Reply to "Isotropy of Speed of Light" by Castano and Hawkins,   arXiv:1103.1620
  In "Isotropy of Speed of Light" by Castano and Hawkins, arXiv:1103.1620, it is claimed, using a flawed theoretical argument, that the speed of light must necessarily be isotropic, independent even of experiment. The key false assumption made is that the round trip time must always be invariant wrt change of direction of the light path. This is shown to be false. More importantly the anisotropy of the speed of light has been repeatedly detected in experiments, beginning with Michelson and Morley in 1887, and with the most recent data being from spacecraft earth-flyby Doppler shift data. Similar misunderstandings critically affect the designs for LIGO and LISA. 
Reginald T. Cahill (Flinders University)
physics.gen-ph