A Demonstration of Precise Calibration of Tropospheric Delay Fluctuations With Water Vapor Radiometers," Geophys

An advanced water vapor radiometer (WVR), currently under development in the DSN Technology Program, will serve as the critical remote sensing instrument in the troposphere calibration subsystem required by the Cassini radio science ex-periments. The ability of WVRs to calibrate changes in tropospheric delay was demonstrated during very long baseline interferometry (VLBI) observations on the 21-km baseline between DSS 13 and DSS 15 at Goldstone, California. WVR mea-surements reduced the observed VLBI post-flt delay residuals over a 13-h period from 43.8 ps to 16.9 ps, a factor of …2.5. When applied to shorter time intervals, an …50-percent reduction in the Allan standard deviation of the site-difierenced residual delay on 100- to 700-s time scales was achieved during conditions of high tropospheric activity. Thermal WVR noise precluded calibration of delay °uctua-tions on shorter time scales and during quiet tropospheric conditions. I