Atmospheric transmittance measurements near 54 and 118 GHz

A zenith-scanning airborne imaging spectrometer, the Millimeter-wave Temperature Sounder (MTS), incorporating eight double-sideband channels centered around the 118.75-GHz oxygen resonance, was employed to measure atmospheric transmittances to space. Simultaneously, a zenith-viewing computer-controlled frequency-stepping spectrometer scanned roughly 3 GHz centered near 53.5 GHz. The observations were obtained continuously as a NASA ER-2 aircraft ascended to -20 km altitude and descended on multiple flights. Nearby radiosondes and air temperature and navigational data from the aircraft flight computer permit comparison of the radiometric data with theoretical transmittance expressions. Sensitivity of the measurements to aircraft altitude and attitude and to temperature and humidity profiles necessitates careful scrutiny of these data. The views of atmospheric radiance against the cosmic background provide sensitivity to conditions in the vicinity of the tropopause. Accurate calibration of the measurements through ascent and descent provides considerable challenges