Disclaimer: I’ve read the FAQ on this, but I’m going to ask again with a focus on the FlightAware Pro Stick Plus, specifically:
Hello. I’m using the FlightAware Pro Stick Plus v1.0, Mode S Receiver using dump1090 --net mode, and I’m trying to understand exactly what is meant by the returned altitude field (in general, or the most frequently returned altitude. I understand that there will be some oddball aircraft out there using different measurement methods, but I’m just concerned with the most common commercial flights). I received data from the ADS-B method and got 39,025 ft, and then I went to the FlightAware live tracker site, found the same aircraft, and it was reporting exactly 39,000 ft. Okay, pretty close. I guess we can call it rounding error?
But then it got me thinking: What does a modern aircraft use when flying? If an aircraft’s goal is to maintain 39,000 ft altitude, would its guidance systems follow the the curve of the ellipsoid using GPS data, or follow the wobbly curve of the mean-sea-level, or is it simply using a barometer (I guess is a variant of mean-sea-level)? Again, I’m talking modern expensive commercial aircrafts, not Indiana Jones in a Cessna).
For reference, this is the style of data string I’m getting from the module: