Winds Aloft readings with an AvMap GPS

[Geoff]

G'day Dynon Support.

I have read the other 3 posts on this subject in late Oct & Nov 2007.

The on screen readings on my 180 appear to be high and relatively inaccurate and I went searching to see if I could find the reason in my AvMap today.

Does your winds aloft readout come from basic data in their GPS or from the AvMap's "Wind Calculation" feature?

If it is the latter, their Winds Calc page needs the OAT to be entered and apparently this needs to be done manually.

Do you know if the AvMap can be made to automatically accept your OAT reading from your sensor, which I have?

I have AvMap's upgrades to late 2007, so if you do not take data from their Wind Calcs page, what do you suggest to make the Winds Aloft reading more accurate in this case?

Best regards

Geoff in OZZZ

 

[dynonsupport]

Ok, so... We use the GPS data to calculate winds aloft along with our OAT probe and magnetic heading. If the latter is not well-calibrated, the wind number/direction will be off. So first check that your FlightDEK-D180 is showing a heading which looks reasonable, and that the OAT looks right as well (it has to be one of the above).

Note that the EFIS doesn't output OAT data, so you'll need to continue to enter this manually on your AvMap.

 

[Geoff]

Thanks for that.

I believe that the OAT is spot-on and the Magnetic Heading is also pretty good ..... so will go back into the AvMap's NMEA output and check that it is set up as discussed in this Forum back in November.

FYI - I flew the aircraft this morning where the winds were variable 10 knots and the display was showing up to 43 knots, so there is definitely an issue.

If I find anything on the Avmap side I'll post it here, but it's an interesting one as the AvMap appears to be spot-on also with it's navigation bearing/track & track prediction displays.

Regards & thanks

Geoff
Wagga Wagga

 

[dynonsupport]

Two more things to look into are the TAS indication on the EFIS (this can be very wrong if OAT is not being reported right), and the ground speed and track as reported by the GPS (this can be seen on the EFIS's HSI page).

 

[Geoff]

Thanks for that.

I reckon that the TAS looks right and I have that displayed on the EFIS all the time, and I'm very pleased with the OAT reading that appears to be right based on ground temps and a review of the local temp trace at elevation in this area after the flight (We have a Met Office in this town that flies balloons for an accurate trace a couple of times each day).

The ground speed and track are displayed on the GPS all the time and also appear to be right or thereabouts. (In yesterday's case it was showing 5 or 6 knots on the nose and when I then flew a reciprocal course the GPS indicated a similar speed from behind). It was just the wind display on the EFIS that was saying 43 knots from the side and looked out of kilter.

My guess is that the output in the NMEA from the AvMap has a problem. Do you think it could be the "direct" or "processed" setting of the NMEA output as was disussed in this forum for AvMaps and EFIS wind aloft back in Nov 07?

Hey ....... does this mean I'm becoming a Beta Tester by default?

Regards & thanks again for your suggestions. I'll get back to you however it'll be a few days before I fly it again, but in the meantime please advise re the above.

Geoff

 

[dynonsupport]

Well, if the GSP ground speed as reported on EFIS HSI screen looks sane, as does the TAS, as does the mag heading, then the last thing to look at is the GPS track direction. This is indicated as the magenta bug on the HSI screen. It should be roughly in front of you when you're moving (the difference between that and the mag heading is the effects due to wind).

If you snapped a digital camera picture of you EFIS with OAT/TAS, and the HSI page up, that might help us shed some light on what you're seeing.

 

[Geoff]

No worries. I'll have a play and a snap and will get back to you, but it may be a week or 2, probably after Easter.

Regards & thanks for your responses so far

Geoff

 

[Etienne]

Is it possible that the GPS is outputting true track instead of magnetic track? I'm not sure if the Dynon handles this (it should)...?

If your magnetic variation is high enough, it may explain the huge discrepancy...

 

[dynonsupport]

It's possible, and in fact a bug that the AvMap units suffered from (one of the NMEA sentences was slightly non-standard, which resulted in true track being outputted in a way that looked like magnetic track) was previously discussed in another thread but was thought to be fixed.

That brings up another good point - make sure you're running your GPS unit's latest firmware release.

One way to check this would be to make sure that magnetic heading is being displayed on the GPS unit somewhere, and compare that with the GPS track information on the EFIS HSI page. They should match.

Note that converting from true to magnetic requires a full magnetic model of the earth. Our products to not have such a model. They rely on the GPS to correctly output magnetic heading.

 

[Geoff]

Thanks Dynon Support and Etienne

I'll have a look at all of those points and get back to you.

