  Interferometry is the primary density control diagnostic for large-scale
fusion devices, including ITER and DEMO. In this paper we present a ray tracing
simulation based on TRAVIS accounting for relativistic effects. The study shows
that measurements will over-estimate the plasma density by as much as 20
degree. In addition, we present a measurement geometry, which will enable
vertical position control during the plasma's ramp-up phase when
gap-reflectometers and neutron cameras are still blind.
