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I'm trying to convert the altitude returned by an ADS-B receiver into its WGS84 altitude. It's been a wild ride trying to determine which type of altitude is returned from ADS-B (I get completely contradicting information depending on which site I look at), but using majority rule, it appears ADS-B is returning uncorrected pressure altitude.

Is there a "reasonable" formula that can convert this to an approximate WGS84 altitude? "Reasonable" meaning I don't have to go fly a weather balloon in the sky and collect a ton of data, and maybe getting to +/- 25 ft?

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From: https://www.faa.gov/nextgen/programs/adsb/faq/#g7

... ADS-B [Out] reports two kinds of altitudes: barometric and geometric. Barometric or pressure altitude is the one pilots know best – this is the altitude that is displayed on the altimeter in the aircraft. Geometric altitude is calculated by GPS (Global Positioning Satellites) as the height of the aircraft above the earth ellipsoid. These two altitudes are not the same, but having both allows for applications that require one or the other as an altitude source and provides a means of verifying correct pressure altitude reporting from aircraft.

Summary: You won't need to correct the GPS-derived altitude. However, the barometric-derived altitude will need to be corrected with the nearest reliable ground weather station pressure value for that date and time.

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  • So maybe the jury is still out on which altitude I am actually receiving. Assuming its the barometer altitude, and I look up the ground weather station pressure value for that date and time, is there an established formula for doing the conversion? Oct 7, 2021 at 12:50
  • I know that ATC adjusts the uncorrected transponder altitudes for all aircraft in their sector in order to create a level playing field (or they used to pre-ADS B). I think that the best source for your question is over at Stack Exchange Aviation.
    – Stu Smith
    Oct 7, 2021 at 22:00

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