Primarily for general aviation discussion, but other aviation topics are also welcome.
#1846774
ls8pilot wrote:I wonder if, because you are on the ground, SE is using location altitude off its internal map? When I get home I'll see what my system shows.... assuming Jab #2 has not turned me into a giant lizard :lol:


OK - tried it at home and got a similar result to you - SkyDemon shows me 140ft ALT, but it also says "on the ground", GNSS Alt is zero, Barometric Alt is 320ft, actual elevation of my home is around 140ft, so I suspect that because it is "on the ground" SD is not using the GNSS altitude.

Maybe someone from the SkyDemon team could comment ?
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By Tim Dawson
SkyDemon developer
#1846777
We correct the ellipsoid altitude reported by SkyEcho by applying a geoid model, which means you end up with an altitude relative to local mean sea level.
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#1915197
TopCat wrote:I had a moment of heart-failure the first time I used mine when it indicated about 800 feet higher than I actually was.


Just had the same thing today. London TMA lowering to 2500ft less than half a mile in front of me, I’m at 1800, glance at the phone screen and it says 2800.

The GPS signal was good and it was mounted in pretty much the best place.. I think. I had just descended fairly rapidly (about 1000fpm) from right around 2800ft (what skydemon was showing) and as I levelled at around 1200ft the skydemon altitude eventually crept down and was accurate again which made me think it was because it was using its own altimeter based on cabin pressure but this thread says that isn’t the case.

Any insight anyone? Does anyone know if I’m able to use my phone GPS as the source for skydemon while still displaying traffic from the skyecho?
#1915377
I've posted this reply here before but it's worth repeating...

GPS reported altitude is always less accurate than the lat-long location. It's all down to the satellite geometry. With satellites to the north, south, east and west you pretty much always get good geometry with multiple position lines all cutting at something approaching right-angles. Problem is, the satellites are all above you (with none below) so there is never going to be as good a set of position lines to provide accurate altitude.

The problem is exacerbated by multi-path signals such as will almost certainly be received by any vehicle (aircraft) internal mounted GPS receiver. Add in the fact that the algorithms in most GPS receivers are optimised for lateral and not vertical navigation and you start to see the problem.

There are also quite complex issues with antennas that reduce the accuracy of altitude reporting. By definition, the antenna in any portable device is 'poor' by quality GPS standards. External 'fixed' GPS antenna are better but still a long way short of what can be achieved.

All that said, in the right conditions, the system is extraordinarily accurate. Reasonably-priced commercial survey-grade GPS should be able to provide 30mm in the X-Y axis and about 100mm in Z. With top quality post-processed data, millimetre accuracy is achievable. And expensive!

In very simple terms the limitations of the GPS system and especially the antenna used in all small devices suggest that the results people are seeing are entirely predictable.

P
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