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I am working on a project and need to determine the altitudes of airplanes (trying to determine when they are landing/doing a touch and go). I have an ADS-B receiver receiving data from the airplanes. At first I was confused as the reported altitude went below the known altitude of the airport. After further research I found the reported altitude from ADS-B was the uncorrected pressure altitude. Where I got confused was, how do I correct it to get the true altitude? I know the elevation and barometric pressure at the airport. I saw conflicting answers online of how to calculate this

Spencer Thomas
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  • I believe I have found the correct answer but would like some confirmation:

    "True Altitude = Reported Altitude + Airport Altitude - (29.92 - Airport Pressure)*1000"

    that airport pressure is the pressure at the airport altitude, not referenced to sea level (as that is the data I have)

    Does this seem correct?

     – Spencer Thomas Feb 23 '24 at 17:41
  • Do you need geometric altitude instead of barometric altitude? It is being supplied by ADS-B as well. – DeltaLima Feb 23 '24 at 17:46
  • I dont see geometric altitude being received. I can take more of a look and see if its there somewhere I am not logging. I really just want the altitude to match up to the altitude of the airport. It can cause confusion among people if the altitude reported is lower than the altitude of the airport – Spencer Thomas Feb 23 '24 at 17:48
  • Do you have the QNH of the airport? What kind of ADS-B are we talking about: UAT or 1090ES? – DeltaLima Feb 23 '24 at 20:16
  • I dont have the QNH, but do have the altimiter reading at the airport, so QNH should be able to be calculated. I am using dump1090 to get the ADS-B signals. I reviewed the fields I am receiving, and there is just one for altitude. I believe the equation I posted above seems to work in my testing – Spencer Thomas Feb 23 '24 at 21:40
  • So you have your own receiver which is connected to dump1090? What is it what you are trying to do with the data? Dump1090 is not including all data that is being sent by the aircraft, I may be able to point you to a different solution if I understand what you want to do. – DeltaLima Feb 24 '24 at 08:35
  • Yes we have our own receiver connected to dump1090. Our measurements are for a lawsuit so I can't share all the details, but we just want to accurately determine where the aircraft are in relation to a sound level meter we have. The lawyers were confused that the aircraft went below ground level so I am working on correcting that to get a more accurate altitude – Spencer Thomas Feb 26 '24 at 15:04
  • No, they calculate it there based on temperature as they say you probably wont have barometric pressure at the ground, but I do not have temperature and do have barometric pressure on the ground – Spencer Thomas Feb 27 '24 at 22:57
  • See also the second half of the answer to Is there a place where altitude while flying is negative? which deals with ADS-B receivers altitude. – mins Feb 29 '24 at 15:15

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