Several pre-operational tests have been performed at NRL. Control runs consisting of the current operational system (3DVar + 3 AMSU + geostationary winds) set the baseline performance. In the first experiment, data from SSMIS channels sensitive to the tropospheric and stratospheric temperature are added to the operational systems, and evaluated after one month of assimilation. In the second month-long experiment, the data from NOAA-15 AMSU-A is removed from the system, and replaced by SSMIS. This scenario is a risk-reduction test, and will tell us whether SSMIS can substitute for a failed AMSU instrument. Performance of the SSMIS instrument may prove crucial to NWP efforts, given the gap between the expected end of life of the NOAA operational satellites and the launch of the National Polar-orbiting Operational Environmental Satellite System (NPOESS) and NPOESS Preparatory Project (NPP) satellite systems. Detailed descriptions and forecast impacts of these experiments will be presented.