Abstract-Millimeter-wave radiometric measurements obtained from the NASA ER-2 aircraft over the arctic region on May 20, 1998, were used to estimate precipitable water (PW) in the range 0.60 g/cm 2 . The approach is a modified version of the recent work by Miao [1], which utilized the radiometric measurements at 150, 183.3 3, and 183.3 7 GHz of the SSM/T-2 sensor to retrieve PW over the antarctic region. However, Miao has implicitly assumed a surface emissivity that is frequency independent over the 150-183 GHz range. This assumption turns out not to be a good one based on the airborne measurements described below, and the errors introduced in the PW estimation were substantial in many cases. It is shown below that four-frequency radiometric measurements in the frequency range of 150-220 GHz provided a robust retrieval of PW, while allowing for a surface emissivity that varied linearly with frequency. The retrieved PW compared favorably with that calculated from rawinsonde data at two widely separated locations. The differences between the retrieved and calculated values are not more than 0.02 g/cm 2 , which is smaller than errors associated with measurement uncertainty. It is found necessary to account for the double side-band nature of the 183.3 GHz measurements in the raditive transfer calculations for development of the retrieval algorithm. The PW values estimated from the algorithm developed from single side band, 183.3 GHz radiative transfer calculations could be in error by as much as 0.10 g/cm 2 . Finally, the effect of surface temperature variations is shown to introduce only a small error in the estimation of PW.
Abstract-The NASA Earth Science System Pathfinder (ESSP) mission Aquarius, will measure global ocean surface salinity with -120 km spatial resolution every 7-days with an average monthly salinity accuracy of 0.2 psu (parts per thousand) [l]. This requires an L-band low-noise radiometer with the long-term calibration stability of 10.15 K over 7 days. The instrument utilizes a push-broom configuration which makes it impractical to use a traditional warm load and cold plate in front of the feedhorns. Therefore, to achieve the necessary performance Aquarius utilizes a Dicke radiometer with noise injection to perform a warm -hot calibration. The radiometer sequence between antenna, Dicke load, and noise diode has been optimized to maximize antenna observations and therefore minimize NEDT. This is possible due the ability to thermally control the radiometer electronics and front-end components to 0.1 'Crms over 7 days.
A seven channel microwave radiometer has recently been developed for the Department of Energy Atmospheric Radiation Measurement Unmanned Aerospace Vehicle program. The instrument is designed to measure precipitable water vapor and cloud liquid water from board a DHC-6 (Twin Otter), chase plane for a remotely piloted vehicle. Five of the seven channels are centered about the 22.235 GHz water vapor absorption line. The other two channels are centered about 36.5 GHz and 89.0 GHz. Four calibration references exist for each channel within the radiometer. A digital computer is used to synchronize the switching of the circulators to the sampling of the analog-to-digital converters, thus, the timing of the data acquisition is programmable. By shortening the duty cycle of the calibration sequence and averaging the reference samples over an extended period of time radiometric precision approaching that of a total power radiometer is achieved. The channel selection along with its novel calibration scheme make this instrument a unique research tool.
Abstract-An airborne Conical Scanning Millimeter-waveImaging Radiometer (CoSMIR) for high-altitude observations from the NASA ER-2 is discussed. The primary application of the CoSMIR is water vapor profile remote sensing. Four radiometers operating at 50 (3 channels), 92, 150, and 183 (3 channels) GHz provide spectral coverage identical to nine of the SSMIS high-frequency channels. Constant polarization-basis conical and cross-track scanning capabilities are achieved using an elevation-under-azimuth two-axis gimbals.
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