[Cub]

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).
.

What sort of margin are you experiencing?

[C1FF]

The dynamic pressure of air at 120 knots (S/L) is equivalent to altitude pressure change of approx 600ft. Although it is highly unlikely that the aircraft cockpit design will result in all the dynamic pressure being converted into cabin static pressure (positive or negaitive) there is likely to be a difference between aircraft static and cockpit static pressures. As the dynamic air pressure is a function of the square of the air speed, faster aircraft are likely to see a greater error / differences. Something to keep an eye on.

[Charliesixtysix]

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).
.

What sort of margin are you experiencing?

Only tested it once to verify a hunch I had - I recall seeing +/-300ft with a (grossly?) simplified method of taking a spare altimeter in the cockpit for comparison.

I don't use any barometric pressure reporting equipment that is not plumbed in so no issue for me, but the potential for discrepancy is something to be aware of.

[Miscellaneous]

...the potential for discrepancy is something to be aware of.

In terms of the warnings from EC is that discrepancy going to be any more significant than the GPS error/difference between two units?

[Boxkite]

The point is, for the purposes of the OP's questions, the Ipad/Skydemon is using GPS altitude from the Skyecho. So any discussion on baro pressure in the cabin is irrelevant to his altitude discrepancies.

[Miscellaneous]

@Boxkite surely not if the baro is what the output is using?

[Boxkite]

@Boxkite surely not if the baro is what the output is using?

Check Cub's post, which is what I would have logically expected.

[Cub]

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).
.

What sort of margin are you experiencing?

Only tested it once to verify a hunch I had - I recall seeing +/-300ft with a (grossly?) simplified method of taking a spare altimeter in the cockpit for comparison.

I don't use any barometric pressure reporting equipment that is not plumbed in so no issue for me, but the potential for discrepancy is something to be aware of.

A 10 Mbs difference? That seems a very large value.

[GrahamB]

AIUI, SkyDemon will plot ownship altitude on the virtual radar using SE2’s GNSS altitude (or that of the tablet internal GNSS if no external GNSS device is connected/selected). It will also use it as the basis of the numbers on the ALT/HGT navigation tool.

When it comes to displaying traffic vertical separation, it will use the pressure altitude passed by SE2 to SD for both traffic and ownship messages.

[C1FF]

Its interesting that the regulators have given considerable thought to ADSB GPS standard and SIL and yet there appears to be little consideration of the baro static source for portable units.

[Dodo]

So thanks everyone. OP here again. I have been playing with my SE2 (with the adsb transmit disabled), and there appears to be a discrepancy between the GNSS altitude shown on the SE2 webpage (on my phone) and the altitude showing on SD (using the SE2 as the GPS source).

See picture..


You can see the SE2 is reporting a GNSS altitude of 279 ft and SD is showing it as 130ft despite using it as the GPS source.

So I am still confused. Any bright ideas?

And yes I know, you can all work out where I live, but that is a risk I shall have to take.

[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

[FlightDek]

Does SD compensate for the GNSS geoid being used by the SE? ie SE is reporting altitude compared to the standard geoid, whereas SD is reporting altitude compared to a geoid that better represents sea level in the UK?

Just thinking out loud (I've got a noisy keyboard!)

[davegill]

What sort of margin are you experiencing?

Only tested it once to verify a hunch I had - I recall seeing +/-300ft with a (grossly?) simplified method of taking a spare altimeter in the cockpit for comparison.

I don't use any barometric pressure reporting equipment that is not plumbed in so no issue for me, but the potential for discrepancy is something to be aware of.

A 10 Mbs difference? That seems a very large value.

In my RV9 I see approximately 200ft difference between cabin and static at cruise.

[GrahamB]

You can see the SE2 is reporting a GNSS altitude of 279 ft and SD is showing it as 130ft despite using it as the GPS source.

So I am still confused. Any bright ideas?

SkyDemon will apply the Geoid Undulation to the raw GNSS altitude to derive an effective 'true' altitude. You should ignore the one on the SE2 webpage (unless you happen to be in a location where GUND=0).

Edit to add: the GUND at your secret location is consistently 148' across the whole airfield. You can often find it on the CAA Aerodrome Chart where there are instrument approaches published.