The Conical Scanning Millimeter-wave Imaging Radiometer (CoSMIR) instrument is an airborne, 9-channel total power radiometer originally developed to calibrate and validate the Special Sensor Microwave/Imager/Sounder (SSMIS) for the Defense Meteorological Satellite Project (DMSP) F-series satellites. CoSMIR has four receivers near 50, 91, 150, and 183 GHz which measure horizontally polarized radiation with vertically polarized measurement capability at 89.0 and 165.5 GHz. CoSMIR was flown for calibration/validation of the SSMIS in 2004-2005 off coastal California. Currently, CoSMIR has been modified to play the role as an airborne high-frequency simulator for the GPM Microwave Imager (GMI). The 9 channels of CoSMIR for GMI validation are at the frequencies of 50.3, 52.6, 89 (H & V), 165.5 (H & V), 183.3±1, 183.3±3, and 183.3±7 GHz. All channels besides 89 and 165.5 GHz will be horizontally polarized.