GPS Error?

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Colin
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GPS Error?

Post by Colin »

Last night I was flying CYOW to KBTV. Little bit of weather over Burlington, so I was on the instrument approach to runway 01, the GPS. I was direct JENAX. I have ForeFlight on an iPad mini mounted to the left of my PFD. The regular moving map on the MFD with the approach loaded, so the points are all in there. I arrive at JENAX and start to turn toward FOBUX.

The controller said, "2RD, I show you a mile north of FOBUX, but paralleling the course. Do you want to be vectored back around for the approach?

Oddly, on the geo-referenced approach plate on ForeFlight (fed from the Stratus in the luggage area) he is correct. But on my MFD I am lined up EXACTLY. Not a ten foot error even. I tell him I'm fine and will correct on the turn to the approach, no need for a vector. I watch the GFC700 making the most beautiful fillet turn onto the approach. The tower says they watched the turn and I am left of course almost a mile. ForeFlight agrees. I decide to trust ForeFlight and use it to fly the rest of the approach using the heading bug.

Has anyone every experienced anything like this? The track is up on FlightAware if you want to look at it. I don't know how to download it and put it onto the approach plate. I didn't have the time to take screenshots and photos of the MFD. When I took off to fly BTV-OWD I checked the satellites and I had a dozen of them, all bright green. So I don't think it was satellite signal. I wasn't IMC until just before the CESAL intersection, so I wasn't very nervous. There's some terrain along the approach, but it is all right of course.
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gyro3816
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Re: GPS Error?

Post by gyro3816 »

BTV is 352M (nearly due north from FOBUX. Any chance you were on NAV2 and AP made the turn to the VOR ?
Otherwise no idea why that would occur. Just for kicks, I overlayed that approach from GP and foreflight and FAA plate and all GPS coordinates align.
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Boatguy
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Re: GPS Error?

Post by Boatguy »

Since the Stratus / FF combo agreed with radar, it's not likely a GPS system (i.e., sat signals) problem. The course correction, when you switched from G1000 to FF is clearly visible on FlightAware which is from your ADS-B out data. That means your G1000 GPS position was correct.

That leaves a Garmin database error. Those are unfortunately pretty common. Garmin currently has this notice:

The geo-referencing for multiple approach charts in the US is incorrect. This affects US and North America FliteCharts databases for cycle 2208.

https://www.garmin.com/en-US/aviationalerts/

See the attachment. This problem is severe enough that they don't even list all the approaches, and they have issued a mid-cycle database update. They don't say it affects navigation, but it reads to me that the underlying data for the approach is incorrect. It's not a plotting error (i.e., good data, bad program) or it would affect all plates.

My guess is that whatever data (I think it's from Jeppesen) is used to geo-reference the FliteCharts is also what is being fed to the FD and displayed on the MFD, even though you probably don't have a FliteCharts subscription. So my conclusion is that your airplane knew where it was, but the navigation database was bad and your plane was flying a course off by 1nm because the database has an error.

I'm speculating that your iPad had the FAA charts, while the G1000 was flying to Jeppesen data. I use Jeppesen charts on my iPad, but I may reconsider that as if this speculation is correct. The cross check of the two sources contributed to a good outcome for your flight. This is pretty scary considering how we depend on that approach data.

In any case, you may want to update your Garmin databases right now. And you could report this error to Garmin at:

https://supportforms.garmin.com/en-US/f ... ase-error/
Attachments
FliteChartsDataAlert_2022_08_22_US_GeoRef_22077.pdf
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Rich
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Re: GPS Error?

Post by Rich »

Colin, which service are you using for your nav data and updates? Garmin or Jeppesen? Also, on FF there are two ways one displays the approach: With and without the plate itself overlaid on the map. I often don't have the overlay in place. Which technique are you using?

FF assembles its own nav data from various sources, but the chart is either Jepp or gov supplied. It's unlikely that Garmin nav data is Jepp data. FWIW there is no such alert on the Jepp site.
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Colin
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Re: GPS Error?

Post by Colin »

iPad has the Jepp charts. But I think you are probably correct. I fly this route enough that I will update when I am next at the airport and try it again even if it is just in VMC to make sure it is correct.

I don't have any actual charts on my G1000, just the loaded approaches.

I sat on the ground for a bit (waiting for customs anyway) and could just not figure out how the position agreed SO often during the flight but was then incorrect just for the approach. I mean, I was sitting there in the customs box and it had my position match on ForeFlight and the MFD. And I knew there was no adjustment of the position for the GPS hardware, it's math and radio signals, so it just is what it is.

If anyone else is up at BTV and wants to fly the approach to check it, I'd be curious what they see. I wish I had my database cycle handy.
Colin Summers, PP Multi-Engine IFR, ~3,000hrs
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Colin
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Re: GPS Error?

Post by Colin »

I update ForeFlight with the Jeppesen data that FF sells. The G1000 I update with Garmin's site (and app, I think).

When I fly an approach I have the MFD showing the moving map with terrain awareness on. I have the synthetic vision up on the PFD. On FF I am watching it on the moving map (no plate displayed), often tapping over to the geo-referenced approach plate to take a look. I was watching the plate when the controller complained and I could see immediately that on the FF plate I was 1nm north of the course, just like he was saying.
Colin Summers, PP Multi-Engine IFR, ~3,000hrs
colin@mightycheese.com * send email rather than PM
http://www.flyingsummers.com
N972RD DA42 G1000 2.0 s/n 42.AC100 (sold!)
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Rich
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Re: GPS Error?

Post by Rich »

Going to be a trick to reproduce. 2008 is current for only a couple more days.
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