S100 and Oudie N altitude.

[K Urban]

Quick question, While flying last week Altitude displays on S100, OudieN, Trig TX, and Winter showed field altitude at take off (5022MSL) but climbing thru 17,000 had a difference of 800FT between S100 and the Oudie. On the S100 manual the altitude is listed as "Altitude in Feet" under the description. I always set the departure elevation and QNH. In the Oudie guide it is listed as Feet above MSL. After landing they both showed field elevation again. 

The TX was just tested and signed off. It was in line with the S100. Has anyone experienced this before? I'm assuming the S100 is pressure altitude but the manual didn't quite say this. Is this a correct assumption?

Thanks,

Kirk

[Dirk Darling]

Was the s100 agreeing with the trig, or the winter, or neither?

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[Andy Blackburn]

I’d go with the higher one.

Andy

Sent from my iPhone

On Jul 31, 2023, at 9:19 PM, K Urban <ka7...@gmail.com> wrote:

Quick question, While flying last week Altitude displays on S100, OudieN, Trig TX, and Winter showed field altitude at take off (5022MSL) but climbing thru 17,000 had a difference of 800FT between S100 and the Oudie. On the S100 manual the altitude is listed as "Altitude in Feet" under the description. I always set the departure elevation and QNH. In the Oudie guide it is listed as Feet above MSL. After landing they both showed field elevation again. 

The TX was just tested and signed off. It was in line with the S100. Has anyone experienced this before? I'm assuming the S100 is pressure altitude but the manual didn't quite say this. Is this a correct assumption?

Thanks,

Kirk

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[K Urban]

Thanks for the responses. The Trig which has a blind encoder (Laughing) but it's not too blind because it shows the altitude it is reporting and the S100 were 16200. The OudieN was 17K. I don't use the Winter much in flight because it's hard to read and not that accurate. I stayed under 17K on the TX. The IGC file I sent to OLC which came straight from the OudieN and showed in the mid 16,s while the altitude on the Nav Box showed hundreds of feet higher. Sitting on my couch tonight the difference between the MSL altitude and the GPS altitude is only a few feet on the Oudie. 

Kirk

[Andy Blackburn]

If the Oudie N was the outlier where was it getting static pressure from?

Andy

Sent from my iPhone

On Jul 31, 2023, at 11:07 PM, K Urban <ka7...@gmail.com> wrote:



[*Eric Greenwell1*]

The Trig is reporting Flight Level, not MSL. It would be unusual for it to show the same value as an altimeter that reports in MSL pressure altitude, like the Winter or (normally) the S100. Perhaps the S100 has been set to report Flight Level  instead of MSL? GPS altitude can also vary considerably from MSL pressure altitude at high altitudes, like you were flying.

Eric

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[K Urban]

Oops! Sorry for the confusion Eric. What I should have said was the readout on the Trig corrected to the local altimeter. This typically gets me pretty close to what the S100 is showing. I don't know where the static pressure is for the Oudie. I am assuming the QNH comes from the S100 while it is paired via BT. I wish the LXNav and Navatier manuals could explain some of this.

Kirk

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[Morgan Hall]

This sounds like a fairly typical discrepancy between barometric altitude and GPS altitude.  I suspect your Oudie is displaying GPS altitude.  My old XCSoar setup displayed GPS altitude and it would often be 500+ higher than my barometric altitude when high along the Whites or in the Great Basin.  I've also had people text me for "violating Class A" because the GPS altitude reported by my InReach was showing 18500 or something like that, when I was using local Altimeter settings for Bishop along the Whites for example.

From an airspace perspective, your altimeter with a local setting is the critical value since below 18k that is the expected source of truth.

I suspect that the GPS altitude is actually the accurate value above MSL, but the barometric altitude is what we go off from a reporting standpoint.  I believe the LX equipment uses the barometric altitude for final glide calculation.  XCSoar used GPS altitude if that was all it had. Normally I'm not too worried about where I can reach when I'm 17k plus, so the discrepancy was never really a source of concern, but it is nice when things align.

Morgan

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[*Eric Greenwell1*]

The Oudie N has it's own high resolution pressure sensor; possibly, the QNH needs to set on the Oudie N directly.

Eric

[lonkelly]

I suggest comparing the S100 and Oudie IGC files. Each position record in an IGC file will have the pressure altitude (ICAO international standard atmosphere based) and GPS height (assuming no GPS drop outs). Also a timestamp so you can closely sync the two data streams. If the Oudie is using its own GPS and pressure sensor you should get a pretty good idea of what is happening. It's my understanding that good quality digital pressure transducers tend to be notably more reliable than the mechanical altimeter.

[lonkelly]

You can also download some of your ADS-B data for completed flights from FlightRadar 24 if you or someone else has a subscription. The CSV files from FR24 have GPS timestamps, time, registration, position, "altitude", speed and course. I think altitude means pressure altitude, but ADSB can transmit pressure altitude and GNSS height.