 ::To clarify things further, the two-way speed of light can be shown experimentally to be the same in any inertial frame.  The one-way speed is dependent on the clock synchronisation scheme used.  It can, however, be shown experimentally that the one-way speed of light is independent of the motion, inertial or otherwise, of the source.  This is explained in more detail at the One-way speed of light article.   