The "point" in our case is the pilot signal traveling from 0 to O'. In order to be able to speak of the velocity of this signal, we must assume that it is a wave train of finite length, but since the length is arbitrary, the result holds for a CW signal. The velocity of the wave train is c, the group (and phase) velocity of plane EM waves in empty space (atmospheric and plasma effects are neglected). Therefore, the velocity components in the X, Y, Z CS are: Applying these to (Bia) and (Bib) (we drop []), we get
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