Originally Posted by
Dont Hang Up
No, the ADS-B barometric is always referenced to 1013mb regardless of height. So you would need to apply QNH for all ADS-B reports.
So, if I understand correctly, the QNH corrected altitudes set in the onboard altimeter are NOT reported in ADS-B and are applied on the data by the ground station?
This could explain why sometimes I notice reported altitudes hundreds of feet below ground when the aircraft has not landed yet.
Meaning that I don't have to mess with TA and TRL neither?
Applying QNH to all the reported barometric altitudes would simplify things quite a bit.
This is always going to be approximate. Even assuming you have access to all the QNH data it is not super accurate - it may be a worst case period forecast for a region. Then the deviation of the real atmospheric conditions from the Standard Atmosphere will also add error.
Analysing historic QNH values from surrounding airports didn't show big differences for 2012-2015.
I know it'll be approximate, and I even doubt it will make any difference in what I intend to do - calculate bistatic Doppler signals from flight data and compare them with real signals in radio spectrograms.
Simulations showed that the altitude parameter is by far least affecting the shape of the signals.
Doppler signals of aircraft mess up spectrograms intended for the study and counting of radio meteors.
If I can accurately calculate them using real flight data, I could also erase them.
So far scientists use sophisticated signal processing algorithms that give too many false positives or negatives to be useful, all due to an airspace cluttered with aircraft.
Where it is available use "difference from barometric" in the ADS-B velocity reports as this will give you a true measure of GPS geometric height to the best accuracy the aircraft can provide - which again is not super accurate but perhaps better than the multiple approximations that the barometric conversion will give you.
This data is not available in Plane Plotter logs.
So, rephrasing the rules I intend to apply:
ADS-B reports barometric altitudes that are expressed in geopotential height.
To convert these into orthometric:
1) Convert geopotential to geometric for all reported altitudes.
2) Apply QNH to all reported altitudes using the barometric and inverse (hypsometric) equations derived from ICAO Doc 7488/3 expressed in geometric (orthometric) heights.
Is this correct?