Alternative to the principle of constant speed of light
  To consider a medium carrying light and electromagnetic waves is impossible, when this medium shall have properties according to the principle of constant speed of light, that is, isotropy of speed of light in every system of reference. Therefore, with principle of constant speed of light abandoned, the so-called medium synchronization of clocks is defined, yielding isotropy solely in the system at rest and anisotropy in all moving systems. From medium synchronization the appropriate coordinate transformation is developed, the so-called medium transformation, a Galilean type of transformation, thus maintaining simultaneity between all participant systems of reference and changing the form of Maxwells equations to the extended Maxwell equations. Despite the fact that medium transformation violates Lorentz invariance and the principle of relativity, it is demonstrated that the results from medium transformation and extended Maxwell equations are fully compliant with observable phenomena. Moreover, it is shown that the concept of a medium, a preferred frame of reference, is compatible not only with medium transformation, but also with Lorentz transformation and with theory of special relativity, so it is quite possible now to consider a realistic medium. Finally, a discrepancy is exposed within theory of special relativity, giving rise to assume a preferred frame of reference even in special relativity. 
Herbert Weiss