Pressure calibration from airport data

p0wer shared this idea 6 years ago
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I've seen discussions about how to calibrate pressure sensor. One of the apps from Google Play is using QNH data from closest airport. Seems like a good idea, we should give it a try.

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QNH IS usually used in pressure altimeters, but pilots must be aware about errors at extreme temperatures.

QFF would be better, as QNH brings deviations if airfieed temprerature is different from 15 - 0.0065 . h [m].


That means, if temperature is very different to than 15 - 0.0065 . h [m], and if airport altitude is very different to the one of current place, you may end with systematic altitude error bigger than error of mean altitude value of GPS. But for less extreme conditions it can be useful.

For illustration, if an airfield at sea level has temperature 30 deg C instead of standard 15 deg C, it is about 5% relative difference in absolute temperature. This makes about 5% altitude error. Barometric altimeter calibrated to sea level airfield would show at above circumstances for real 500 m above sea about 475 m value, as pressure is higher than would be at standard conditions. Or about 525 m if there is about 0 deg C at sea level.

Additional error can be caused by horizontal presure gradient near atmospherical fronts or pressure high centers, if the airfield is not near enough, or if data are not fresh.

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