GPS - incorrect position?

[RobertPBham]

Hi all,

Is there any where in the PAW log file to ascertain your own GPS position?

I have recently replaced my GPS dongle due to two issues I experienced in December - the first was lots of GPS dropouts on the flight (think this has been fixed by changing the auto setting to PAW within the config page) and the second was due to a significant position error I saw on SkyDemon. Basically, SkyDemon was reporting Gaydon was on my left/straight ahead when in fact it was on my right.

I originally thought that it may have been a mapping error or caused somehow by the GPS dropouts (is that possible) as I had only just pulled the latest maps the previous day. I have had a map update since then and the SkyDemon log still presents Gaydon to the left/staightahead.

I have not had chance to fly with the new dongle as it would be easy to replicate the return route to Wellesbourne via Gaydon but was wondering if the GPS positions are stored in the track file and if so, how can these be deciphered? I can then just plot the cooridinates on Google Earth and double check everything to ascertain where the problem was. I'm assuming SkyDemon uses GPS coordinates within the log file? Hence if it was a mapping issue, this would appear fixed with an updated map?

I should add that the rest of the flight appeared fine although I was doing an RNAV approach in to Cambridge so was reliant on the onboard GNS430 for a significant part of the flight there and back so wasn't double checking SkyDemon.

Thanks for all your help
Rob

[Admin]

Yes, the track file contains the nmea gps messages
Thx
Lee

[grahambaker]

What does the SD log itself show on the chart?

The current SD chart certainly appears to map Gaydon more or less correctly.

Looking at the aerial photography for the Gaydon disused symbol as marked on the chart appears to centre it in the large house on the south-east edge of the disused airfield.

[RobertPBham]

The SkyDemon log shows the incorrect track and I thought this might change with a map update.

I’ve tried decoding the NMEA GPS data but it isn’t working as hoped and Google Earth is giving me locations in the sea.... :-) (that’s with trying various conversations)

So I’m going to go for the easy option and just fly and see what the results are - I’ll use specific landmarks to confirm.

I’m also going to check Flight Radar or similar to see what that reports - I know it’s not 100% but will give me an idea.

Thanks
Rob

[grahambaker]

How close to Gaydon were you and was it a very windy day?

[Admin]

Robert have you tried loading the track file into the viewer ?
https://aircrew.co.uk/playback/

Thx
Lee

[grahambaker]

Perhaps it is a mapping issue

The image below shows a route heading eastbound from Wellesbourne at a scale of 1:250k. I’ve dropped a waypoint onto the centre of the Gaydon former runway, and you can clearly see that it’s possible to be right of that but left of the ‘disused’ symbol when using a normal aviation chart scale.

A note to Tim Dawson requesting that he review his mapping data might be in order.

[Ian Melville]

0kb, that image is corrupt.

IIIRC Tim uses the official source for positions, perhaps you need to check what that says before to talk to him?

[grahambaker]

Thanks, I hadn’t spotted it hadn’t uploaded properly.

Indeed, it could be bad data that Tim D has received, or correct but misleading data.

[RobertPBham]

Thanks all - I think Graham has solved the conundrum and I picked the worst place to check my GPS position based on the map data! :-)

I've included a screenshot from aircrew (thank you to Lee for pointing out that fantastic resource) and SkyDemon together.

As you can see, there is a 'difference' to where I would expect Gaydon's position to be mapped. I've never noticed it before all this time flying over it, but I guess as I've been looking for GPS issues due to the dropouts, this pointed to a problem that didn't exist and is just a small map discrepancy (or maybe not depending on where you think the disused symbol should be - I think the center of the airfield, but maybe that is the center if they own huge amounts of land around it!?)

Thanks for everyone's help - it's reassuring to know everything is working correctly! :-)

Thanks
Rob

[exfirepro]

Hi Rob,

I have been scratching my head trying to work out what was going on. The only thing I could think of was that it was something to do with your heading due to wind direction - which Graham alluded to above. I have certainly experienced this to some degree in my flexwing on a few occasions, particularly in strong crosswind conditions but didn’t want to comment in case I just confused the issue. The track shots definitely support this as the most likely situation, rather than a GPS fault.

Glad you have got to the bottom of this.

Best Regards

Peter