Possible problem with GPSout NMEA TRACK

[captaingps]

Peter,

In another thread, I discussed using your GPSout program with the free Lowrance emulators.

I put 36 knot wind from 297. When flying a heading of 010 at about 97 knots (indicated airspeed) the GPS in Flight Simulator indicates a track of about 030. I didn't do the math, but this looks about right.

Using GPS out on either the Lowrance 500 or 1000 emulator, it indicated a TRACK of about 010 when it should have been 030 due to the wind. To eliminate this as a problem with the Lowrance emulator, I found the same result when I select GPS tracking in Garmin MapSource.

I was using the following .ini settings:

Sentences=RMA, RMC, GGA,GSA

Speed=4800

Interestingly enought, when I run the same Flight Simulator flight with a Garmin aviation handheld using Aviation in and simulation mode on the GPS and Sentences=AV400 and speed=9600, the TRACK is displayed correctly on the GPS.

My comments are meant strictly in a constructive tone and as an attempt to isolate the problem rather than a rant about a free program that you have taken the time to develop. It's a cool program.

Thanks,

John Bell

[captaingps]

One more thing: I am running FS 9.0.

[Pete Dowson]

In another thread, I discussed using your GPSout program with the free Lowrance emulators.

...

Using GPS out on either the Lowrance 500 or 1000 emulator, it indicated a TRACK of about 010 when it should have been 030 due to the wind.

Ah, I have indevertently answered this in that other thread. Could you refer there, please, to avoid duplication? Thanks.

[LATER]

On further investigation I have found some oddities which I will report on in this thread -- see next message.

Regards,

Pete

[Pete Dowson]

Okay. In investigating this track versus heading business I have found two oddities. One is the error in the RMA sentence, which I don't think would be related to track or heading -- it would simply be an invalid sentence and be discarded.

The other is that there are times (most times I've tested so far) where the MAGNETIC track supplied to me from the GPS part of FS is correct, whilst the TRUE track supplied is wrong, and in some cases wildly so!

Note that all the standard NMEA sentences use the TRUE track, whilst the AV400 format uses Magnetic.

As well as fixing the RMA sentence formatting, I have now discarded the use of the GPS True track reading and only used the Magnetic one. To provide the True track for the NMEA sentences I adjust by the Magnetic Variation, which I already know.

These changes are in version 2.57 of GPSout which I will release today. As it is such a small ZIP I attach it here, too.

Regards,

Pete

GPSout.zip

[captaingps]

Pete,

Thanks. I will give it a try when I get home.

John

[dfournie]

Thanks for the fixes Peter. I think I've stumbled across another one that is nothing you can fix, just an FYI. The FS2002 Magvar tables are obviously 2+ years old. By using just about any moving map with the most current Jepp/NIMA/etc navdata and FS2002, there is a slight difference between the 2002 magvar and 2004 magvar values (as would be expected in some locations). I have noticed 1 - 2 degree differences around the US in Flight Sim 2002. I would suspect FS2004 does not have this problem being it has a newer magvar table to consult.

[Pete Dowson]

Thanks for the fixes Peter. I think I've stumbled across another one that is nothing you can fix, just an FYI. The FS2002 Magvar tables are obviously 2+ years old. By using just about any moving map with the most current Jepp/NIMA/etc navdata and FS2002, there is a slight difference between the 2002 magvar and 2004 magvar values (as would be expected in some locations). I have noticed 1 - 2 degree differences around the US in Flight Sim 2002. I would suspect FS2004 does not have this problem being it has a newer magvar table to consult.

The MAGDEC.BGL (in FS2004 it's in the Scenery\BASE folder) contains all the magnetic variations as 16-bit words (0x8000 = -180 to 0x7fff <= +180) in a 360 x 180 matrix, one value per "square" degree of Lon/Lat. So it is easy enough to update if you have a good source and are handy with VB or something.

Naturally, the FS values, when released, are frozen to match the state of the FS Jepp database for the airport facilities data, and navaids. Normally I think that data is frozen about one year before release, so the FS2004 values would now be over two years old and the FS2002 values over four.

