Originally Posted by
Pilot DAR
What remains unclear to me, is the "groundspeed" being reported, two dimensional, parallel to the surface of the earth?
The link is more than an article, its a complete book. The link below has an index of all chapters which may possibly contain your answers. If not, perhaps reach out to the author for those answers? I follow the theory side mainly to assist with troubleshooting plus had a unique opportunity to learn the workings of ADSB when the company I worked for participated in the FAA's 2nd ADSB critical service test back in 2009 or 10. Beyond that I don't have much to offer except point you in the different directions for your possible answers.
https://mode-s.org/decode/index.html