The feasibility of a microwave vertical sensor depends on the uniformity of the radiometric temperature of the earth's O2 mantle at some operating frequency in the 60-GHz(5-mm wavelength) range. The essential properties of passive vertical sensors are examined. The characteristics of the 5 mm O2-band emission from the earth's atmosphere are analyzed. The potential accuracy of a 60 GHz O2-band vertical sensor is determined, considering both instrumental and O2 mantle uniformity limitations. To take the data required to evaluate the O2 mantle uniformity, a downward-looking radiometric instrument operating at 60.80 GHz was designed and built by Ewen-Knight Corporation. The design of the instrument is analyzed from the atmospheric physics and the instrument engineering viewpoints. The instrument, designated Experiment CRLF 739, was accepted for flight aboard the OAR satellite OV1-86; the details of interfacing the instrument with OV1-86 are discussed. (Author).