Regards,

Pete

[scrayton]

I am getting the same problem with there being exactly 8 degrees difference between the GPS (RealityXP GNS530) track and the Heading as indicated on the HDI gauge on my PFD. I tried downloading the GPSOUT.zip, but get a message that it is "Not A Valid Archive."

Help!!!

[scrayton]

Well, I got the GPSOUT to download and will give it a try. Thanks...

[captaingps]

Pete,

I think your updates to GPSout worked well.

I just ran the latest version of GPSout. I ran the Lowrance AirMap 500 emulator on a laptop with GPSout 2.57. I had also taken your suggestion and eliminated the redundant fields. I used Sentences=RMC,GGA,GSA and Speed=4800.

I used the same scenario of a heavy crosswind. When I flew in a manner so that the AirMap 500 TRACK matched the AirMap 500 BEARING, the XTK (cross track error) remained essentially zero. This is how I recommend navigating in the real airplane. Thus, I consider your update to be a success -- thank you.

As a matter of interest, I also observed the magnetic variation difference that David Fournie had observed. I found that the magnetic TRACK reported by the AirMap 500 varied by a couple of degrees from the magnetic TRACK indicated by the Flight Simulator GPS. However, from my understading of the issue, any change to GPSout to cure this would cause the GPS to not work in such a way as I just described in the previous paragraph.

Thanks again,

John Bell

[captaingps]

Pete,

I think your updates to GPSout worked well.

I just ran the latest version of GPSout. I ran the Lowrance AirMap 500 emulator on a laptop with GPSout 2.57. I had also taken your suggestion and eliminated the redundant fields. I used Sentences=RMC,GGA,GSA and Speed=4800.

I used the same scenario of a heavy crosswind. When I flew in a manner so that the AirMap 500 TRACK matched the AirMap 500 BEARING, the XTK (cross track error) remained essentially zero. This is how I recommend navigating in the real airplane. Thus, I consider your update to be a success -- thank you.

As a matter of interest, I also observed the magnetic variation difference that David Fournie had observed. I found that the magnetic TRACK reported by the AirMap 500 varied by a couple of degrees from the magnetic TRACK indicated by the Flight Simulator GPS. However, from my understading of the issue, any change to GPSout to cure this would cause the GPS to not work in such a way as I just described in the previous paragraph.

Thanks again,

John Bell

[dfournie]

I agree, there seems to be no track error problem now. The only reason I even brought up the Magvar issue with FS2002 is because I am hardware limited on the laptop I am runnng it on. The machine doesn't quite have enough power to run FS2004, yet is more than capable of running FS2002. Using the magdec.bgl from FS2004 wouldn't be possible would it?

[Pete Dowson]

However, from my understading of the issue, any change to GPSout to cure this would cause the GPS to not work in such a way as I just described in the previous paragraph.

Well, yes, true -- because I have to use the magnetic track instead of the true track.

I would certainly not entertain adding "fiddles" to GPSout to try to adjust the Magnetic Variation for FS's data freeze date to match another arbitrary date. The changes will be varied all over the world, and will still be changing in any case. Furthermore the main use of GPSout is to provide a correct moving map to match FS's scenery and NavAid data, not a new real-world flight GPS.

The only, and correct way, to update FS's navaids and magnetic variation is to make or purchase updated scenery files. As I said, all of the magnetic variations are tabulated for every square degree in MAGDEC.BGL.

Regards,

Pete

[bnepethomas]

This is great stuff, I went and purchased a Lowrance 2000, and tied it in with GPSOut, works a treat. Thanks for the cool utility Pete.

cheers

Peter

[Pete Dowson]

Using the magdec.bgl from FS2004 wouldn't be possible would it?

Try it. I doubt the format has changed. If it doesn't work you just revert to your carefully saved copy of the FS2002 one.

Regards,

Pete

[jpmejia]

Hi bnepethomas,

I'm trying to do the same with my Airmap 500 and I don't get it to work.

I created a serial cable (5 GND - 3 TX) going to the Airmap (1 GND - 4 RX).

Sentences=RMC,GGA,GSA

Speed=4800

My Airmap with:

Baud Rate: 4800

MNEA GPS Input

--> RMC and RMB, GCA, GSA and GSV

Any ideas?

Thanks, JP