A multi-channel microwave radiometer uses reference averaging for calibration: Precision approaches that of a total power radiometer

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.