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Garmin GLO/Android altitude discrepancy


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StuartW
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Hi,

New user here.  My setup is as follows:

Huawei MediaPad T3 8"
Garmin GLO bluetooth GPS

I've paired the Garmin and setup Bluetooth GPS as mock GPS provider, however SkyDemon is showing an altitude discrepancy.

Bluetooth GPS says 75.5ft
Sky Demon says -70ft

whilst on the ground.
I've checked that Bluetooth GPS is NOT doing Altitude correction as mentioned in other threads.

If using the internal GPS, SkyDemon appears to be correct.

What am I missing please.

StuartW

Tim Dawson
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It's doing altitude correction. Really. Probably the mock GPS app you're using.
StuartW
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Tim Dawson - 6/19/2020 8:25:43 AM
It's doing altitude correction. Really. Probably the mock GPS app you're using.

I've now tried two different mock gps apps - Bluetooth GPS and Bluetooth GPS Provider.  Both have the option to disable correction, both report the same negative figure in SkyDemon when correction is disabled.

Strangely, as an experiment I set the providers correction to be ENABLED, now the discrepancy between the providers and Skydemon is less (around 20'), SkyDemon is now showing a positive figure, but - here's the catch - connecting an iPad to the Garmin directly and SkyDemon on iPad reports exactly the same altitude as SkyDemon on android.

Is there even an issue here, or am I over thinking it? 

PaulSS
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Given the accuracy of a Garmin Glo is 3m, I would say an altitude discrepancy of 5.5 ft is over-thinking things. I'm having difficulty envisaging a scenario when I'd be using GPS altitude, except in the case of an instrument/static pressure failure. In that case I would not be worrying about a 5.5 ft inaccuracy.
StuartW
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PaulSS - 6/19/2020 10:01:54 AM
Given the accuracy of a Garmin Glo is 3m, I would say an altitude discrepancy of 5.5 ft is over-thinking things. I'm having difficulty envisaging a scenario when I'd be using GPS altitude, except in the case of an instrument/static pressure failure. In that case I would not be worrying about a 5.5 ft inaccuracy.

I think you have misread the discrepancy Paul:

Bluetooth GPS says 75.5ft
Sky Demon says -70ft (minus 70ft)

Thats about a 145ft drift...

grahamb
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Geoid undulation varies from place to place, but 145ft would not be an atypical value for somewhere in the UK. Heathrow is 151ft.
StuartW
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grahamb - 6/19/2020 10:47:09 AM
Geoid undulation varies from place to place, but 145ft would not be an atypical value for somewhere in the UK. Heathrow is 151ft.

Agreed - but in the perspective of this issue I'm still confused.

I have found that by ENABLING correction (which I believe is incorrect) I can make SkyDemon report the same on android (using the GLO2 via Bluetooth GPS Provide/Mock GPS) report the same as an iPad connected directly to the GLO2, and I mean EXACTLY the same - they even change almost synchronously.

So is SKyDemon NOT doing correction in the latest version?

PaulSS
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You're absolutely correct, I did miss the minus sign.
In the big scheme of things I do think it is not very important, practically, but I am interested in the technical side of the problem/solution.

StuartW
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PaulSS - 6/19/2020 3:54:30 PM
You're absolutely correct, I did miss the minus sign.
In the big scheme of things I do think it is not very important, practically, but I am interested in the technical side of the problem/solution.

Perhaps its cause I haven't actually been in the air with SkyDemon yet, but 140' drift could be the difference between busting airspace and not?  

I too however am interested in what is technically going on here, as it's certainly not working how I've seen suggested elsewhere...

PaulSS
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HMmmm, I don't think you'll be flying looking at the SkyDemon 'radar' display to determine your vertical clearance from controlled airspace, so the accuracy of the GPS altitude (IF this is what SkyDemon uses) really shouldn't be a snag.

SkyDemon tells you at CTA goes from 2500-6000 ft. You fly at 2300' on your altimeter. Snagless :-)

GO

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