  Based on the UrQMD transport model, the transverse momentum and the rapidity
dependence of the Hanbury-Brown-Twiss (HBT) radii $R_L$, $R_O$, $R_S$ as well
as the cross term $R_{OL}$ at SPS energies are investigated and compared with
the experimental NA49 and CERES data. The rapidity dependence of the $R_L$,
$R_O$, $R_S$ is weak while the $R_{OL}$ is significantly increased at large
rapidities and small transverse momenta. The HBT "life-time" issue (the
phenomenon that the calculated $\sqrt{R_O^{2}-R_S^{2}}$ value is larger than
the correspondingly extracted experimental data) is also present at SPS
energies.
