The Integrated Multi-satellitE Retrievals for the Global Precipitation Measurement (GPM) mission (IMERG) combines precipitation retrievals from all available low-orbit passive microwave (PMW) data, together with geosynchronous infrared (GEO-IR) providing baseline retrievals to compute merged global precipitation every half hour. The IMERG Early Run includes forward propagation of PMW data, while the Late Run also uses backward propagation. IMERG includes the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager/Sounder (SSMIS) constellation operated by the U.S. Department of Defense (DoD). DMSP satellites F16, F17, and F18 are currently operational, but the entire program is scheduled to be retired in September 2026. At the same time, the DoD’s new Weather System Follow-on – Microwave satellite 1 (WSF-M1) is being operationalized (currently planned for late September 2025), providing MicroWave Imager (MWI) data. Using December 2023 as a test case, this study examines the impact of eliminating the SSMIS constellation, and then using the F17 SSMIS to simulate the addition of MWI since F17 and WSF-M1 occupy nearly the same orbital slot.