds/millimeters movement of the plane is causing the GPS GROUND TRACK simvar to flip 180 degrees. That is obviously not what any useful track simvar would do, and itā€™s nothing to do with any gauge. The MSFS GPS ground track simvar calculation was written before the jitter was introduced by the new ground physics of the plane moving on its tyres etc, so it assumed the other variables it depends on would be sufficiently smooth that it didnā€™t need its own smoothing. Simple as that.

Suggesting a real-world aircraft GPS would flip its ground track calculation by 180 degrees every few milliseconds is simply a misunderstanding of how real-world instruments work, or indeed what GPS is capable of.

The discussion of the availability or otherwise of a usable map for 3rd-party development in the sim is a completely different conversation.

On a more practical note, GPS GROUND TRUE TRACK could be smoothed, but TBH it would be a useful but simpler fix to flip it to GPS GROUND TRUE HEADING below some speed threshold. This can be done within gauges but there is code out there that pre-dates the physics changes that have affected this simvar.

4

The track has always been the instantaneous vector derived from GPS position sampling and that code has not changed in the sim, and remains the same as in FSX and earlier.

Weā€™ve been working around that in instrument code since the release of the sim (again, as real instruments so), and it is not related to any physics changes. It has been noisy at low speeds and with any plane vibration since then.

5

Iā€™d venture that youā€™re both correct to an extent. Itā€™s true that GPS ground track is the direction of the vector connecting successive GPS positions and noisy positions will yeild even noisier headings, but real GPSs also donā€™t compute fixes at millisecond intervals. A 10 Hz GPS computes a fix every 100 ms. Jiggling the heading around every couple of milliseconds implies a several hundred Hz GPS.

6

Hi @MSGamerTag01,

Iā€™ve moved your topic into SDK - to raise bug reports on the SDK please use the dedicated Dev Support platform: