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On FlightRadar24, when a user clicks on an airplane icon, the data panel shown above is displayed. Note the figure labelled "calibrated altitude".

For an airplane flying below the "transition altitude" (18,000' in the US), would this value better be labelled "pressure altitude" in most cases?

Meaning that this is the figure that we'd see on a barometric altimeter with the Kollsman window set to 29.92 inches Hg (or 1013.2 milli-bars)? And meaning that a correction for the altimeter setting at a nearby station should be applied to get the MSL value that the pilot would actually be seeing on the aircraft's barometric altimeter?

Relevant to this question, would be the following questions:

a) Is the figure called "calibrated altitude" on the FlightRadar24 data panel based on data from the aircraft's ADS-B transponder?

b) Does the aircraft's ADS-B transponder always transmit "pressure altitude", with no correction for non-standard meteorological conditions?

c) If the answers to "a" and "b" are both "yes", then does FlightRadar24 apply any correction for non-standard meteorological conditions, based on the altimeter setting values for nearby stations, to this data?

(Other ASE answers to related questions seem to have established that the answers to (a) and (b) are both "yes" -- so it seems that (c) is the crucial unresolved question.)

It would seem that if the answers to these questions were "yes", "yes", and "no", respectively, then the value titled "calibrated altitude" would indeed be better titled "pressure altitude". Is this in fact the actual situation? Does FlightRadar24 in fact not apply any correction to the received altitude data, to compensate for non-standard meteorological conditions, where the correct altimeter setting for the area is other than 29.92 inches Hg (1013.2 millibars)?

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FlightRadar24 gets aircraft data from multiple sources: ADSB is primary (terrestrial and satellite), other are MLAT (mode S xpdr triangulation), Radar (xpdr alitude), Open Glider Network (aggregate of multiple sources) and WAGs (position estimate when destination is known).

The data transmitted via ADSB is barometric pressure altitude, as is data via XPDR.

FlightRadar24's definition of altitude is as reported by the aircraft:

Altitude For each flight tracked on Flightradar24 the calibrated altitude reported from the aircraft, which is a pressure-derived value, is displayed. Extended Mode S Data received from some aircraft also includes the GPS-derived altitude of the aircraft.

Their description of "how flight tracking works" also describes no corrections to any of the data received.

So the calibrated altitude displayed is pressure altitude, from multiple sources.

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