Regards Geoff

 

[Geoff]

Have ordered the latest AvMap firmware and data base upgrade, so will fit that when it comes next week and before I respond to you further.

HOWEVER, I note in my original installation instructions that you wanted the NMEA sentences to be RMC, RMB, GGA, APB and BOD.

Yet in your latest installation instructions you want the first 3 plus only one of either APB or BOD.

Could it therefore be a part of this problem that I still have both APB and BOD enabled?

Regards Geoff

 

[Etienne]

Note that converting from true to magnetic requires a full magnetic model of the earth. Our products to not have such a model. They rely on the GPS to correctly output magnetic heading.

At DynonSupport: Which sentences do you guys use to get the velocity?
RMC supplies True heading, and Magnetic variation
VTG supplies True heading and Magnetic heading

At Geoff: If you find that the track from the GPS (when displayed on your Dynon) is wrong compared to your compass heading, then you may be able to find out if there's an error in the stream from the AvMap by looking at these two sentences, and comparing it to the variation displayed on a map...

If you need some help with the actual decoding, look at http://www.gpsinformation.org/dale/nmea.htm#RMC or post a couple of output lines from the GPS here

 

[jakej]

Hi Geoff

From my experience last week there are still some issues with the Avmap with the lastest software version (Vers confirmed by Avmap,Italy) eg when engaging an autopilot it would initially start to intercept a track but then start wandering followed by going in circles.  So there appears to be a datastream or setting problem.

This symptom has been experienced by one other owner (that I'm aware of) however after some experimentation (he can't remember what he did    ) the problem was fixed and now rock solid - I'll be doing further testing here in South Australia in the next 2 days and will report back if you like ?

regards,  jake

 

[Geoff]

Thanks Jake

I would certainly be interested to hear of anything you learn in your testing over the coming days.

Have you mucked around with the NMEA setting options "Direct" & "Processed" that are included in the latest software, which AvMap's website says "nmea output comes from GPS to outport being processed by background task. (This setting solves the problem with TRIO autopilots)"?

Thanks for your last post Etienne. But I'm merely a simple Mechanical Engineer who is just a basic Lever & Fulcrum type of guy. When I was training, we ued to leave the electrical and instrumentation stuff to the smarter members of our year while we went to the cafe or a bar, so while this is very "interesting", analysing NMEA sentences is a bit out of my league.

You might know the old joke:

Question - What do mechanical engineers use as contraceptives?

Answer - Their personalities.

Regards & thanks again to you all

Geoff

 

[dynonsupport]

Etienne - We use the RMC sentence for velocity. We do not make use of the VTB sentence at all.

The hopefully solved issue with the AvMap, by the way, was that a data field that should have had magnetic variation didn't, and hence we did not have a correction to apply to the various GPS headings reported in true.

 

[Geoff]

We use the RMC sentence for velocity. We do not make use of the VTB sentence at all.

The hopefully solved issue with the AvMap,

Sorry, don't understand, specifically what do you want me to do to solve it?

 

[jakej]

Hi geoff

The problems with the autopilots are not unique to them only otherwise why would we talking about this on the Dynon site - see my point

Went flying today and discovered that initially the a/p would lock on ok for 2 goto way points then the plane would go around in circles. This happened on different ocassions each time after activating goto twice.

It's as though there is a processor in the AVmap that can't handle the load - I'm not into that side of technology so don't know if I'm on the right track. Maybe someone else could elaborate here ?

I've contacted Avmap with all the settings and software version and had the settings confirmed as done so it's back to the drawing board for me. Will advise when more is known.

cheers, jake

 

[dynonsupport]

Geoff - To summarize:

It seems that some versions of AvMap's GPS data output stream is a little bit atypical since it omits magnetic variation, and this confuses Dynon products.

It looks like this may have been solved in on AvMap's end. See this thread for details.

We do think that the next version of our firmware will work better with even the nonstandard AvMap stream, but it will be a little while before this goes into our beta test cycle.

Updating to the latest AvMap firmware, and configuring it per the customer's advice in the above thread, may solve it now though.

 

[Geoff]

The issue appears to have been solved as readings are now correct for wind strength & direction.

It was fixed by either (1) installing the latest software from AvMap or (2) Deleting the BOD sentence from the AvMap's NMEA stream or (3) both of the above.

Over the next week or so I will reinstate the BOD sentence and see if it still reads correctly. If there is any change that allows us to know exactly what fixed it I'll post again here.

But might I make a suggestion, and that is when Dynon makes a change to the installation instructions, such as this one which needs a change to the NMEA sentences, can you maybe highlight that in red or some other way that catches an owner's eye?

Regards & thanks for your help on this issue.

Regards Geoff in OZZZ