The National Polar-orbiting Operational Environmental Satellite System (NPOESS) Atmospheric Sounding Testbed - Microwave Temperature Sounder (NAST-MTS) contains two microwave radiometer systems covering the spectral ranges of 50-56 GHz with eight single-sideband channels, and 115-123 Ghz with nine double-sideband channels centered on the 118.75 Ghz oxygen line. Both radiometers scan +/- 65 degrees from nadir and also view two black-body targets and zenith for calibration. The temperature weighting functions cover altitudes from the surface to ER-2 altitude. A video camera in the MTS package provides ancillary digitized stereo images for cloud location.