ft,
I pondered weighing in again on this, since there appears to be a certain amount of noise on the channel which I don't wish to amplify. Your proposal raises a question.
Originally Posted by ft
I'd say that ground speed should be defined as the speed of the aircraft over level ground, as defined by the rate of change of the position of the aircraft as projected vertically onto the same level ground.
Yes, but it isn't so defined by the nav community, as we have seen. They take the angular velocity as definitive and multiply by the radius of the flight above the nominal centre of the earth (radius of earth + altitude).
Definitions are usually given for a purpose. If you say the definition "should" be as you say, what is the purpose you have in mind?
Originally Posted by ft
Then again, "vertically" is not constant and usually not "directly towards the center of the earth", nor is "level" usually "perpendicular to a vector through the center of the earth".
Indeed so. But these difficulties apply to all reasonable attempts to define it.
PBL