waito
I think I covered some of that in an earlier post, but I can't find it any longer.
Of the parameters obtainable from FR24, the timing data is probably the least reliable, for the reasons of latency already described. That's easy to see from a chronological timeseries of, for example, altitude or track, and results in the jagged charts that have been posted. There are ways to mitigate that, but beyond the scope of this post.
As for the rest:
Altitude is normally baro-derived, almost always to the nearest 25', and based on a 1013.2 hPa datum (as with Flight Levels). FR24 sees exactly the same data as ATC uses.
Horizontal position (lat/lon) is nowadays almost always GPS-derived, with only a very few aircraft sending inertially-derived data. This aircraft had GPS.
Speed (for FR24 purposes) is always horizontal groundspeed, not airspeed, and is resolved by the receiving station from N-S and E-W components. It comes from the same source as track and position.
Track (not heading) is true, not magnetic, and is likewise resolved from those two components.
Frustratingly, there are a bunch of other Mode S EHS (not ADS-B) parameters that ATC can interrogate but which FR24 doesn't seem to have captured on this occasion, including TAS, IAS, magnetic heading, roll angle and track angle rate
HTH