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The National Polar-orbiting Operational Environmental Satellite System (NPOESS) Aircraft Sounder Testbed-Microwave, or NAST-M , has added two spectrometers to its original suite of spectrometers operating near the oxygen lines at 50-57 GHz and the oxygen line at 118.75 GHz. The NAST-M suite now includes a spectrometer that is centered on the water vapor absorption line at 183.31 GHz, and another that is centered on the oxygen absorption line at 424.76 GHz. This addition has increases the ability of NAST-M to retrieve humidity profiles and to sense smaller-diameter hydrometeors. All four of the spectrometers' horns are collocated, have 3-dB (full-width at half-max) points of 7.5°, and are directed at a single scanning mirror, which produces a cross-track scan beneath the aircraft with a swath width of approximately 100 km. Aircraft carrying NAST-M cruise at altitudes of 17 to 20 km, yielding a nadir surface spatial resolution of ∼ 2.6 km.