Originally Posted by
ATC Watcher
You do not need Mode S to get GS. Any radar can give you GS, if you know the Rotation RPM of the antenna, the distance covered between 2 update and bingo: GS. In a multi radar environment, the GS is very accurate. It is often displayed on the labels of ATC radar displays.
Ah, I'm with you now. I misinterpreted your post as meaning that it was Mode S that the GS was derived from.
Yes, I understand how GS can be derived from successive radar plots (or of course extracted direct from EHS or ADS-B transmissions from aircraft thus equipped).