Estimation of the propagation delay through the troposphere from microwave radiometer data

Uncertainty in the estimate of the microwave propagation delay through the troposphere is a principal limiting factor to the accuracy of measurements made with very long baseline interferometry. This uncertainty is due primarily to tropospheric water vapor, the total amount and vertical distribution of which are variable. Because water vapor both delays and attenuates microwave signals, the propagation delay, or wet path length, can be estimated from the microwave brightness temperature near the 22.235‐GHz transition of water vapor. We analyzed the data from a total of 240 radiosonde launches taken simultaneously at Chatham, Massachusetts, Albany, New York, and Portland, Maine, in 1974. Estimates of brightness temperature at 19 and 22 GHz and wet path length were made from these data. The wet path length in the zenith direction could be estimated from the surface water vapor density to an accuracy of 5 cm for the summer data and 2 cm for winter data. Using the brightness temperatures, the zenith wet path length could be estimated to an accuracy of 0.3 cm. Two dual‐frequency radiometers constructed by the National Radio Astronomy Observatory (NRAO) were rebuilt and used to determine how accurately the path length can be predicted from real radiometry data. These radiometers were capable of measuring the difference in the brightness temperatures at 30° elevation angle and at the zenith to an accuracy of about 1°K. In August 1975, 45 radiosondes were launched from Haystack Observatory over an 11‐day period. Brightness temperature measurements were made simultaneously at 19 and 22 GHz with the NRAO radiometers. The rms error for the estimation of wet path length from surface meteorological parameters was 3.2 cm, while from the radiometer brightness temperatures, the rms error was 1.5 cm.