[ls8pilot]

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

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 ?

[Tim Dawson]

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.

[ZacKaplowitz]

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?

[Peter Kelly]

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

[defcribed]

Reviving an old thread....

I was manoeuvring just northeast of Banbury yesterday, chasing myself around some clouds. Now if you start a climb in an RV it's not unknown for SkyDemon to go nuts with impending airspace warnings because it doesn't take long on that trajectory before you infringe. You make sure you know where you are and get used to the warnings.

I knew I still had a thousand feet or so of Class G above me and was just levelling off to swing around this particular cloud when SkyDemon upped the ante and declared 'ENTERED DAVENTRY CTA'. I didn't stop to argue with it, closed the throttle and stuffed the nose down.

This was SkyDemon on my iPad, getting position data from my SkyEcho 2.

Was wearing an Oxford FMC and listening out. Sweating a little, I kept the radio and txpndr as they were for long enough for London Control to pick up the phone and say "Oi, Oxford...." and then the radio call to come to me.... but nothing came.

Reviewing the flight later on the ground, there was a big difference between the altitudes on my iPad (SE2) and my phone (internal GPS). They disagreed by about 1,500ft on how high I got. The iPad showed a long climb continuing after entering the CTA, well after I'd stuffed the nose down, and then an almost vertical drop of 2,000ft. The phone showed what I expected - the short steady climb followed by levelling off then a sudden descent, never getting closer than about 1,000ft from the base of the CTA above.

Yes, yes, yes......low QNH yesterday. But the CTA it thinks I infringed is defined by an altitude, not a flight level. The profile on the iPad is also implausible - you can't dive an RV that quickly without getting to about twice VNE.

My take on it is the SE2 had a brain fart of some sort. Another good reason to keep my phone going as a backup.

[Miscellaneous]

It's only 20 mins since I sent UAvionix an email re unreliability of SE. It seems to get worse every flight. I gave up on it last flight.

[GrahamB]

I'm not particulalry impressed with SE2 as it seems selective in what it displays. Using it on Saturday, travelling the same routes just behind, above, below and above another Grumman with certified ADSB-out, I never saw him once over the course of the day, despite seeing lots of other ADSB-out targets. Even when following him in to our destination, I could see other targets in the circuit, but not him.

He was definitely broadcasting, as he appears on both PAW Vector, and FlightAware. The week before, when using my PAW (which is subsequently OOS due to a problem with the RPi board, hence using the SE2) I saw him clear as a bell.

[Miscellaneous]

Well, I'm not only not impressed with the SE2, I am less impressed with the support.

In response to my message on Tuesday I received an automated email reply providing a ticket number and advising a support specialist would be in touch within 24 business hours.

So far nothing!

[999Pilot]

What am I missing here? Why does it really matter? Why not use the aircraft’s altimeter set to the most relevant setting to establish your altitude, height or Flight Level. Are we in GA becoming so reliant on technology that the fundamentals seem to be unimportant. You could link this to a number of threads on this Forum. (GNSS and AAIB ones to name but 2).

[Charliesixtysix]

It is worth understanding that devices utilising cockpit static can give rise to significant discrepancies in height readings compared with instruments utilising the aircraft static system. Probably not anywhere near enough to account for the 1500 ft described by defcribed above but flight tests carried out with two freshly calibrated altimeters in our RV7 demonstrated that the indicated difference between cockpit static and aircraft static could easily be in excess of two hundred feet in level flight at high speed.

[johnm]

This analysis of an infringement highlights some issues in respect of altimeters.

Just ignore the pompous ass style in which it’s written

https://airspacesafety.com/infringement-occurrences/

[Tim Dawson]

Wow. Whoever wrote that definitely has a bee in their bonnet about the unnamed VFR moving map system being “unregulated”.

[Hanworth]

This analysis of an infringement highlights some issues in respect of altimeters.

Just ignore the pompous ass style in which it’s written

https://airspacesafety.com/infringement-occurrences/

Thanks for pointing to that John.

'Pompous ass' is a tad unfair methinks. 'Officialese' would be fairer.

I don't know about 'issues with altimeters' either tbh. Matey didn't reset to 1013 even though he was above TA and flying intentionally very close to the base of airspace defined by flight level.
That's a pretty basic mistake.

@999Pilot has it right above. Use the basic stuff that we all carry first. Too many folk ignore the basics because they've got the bells and whistles of modern tech. This is an example of how doing that can get you into trouble.

[Sooty25]

Seems to me that the cause of a lot of this is the lack of reliable altitude/ static pressure data into the Moving Map Systems.

GPS is not reliable, nor is it referenced to atmospheric pressure. On top of that, it just not the recognised way of doing it.

Cockpit static we know will have variations from aeroplane to aeroplane, so any device that relies on that, creates errors.

Really, we need a sensor in the aircraft static line, that transmits to the tablet hosting the MMS. Until we do that, so the tablet is on the same static as the altimeter, we will always have errors between the two.

[malcolmfrost]

Controlled airspace altitude limits are defined by certified pressure altimeters, not GPS!!