Atmospheric calibration for precision Doppler tracking of spacecraft

The signals from the Cassini spacecraft that will be used for radio science experiments will be affected by delay fluctuations in the Earths atmosphere. These fluctuations are dominated by water vapor in the troposphere, and in the case of the gravitational wave experiment (GWE), they are likely to be a limiting error source. A passive remote sensing system, centered around a water vapor radiometer (WVR), has been developed to provide calibrations of water vapor fluctuations during radio science experiments. During the past two years, most of the technical challenges involved in the design of this instrument have been overcome and we are ready to begin implementation. We will discuss the performance that has been demonstrated with the current generation of WVR instrumentation and the general design of the package that will be installed at the Goldstone tracking site. In addition, recent results obtained for a comparison of co-located WVRs and global positioning system (GPS) receivers will be presented. The possibility of using these results to estimate gradients and⧹or fluctuations in the dry atmosphere will be discussed.