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The National Polar-orbiting Operational Environmental Satellite System (NPOESS) Aircraft Sounder Testbed-Microwave (NAST-M) includes spectrometers operating near the oxygen lines at 50-57, 118.75, and 424.76 GHz, and a spectrometer centered on the water vapor absorption line at 183.31 GHz. All four of the spectrometers' antenna horns are collocated, have 3-dB (full-width at half-maximum) beamwidths of 7.5°, and are directed at a single mirror that scans cross-track beneath the aircraft with a swath up to 100-km wide. The first part of the paper describes the instrumentation and calibration for the newly installed spectrometers at 183.31 and 424.76 GHz. The second part demonstrates the potential performance of NAST-M, by presenting radiance images and precipitation rate and cell-top retrievals obtained during overflights of isolated convective storm cells, and by comparing these results with coincident visible images. NAST-M radiances are also compared with visible, infrared, and radar images. The nonlinear retrieval method was trained with a simple precipitation model. The data were obtained during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE 2002) and the Pacific THORpex (THe Observing-system Research and predictability experiment) Observing System Test (PTOST 2003).