AN AIRBORNE MILLIMETER-WAVE IMAGING RADIOMETER FOR CLOUD, PRECIPITATION, AND ATMOSPHERIC WATER-VAPOR STUDIES

A six-channel airborne total-power Millimeter-wave Imaging Radiometer (MIR) was recently built to provide measurements of atmospheric water vapor, clouds, and precipitation The instrument is a cross-track scann er that has a 3-dB beamwidth of 3.5 degrees and an angular swath of 10 0 degrees. It measures radiation at the frequencies of 89, 150, 183.3 +/- 1, 183.3 +/- 3, 183.3 +/- 7, and 220 GHz. The inclusion of the 220 -GHz receiver makes this instrument unique; no other instrument has ma de atmospheric radiation measurements using this combination of freque ncies. The temperature sensitivities Delta T, based on the actual flig ht data with a 6.8-ms integration time, are found to be 0.44, 0.44, 1. 31, 1.30, 1.02, and 1.07 K. The instrument has two external calibratio n loads maintained at the temperatures of 330 and 250 K (the ambient t emperature at an aircraft altitude of 20 Bm). These calibration load t emperatures are monitored precisely so that the radiometric measuremen ts of the instrument could be made to better than 1 K of accuracy in t he brightness temperature range of 240-300 K. Measurements made with a calibration target emmersed in liquid nitrogen indicate a measurement accuracy of 2-4 K for brightness temperatures below 100 K. The instru ment has flown successfully aboard the National Aeronautics and Space Administration (NASA) ER-2 aircraft for more than 130 h. This paper is an overview of the system design, calibration, and measurement capabi lities.