Reported QNH is off for temperatures other than 16deg

[rkusa]

@talbotmcinnis I hope you don't mind that I am pulling this discussion into a Github issue here. This helps me keep track of why certain changes were made and also to reference to them if it comes up again in the future.

As you've reported, the QNH doesn't seem to be correct, tested via:

adjust the altimeter setting so that the altimeter reads the correct known elevation of whatever point you are at

This seem to also match what was reported here https://forums.eagle.ru/topic/256057-altitude-qnh-error-bug/
When I change the mission temperature to 16 deg, the QNH seems to be correct, but for other temperatures it deviates.

I'd be open to fix this for DATIS, despite the DCS mission briefing reporting something else then, however, I don't know yet how to fix it. While I can get the pressure and temperature on both sea level and the actual altitude of the airplane, using a barometric formula doesn't seem to provide a accurate result (the linked error above also suggests that DCS is using a strange pressure system).

Example:

Parked at Beslan, Mission set to 8.4 deg and QNH 2992.
Altitude in parking position is 1727ft, while F18 reports 1770ft (with the QNH of 2992 dialed in).

DCS reports the following information:

• At aircraft altitude: temp 4.99 deg C, pressure 95034.4 Pa
• At sea level: temp 8.4 deg C, pressure 101325 Pa

Throwing this into the barometric formula (which I might have done wrong) results in an altitude of 1722.47, which is closer to the actual 1727ft the mission editor reports, but still off by ~4.5ft (compared to the previous error of 43ft). Though I am not sure how much margin of error is acceptable here.

@CaptainBandages do you happen to have an opinion here too - when sitting on the airfield, having the QNH dialed in, how much difference in altitude compared to your actual altitude is acceptable in your opinion?

[CaptainBandages]

QNH is sea level pressure corrected for non standard temperature so the altimeter reads elevation on the ground. Its also might be why the tankers are not at their stated indicated altitude in DCS, they are erroneously using True Altitude in the mission editor.

You can have 75' variance for it to be acceptable on the ground. In the air the difference between 2 altimeters can be up to an exceptable tolerance of 200'.

DCS gives out QFE in the mission brief which very few states use which the altimeter reads zero at the airport. Only time I see it used is for airshows.

[CaptainBandages]

MSL will be different from QNH because QNH is MSL corrected for non standard temperature at the airport elevation that the altimeter setting was issued for. So you'll see a deviation from MSL and ASL in non standard temperatures. The important part is that the altimeter reads the correct elevation at the airport reference point. Soon as you climb from that reference you introduce errors but the important part is that everyone in the vicinity uses the same reference in the same volume of air. We dont have to worry about temperature corrections for deconfliction from other aircraft. We can however if we are concerned about avoiding terrain, calculate true altitude. Basically it only a problem when its colder than standard. "From High to Low, Lookout Below" also applies to flying in different pressure regions without setting the altimeter.

[talbotmcinnis]

I'll see if I can get extra opinions too. Thanks for looking into it, and good job even figuring out that temperature was the factor!

[talbotmcinnis]

Tested the fix and I definitely think its going in the right direction, ie compensating for temperature, but I did still notice a deviation at nellis. I was a bit rushed so I didn't get exact data but I will repeat my test and get you actual numbers. Code looks sound so not sure how practical it is going to be to tweak, but lets hope the data reveals a pattern.

[rkusa]

For most tests, I saw a difference of +10ft from what would have been an exact match (as exact as it can be anyway). E.g. sitting at Nellis at 1848ft ASL, the altimeter setting report from ATIS brings my altitude to 1860, while 1850 might have been the even better result. While I'd say that 20ft is within a reasonable margin of error, I'd be fine to always substract 10ft in case you can confirm that this is the difference you see most of the time too.

[talbotmcinnis]

Hey Markus. So I tried more combinations and here is what I found: [image: image.png] Totally unrelated, but when the ME temperature is "minus 5", datis reads out "5 degrees celsius", leaving out the "minus" or "negative" phrasing. As before, at 16 degrees it's effectively spot on, but as the temperature moves away from 16 the error does seem to grow. At +50C it is the most off at nearly 70' over. I would suggest a simple linear adjustment based on this data would be sufficient because who knows what DCS internals are doing, and yes I agree that a zero-difference is unachievable. Quick math attempt, I would suggest a 0C we can approximate zero error, and at +50 I observed the "correct" altimeter setting for field elevation is 29.60, so DATIS announcement was high by 0.07 in inHG. So subtracting 0.07 * temp_c / 50 from the readout should put it right on the money. Of course there is a risk this correct does not hold at different altitudes, but it works great at Nellis which is reasonably typical altitude wise. The good news too is that at least in Canada, regulations for VFR are within 100' and IFR within 75', so while I love the pursuit of perfection here, I think you can sign off once it's within 10! Once again exciting progress, great job.

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On Fri, Apr 30, 2021 at 3:58 AM Markus Ast ***@***.***> wrote: For most tests, I saw a difference of +10ft from what would have been an exact match (as exact as it can be anyway). E.g. sitting at Nellis at 1848ft ASL, the altimeter setting report from ATIS brings my altitude to 1860, while 1850 might have been the even better result. While I'd say that 20ft is within a reasonable margin of error, I'd be fine to always substract 10ft in case you can confirm that this is the difference you see most of the time too. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#85 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAYYIGVG2U46YQAD27E37YLTLJPKFANCNFSM4WSDCNVQ> .

[CaptainBandages]

https://www.weather.gov/media/epz/wxcalc/altimeterSetting.pdf

[talbotmcinnis]

Retry at uploading the image.

[rkusa]

@talbotmcinnis Thanks a lot for testing the changes! I've found some more deviations for low/high temperatures on different pressure settings in the ME and different airport altitudes. I was unable to fix them with a linear adjustments, but was able to find another approach. I am now calculating QNH based on the pressure values DCS reports for sea- and ground-level. This looks quite promising and I didn't notice any deviations of more than 10ft during testing. I've already released the changes as part of v3.0.1, but please feel free to report higher deviations if you still notice any.

[talbotmcinnis]

Excellent, can't wait to test it out, but I'm confident you've got it now. Great job!

[CaptainBandages]

I think you guys are doing the temperature corrections wrong. The Altimeter settings are way out now compared to the previous version by .04".

[talbotmcinnis]

Seems like its going to be challenging to handle all the variables in play here. I suggest we use some test-driven development, and come up with unit tests of all this that can run without dcs so changes to the core model can be validated. Just a thought.