  This article is devoted to results on pion-pion -Bose-Einstein correlations
in central Pb+Pb collisions measured by the NA49 experiment at the CERN SPS.
Rapidity as well as transverse momentum dependences of the correlation lengths
will be shown for collisions at 20A, 30A, 40A, 80A, and 158A GeV beam energy.
Only a weak energy dependence of the radii is observed at SPS energies. The
kt-dependence of the correlation lengths as well as the single particle
mt-spectra will be compared to model calculations. The rapidity dependence is
analysed in a range of 2.5 units of rapidity starting at the center of mass
rapidity at each beam energy. The correlation lengths measured in the
longitudinally comoving system show only a weak dependence on rapidity.
