  In the infrared part of the electromagnetic spectrum, the relativistic
dispersion theory is applied to the SS433 jets, to describe its dispersion
properties. In particular, we here investigate the dispersion Doppler effect of
the radiation in the relativistic cold plasma approximation for this
micro-quasar.
  We find that the Doppler plasma line displacements in SS433 are affected by
the plasma dispersion only in a narrow frequency band; in the far infrared
(far-IR) one. As a consequence, although the shift $Z$ modulation due to the
precession of the SS433 jets is well described by previous works in the optical
range, it has to be corrected by plasma dispersion effects in the far-IR range.
