Primarily for general aviation discussion, but other aviation topics are also welcome.
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By Dodo
FLYER Club Member  FLYER Club Member
#1846506
If the mods think this should be tacked on to one of the other existing EC threads, please feel free to move it.

I have a query about the GPS altitude shown on my ipad (using Skydemon) using the Sky Echo 2as my location source.

Before I had my Sky echo 2 device I noticed that the GPS altitude on my ipad mini 3 (using its own GPS), my Garmin 295, the altimeter and the Displayed Flight Level on my transponder screen (with an appropriate mental arithmetic calculation for the difference between QNH and 1013) tended to agree most of the time.

Now I use Sky Echo 2 as my external GPS for the ipad, I find that GPS altitude sometimes varies significantly in level flight from minute to minute (and by several hundred feet) and is usually the odd one, in that the Garmin 295, the altimeter and the Transponder displayed FL agree with each other.

So I have 3 related questions,

1) Could the GPS module in my Sky Echo 2 be faulty in altitude only (seems unlikely) or does it just have a lousy view of the sky.

2) Does the altitude output in my Sky Echo's ADSB broadcast come from the Sky Echo's GPS, or from its built in altitude encoder.

3) Is the traffic vertical separation against other traffic as shown on my ipad in flight based on my GPS altitudes, (which may be wrong if my GPS is returning the wrong altitude) or from the built in altitude encoder derived altitude.

and one more question. Has anyone else noticed this?
Last edited by Dodo on Wed May 12, 2021 5:55 pm, edited 1 time in total.
By TopCat
FLYER Club Member  FLYER Club Member
#1846509
Dodo wrote:and one more question. Has anyone else noticed this?

I had a moment of heart-failure the first time I used mine when it indicated about 800 feet higher than I actually was.

Having checked the altimeter reading and QNH setting in the next couple of seconds, it was obvious, fortunately, that the tablet was lying. I would certainly have set CAIT off at Heathrow if I'd really been up there.

I've found a better place for the SE2 since then, and the issue hasn't recurred. Mostly it seems to show a few 10s of feet different from my altimeter, which is much the same as when I was using the tablet itself for altitude.

Hard to say whether the mounting location is the issue, as a sample size of 1 isn't really enough.
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By gaznav
#1846531
From the manual:

GDL 90 output (via Wi-Fi)

o Traffic, ownship GPS, barometric pressure, and FIS/TIS data are translated to standard GDL 90 format for Wi-Fi transmission to compatible situational display applications.
By johnm
FLYER Club Member  FLYER Club Member
#1846535
TopCat wrote:If the Sky Echo is outputting barometric pressure, how does it get converted to altitude on the Sky Demon display without knowing what the QNH is?


It will display FL as the transponder does I imagine
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By TopCat
FLYER Club Member  FLYER Club Member
#1846548
johnm wrote:
TopCat wrote:If the Sky Echo is outputting barometric pressure, how does it get converted to altitude on the Sky Demon display without knowing what the QNH is?

It will display FL as the transponder does I imagine

If that's the case, how do CAS alerts work then? Most of the CAS that affects recreational pilots is based on altitude.
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By gaznav
#1846556
@TopCat

I believe they trigger warnings as you get close, not as you enter it. So if it is CAS not based on FL then there could be an error of the exact time of warning if it’s a high or low pressure day and you are above or below the CAS that you are about to infringe. It’s then time for the 27ft/hPa (let’s use 30 for ease) calculation to check it against your altimeter. Obviously, you should be staying clear from CAS with your pressure altimeter and not from the SkyEcho or GPS.
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By Cub
FLYER Club Member  FLYER Club Member
#1846562
I have just checked with Tim because I had forgotten! SkyEcho produces both GPS altitude and Baro level. SkyDemon uses and displays GPS altitude. Any ATC or FISO unit capable of receiving and displaying standalone ADS-B will receive and process the Baro level.
Iceman, TonyC, gaznav liked this
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By Cub
FLYER Club Member  FLYER Club Member
#1846608
C1FF wrote:As SE2 is not connected to the aircraft static system, it will transmit baro level associated with cabin pressure. Typically, what sort of error will this produce?


I believe negligible in most unpressurised GA aircraft operating below FL100.
By johnm
FLYER Club Member  FLYER Club Member
#1846613
Hmmm something else to keep an eye on, fortunately I only use it in receive mode.
#1846623
Cub wrote:
C1FF wrote:As SE2 is not connected to the aircraft static system, it will transmit baro level associated with cabin pressure. Typically, what sort of error will this produce?


I believe negligible in most unpressurised GA aircraft operating below FL100.


Not always.

Some types can have cabin pressure vary significantly from static.

Eg in my experience RV7 cabin pressure is lower than static at cruise speed by some margin ( and varies according to if vents are open or closed).

Beware- this as a potential gotcha with carry on kit that uses barometric pressure whilst not plumbed to a/c static.
Last edited by Charliesixtysix on Thu May 13, 2021 8:09 am, edited 1 time in total.
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By cotterpot
FLYER Club Member  FLYER Club Member
#1846625
..and probably why in the SE2 manual is this - page 15

Altimeter Cross Check
The reported altitude must be cross-checked against the aircraft’s altimeter during pre-flight.



Doesn't mention during flight though so in case of varying with vents open/closed would be a further check?