A Dual-Channel Microwave Radiometer for Measurement of Precipitable Water Vapor and Liquid

The design and performance of a two-channel ground-based microwave radiometer (20.6 and 31.6 GHz) for measurement of total integrated water vapor and cloud liquid in a vertical column are discussed. Unique features of this instrument are a single antenna producing equal beamwidth for the two frequencies, and incorporation of two stable reference loads in a three-way Dicke switching sequence. The latter allows an automatic gain control (AGC) to be applied in a minicomputer. Linear statistical inversion is used to retrieve the precipitable water vapor and liquid quantities which are provided in real time. The instrument has been in reliable operation, continuously and unattended, at the National Weather Service Forecast Office, Denver, CO, for a six-month period. The radiometrically derived precipitable water vapor compares favorably with that obtained from radiosondes.