Test and evaluation of water vapor radiometers and determination of their capability to measure tropospheric propagation path length

Two dual frequency microwave radiometers were refurbished and placed in operation on steerable search light mounts. A total of 330 radiosonde profiles were examined. The wet path length had a mean value of 17 cm with an rms deviation of 7 cm in the summer and a mean value of 5 cm with an rms deviation of 2 cm in the winter. Using surface meteorology or radiosonde profiles at remote sites the wet path could be estimated to an accuracy of 5 cm in the summer and 2 cm in the winter. The estimate from brightness temperature at 19 and 22 GHz was better than 0.3 cm. The brightness temperatures measured with the radiometers were compared with the brightness temperatures calculated from radiosonde profiles during an eleven day experiment in August 1975 during which 45 radiosondes were launched. The rms residual in the correlation between measured brightness temperature and the calculated wet path length in the zenith direction was 1.5 cm.