What a lot of twaddle! Checkboard (and, believe it or not, 'bumpyflight's' first sentence) are absolutely correct. IF you use the term 'ground speed' it is speed relative to the ground, and nothing else, and unless you have a plateau up at 'X+Y' altitude, that will do. If you wish to relate to the velocity along a tangent to the earth at any particular radius you'll need another word! How about we invent one if you have to? 'Tangspeed'? Orbital velocity? Answers on a postcard (positioned in an earth frame continuum) of course. Not forgetting the stamp to be positioned relative to the postcard frame.
As to how you can consider any form of PEC calibration using 'groundspeed' when you have no idea of the motion of the containing airmass.............................. .you'll need to explain.
PBL's elegant maths conclusively prove that the 'angle' subtended by the horizon has nothing to do with Tangspeed.'
