This Flag effectively sets the refractive index
(calculated in refrac.for,
refrac_fnc.f90 in RFM v5)
to unity, although in practice
there are switches in the ray-tracing code for the non-refractive case.
Incompatible with viewing geometries that don't involve
a spherical atmosphere.
This Flag should not be confused with the *GEO alternative
to the *TAN section which specifies
tangent paths by their projected geometric tangent points, but for
which refraction is still considered.