  Hanbury-Brown Twiss (HBT) correlation measurements provide valuable
information about the phase space distribution of matter in ultrarelativistic
heavy-ion collisions. The rapidity dependence of HBT radii arises from a
nontrivial interplay between longitudinal and transverse expansion and the time
dependence of the freeze-out pattern. For a non-accelerating longitudinal
expansion the dependence primarily arises from the amount of radiating matter
per unit rapidity $dN/d\eta$, but for a scenario with strong longitudinal
acceleration additional complications occur. In this paper I explore
schematically what type of dependence can be expected for RHIC conditions under
different model assumptions for the dynamics of spacetime expansion and
freeze-out.
