Determining aircraft's ground track

[SeanMcLeod]

Hi

I'm looking to read the aircraft's current ground track via FSUIPC.

I've found a variable which gives the aircraft's heading, but can't find one for the ground track. Is there one I'm missing, or do I need to calculate it myself based on:

0x3190 Z - forward/back velocity

and

0x3198 X - left/right velocity

Thanks

[Pete Dowson]

I'm looking to read the aircraft's current ground track via FSUIPC.

I've found a variable which gives the aircraft's heading, but can't find one for the ground track. Is there one I'm missing, or do I need to calculate it myself based on:

0x3190 Z - forward/back velocity

and

0x3198 X - left/right velocity

The ground track isn't normally provided. The programs that do show it calculate it from heading, TAS and ambient wind speed and direction. You can't derive it without taking the wind into account.

However, if you are doing this only for FS2004 you can 'cheat' and get it from the GPS data in the 6000-61FF area. Check offset 6040. I think it is valid even with no plan loaded.

Of course in a real airliner it's sort of the other way around. The track is provided by the INS and the wind indicator for the ND is derived from the differences that provides from heading and TAS.

Regards,

Pete

[SeanMcLeod]

Hi

Yes, I'm looking for a solution that will work on FS2004 and FS2002.

The track is provided by the INS...

The FSUIPC SDK docs state that:

0x3190, 0x3198 Z - forward/back velocity, X - left/right velocity

are relative to the world axes.

And I presume the values are the overall result after MSFS has applied the wind to the aircraft?

If so then they would essentially act as INS data that I could use to calculate the track from.

Thanks

[Pete Dowson]

The FSUIPC SDK docs state that:

0x3190, 0x3198 Z - forward/back velocity, X - left/right velocity

are relative to the world axes.

Well, that's what the names of those imply when you save a flight and look in the .FLT file, e.g.

[simVars.0]

...

PVelWorld=0.00016016837602437931

BVelWorld=5.2741978078219892e-005

HVelWorld=-3.1679513659914766e-007

XVelWorld=8.5849995967027069e-005

YVelWorld=-3.6722501463765101e-006

ZVelWorld=0.015182631094688877

Apart from that I've no idea what they are, sorry.

And I presume the values are the overall result after MSFS has applied the wind to the aircraft?

I don't know. It depends what "world axes" means relative to "body axes". The only person I know of that understood all that stuff is Ian Donohoe, and it was his information that originally led to me adding this stuff to the FSUIPC list of values. All I've done since then is track them down on each successive FS version. Those that understand them use them, but I am sorry I am not one of those elite few. :?

If so then they would essentially act as INS data that I could use to calculate the track from.

All I can suggest is try it and see. Try it on FS2004 so you can compare it with the GPS value I pointed you too. If it works, then apply it to FS2002 as well.

Regards,

Pete

[SeanMcLeod]

Seems to work as expected. I've only tested on FS2002 so far. First test was to compare my calculated track with MSFS's heading in the absence of wind, they should be identical and they were to a good number of decimal places.

Then I added some wind via MSFS's weather interface and sure enough there was now a difference between my computed track and the heading I'd selected for the auto-pilot to follow.

I opened the GPS window which displays the ground speed and a value labeled TRK, although as I'd seen mentioned somewhere else the value displayed for TRK is erroneous and is actually the aircraft's heading. So I couldn't use it's value as a cross-reference.

However taking the groundspeed value, the airspeed value I'd entered in the weather interface and performing some simple trig. the angle from the trig. computation matched the difference in the angle between my computed track and the selected heading.

So these variables do seem to be the aircraft's velocity in the world axes after the wind etc. has been applied.

Cheers

[Pete Dowson]

I opened the GPS window which displays the ground speed and a value labeled TRK, although as I'd seen mentioned somewhere else the value displayed for TRK is erroneous and is actually the aircraft's heading. So I couldn't use it's value as a cross-reference.

Oh, I thought it was trackhang on, I'll re-check.

[later]

The value labelled "TRK" in degrees magnetic in the GPS is defitiely track, not heading. I don't know who said it wasn't, but I think they are wrong.

So these variables do seem to be the aircraft's velocity in the world axes after the wind etc. has been applied.

Okay. The terms "body axes" and "world axes" are confusing. Since the aircraft always has zero velocity relative to its own "body" I always assumed that these terms referred not to what the speeds are relative to, but how the axes are oriented -- so world axes would have x, y and z oriented relative to the Earth, but the speed wasn't necessarily relative to a fixed point. Evidently, as you've found, it is. Nice to know. Thanks!

Regards,

Pete

[SeanMcLeod]

The value labelled "TRK" in degrees magnetic in the GPS is defitiely track, not heading. I don't know who said it wasn't, but I think they are wrong.

I was specifically refering to the GPS window in MSFS 2002, I can't test in MSFS 2004 at the moment.

In FS2002 it's definitely the heading that is displayed and not the ground track, e.g. in the autopilot I select a heading of 185 and then add a wind component in the weather interface and after adding the wind component the GPS TRK is still 185.

More than likely this has been fixed with the whole new GPS interface in FS2004.

[Pete Dowson]

More than likely this has been fixed with the whole new GPS interface in FS2004.

Yes, it seems so.

Pete