This article presents the derivation of governing equations for the propagating wave under the influence of magneto-ionic plasma (i.e. Earth's Ionosphere), ready to be used in Finite Difference Time Domain algorithm. The governing equations and constituent relations are derived from the magnetoionic theory given by Appleton-Hartree, discussed by Ratcliffe and Budden. The frequency domain relations, are then converted to time domain suitable for inclusion in the time stepping algorithm used in FDTD method developed by Taflove. Although the intention is to achieve a Full Wave analysis, for waveform attenuation, group velocity, and wavefront direction, but due to limited scope of this paper, only the derivations are detailed. Plane wave formulation is discussed, leaving the spherical wave formulation for later publication.