Retrieval of atmospheric water vapour using a ground-based single-channel microwave radiometer

Estimates of the amount of atmospheric water vapour derived from algorithms for a ground-based single-channel (21.0 GHz) microwave radiometer have been investigated. Ten datasets covering 44 days were used to derive the methods and two other sets (in total 32 days) were used to assess the quality of these. It is shown how the rms estimation error can be reduced by recognizing the rapid variations in sky brightness temperatures during periods when cloud liquid is present. Data was either discarded, guided by the variability, or an adaptive Kalman filter was applied with different parameter values for different degrees of variability. The resulting estimates were compared to the estimates obtained from a dual-channel algorithm (21.0 and 31.4 GHz), which were used as reference. The amount of water vapour was represented as the ‘wet delay’, the excess radio path length due to the atmospheric water vapour. Applying the Kalman filter to the single-frequency estimates reduced the wet delay rms error from 20 mm to 9 and 14 mm for the two datasets. Further reduction of the rms error was achieved by the removal of data in periods with high variability; discarding about 40% of the data led to rms errors of 5 and 7 mm for the two datasets.     

A three-year study of radio wave propagating delays due to tropospheric water vapor

Measurements of the radio emission from the sky at 21.0 and 31.4 GHz have been made from May 1980 to April 1983 at the Onsala Space Observatory on the west coast of Sweden. A total of 483 such observations have been used to calculate the water vapor induced propagation delay of radiowaves penetrating the troposphere. A comparison with simultaneous radiosonde launches made at Gothenburg-Landvetter Airport (37 km away from Ousala) gives a root mean square (rms) difference of 1.3 cm in the zenith direction and 1.1 cm for a certain group of stable weather data. The zenith wet path delays are also calculated with three different models based on traditional meteorological observations at the surface. The lowest rms difference obtained by using a model and compare the results with radiosonde data at the same site was 2.1 cm. A third site on the Swedish west coast was compared with the other two by using old monthly means of radiosonde data together with the three models. The offsets between the models and the radiosonde/water vapor radiometer (WVR) data are dependent on the model, on the site, and on the time of the year.     

Correlations between slant wet delays measured by microwave radiometry

We present an analysis of the correlation between the atmospheric slant wet delays in different directions using data from a microwave radiometer. The correlations between wet delays observed in different directions using different temporal constraints are compared to a model derived from theories of turbulence. The agreement between the model and the radiometer data was good, and the average squared difference between zenith mapped slant wet delays could be predicted with an accuracy of 0.01-0.04 cm/sup 2/. Our analysis shows a large short-term variability which variance has a seasonal dependence of about /spl plusmn/26%, largely depending on the refractivity structure constant C/sub n/. We also demonstrate how the model can be used to characterize the stability of a microwave radiometer.     

Precision Geodesy Using the Mark-III Very-Long-Baseline Interferometer System

Very-long-baseline interferometry (VLBI) has been used to make precise measurements of the vector separation between widely separated antennas. The system for acquiring and processing VLBI data known as Mark-III is described. Tests of the system show it to have millimeter-level accuracy on short baselines; measurements of baselines longer than a few hundred kilometers suggest that accuracy is limited by the uncertainty in the calibration of tropospheric path delay to the level of a few centimeters. VLBI experiments conducted between 1976 and 1983 have demonstrated the stability of the North American plate by showing that there is no change in the distance between easternl-California and Massachusetts at the level of a few millimeters per year or greater. Experiments made from 1980 to 1984 indicate that the distance from Massachusetts to Sweden is increasing by 1.7 ± 1 cm/year where the quoted standard deviation includes the estimated effects of systematic atic errors     

Tropospheric wet path-delay measurements

A dual-channel microwave radiometer measuring the sky brightness temperature at the frequencies 21.0 and 31.4 GHz, an infrared spectral hygrometer (IRSH) measuring the ratio of the radiation from the sun at the wavelengths 931 and 880 nm, and radiosondes have been used simultaneously to determine the excess path length due to water vapor (wet path delay) of radio waves propagating through the troposphere. By a least squares fit of the measured parameters from the microwave radiometer and the infrared spectral hygrometer, respectively, to the wet path delay calculated from the radiosonde profiles, the following root mean square (rms) differences of the wet path delay in the zenith direction were obtained: infrared spectral hygrometer-radiosondes, 1.1 cm; microwave radiometer-radiosondes, 0.7 cm; and 0.5 cm for a selected group of "good weather" data. The wet path delay was also calculated from surface meteorological measurements alone and the rms difference compared with corresponding radiosonde data was 2.0 cm in the zenith direction.