The Media Calibration System for Cassini Radio Science: Part III

Abstract

A new media calibration system has been implemented at DSS 25. It is designed to calibrate the delay of radio signals imposed by the neutral atmosphere. The system provides periodic measurements, with intervals greater than 2 s, of both the static dry and fluctuating wet components of this delay. In particular, a water vapor radiometer calibrates the fluctuations in path delay due to atmospheric water vapor that we believe will dominate the error budget for several radio science and radio astronomy experiments. Previous articles described the instrumentation, observing strategy, data analysis procedures, and weather conditions for 24 interferometric experiments that started in August 1999 and ended in May 2000. In this article, we include an additional nine experiments that were carried out after an instrumental improvement and summarize our results. The evaluation of our data indicates that the new media calibration system likely meets the two-way Allan standard deviation specification of 1.5 x 10^(-15) on time scales of 2000 to 10,000 s, a requirement of the Cassini Gravitational Wave Experiment for two-way Doppler tracking. This conclusion was based on 11 of our experiments. In the rest of the reported experiments, the performance evaluation of the water vapor radiometer appears to be limited by a systematic error in the interferometer that is under investigation.