How is auto QNH calculated?
To accurately know your current flight level altitude, the device needs to be calibrated to the aviation standard pressure of 1013.25 hPa.
To accurately know your current altitude above mean sea level (AMSL) the device needs to be calibrated not to the aviation standard pressure of 1013.25 hPa, but rather to actual atmospheric pressure.
Oudie will correct the current standard pressure altitude to the current altitude above sea level and then calculate the current QNH pressure with that data.
There are three ways Oudie will do this:
1. Determine if there is an airfield nearby (takeoff in the paragliding version) and use that altitude as the source for the QNH calculation. (Note: Oudie will do this only if the check box Auto QNH is not checked.)
2. Should an airport not be available close by, Oudie will use the ground elevation from the current location.
3. If we determine that the ground is not level enough to use method 2 above, Oudie will fall back to a solution which is not scientific but still better than nothing. It will average the GPS altitude for a long time and then apply that altitude as the source for the QNH correction.
Because Oudie calculates altitude from both atmospheric pressure and GPS data, it is possible for Oudie to automatically correct QNH with data already available to it.
By default, the Auto QNH checkbox is checked. The Auto QNH feature is going to average the GPS Altitude for a very long time to eliminate random errors from the GPS Altitude.
Then, it will correct the pressure altitude to the measured GPS Altitude, by changing the QNH Pressure setting. This gives a useful takeoff altitude suggestion as well as an indication of the QNH pressure level.
Note: for this method to work well you need to turn on the Oudie for at least 5 minutes, 10 is better. Once Auto QNH has been determined you can put the Oudie back to sleep. It will then remember the measured values once you turn it back on.)
When is auto QNH available?
It is available by default in the Oudie IGC and Oudie 3 which both have a very reliable and accurate pressure sensor already installed. On an Oudie 2 (and 1) where only GPS altitude is available internally you need some input from pressure sensors to be able to use Auto QNH. You get that if you connect your Oudie to an IGC approved flight recorder or vario such as Flarm, LXNAV V7, Nano, Colibri, CAI 302, Borgelt vario etc etc. The list is long and these are only the most popular solutions out there.