"Groundspeed" is derived from a horizontal vector with reference to the WGS84 ellipsoid (as
Abrahn mentioned), but yes
Pilot DAR I should think your understanding is, for practical purposes, correct.
Jan Olieslagers There is very definitely a standard! The reference I used earlier to research this was
ICAO Aeronautical Telecommunications Annex 10 Vol 4, and
CS-ACNS (as reported above).
wrench1 posted some useful links that give good explanations around ADSB velocity data, these are definitely worth a read. Further useful information can also be found in papers such as
Modeling ADS-B Position and Velocity Errors for Airborne Merging and Spacing in Interval Management Application (Satish C. Mohleji and Ganghuai Wang), another description of ADS-B airborne velocity
here, GNSS time reference info
here, and geodetic detail
here.
While
Pilot DAR hasn't said exactly what this is about I've assumed a light a/c that doesn't transmit extended information?
FP.