Reported groundspeed is GPS derived speed
Okay, I should ask my question a little differently then... Is the GPS (or GNSS) derived [ground] speed considered in two dimensions - parallel to the earth's surface? Or in three dimensions, considering an altitude change as an element in the speed? If the plane is diving at a severe angle, is the ADS-B reported speed considering the vertical speed as an element in the "groundspeed" in the third dimension? IN the extreme, if a plane dived vertically, would it indicate any "groundspeed" on the ADS-B output?