Radiowave Phase Scintillation and Precision Doppler Tracking of Spacecraft

Phase scintillation caused by propagation through irregularities in the solar wind, ionosphere, and troposphere, introduces noise in spacecraft radio science experiments. In precision Doppler tracking observations, scintillation can be the dominant noise source; statistics of this propagation noise are necessary for experiment planning and in design of optimum signal processing procedures. Here high-pwision tracking data taken with operational spacecraft (Mars Observer, Galileo, Mars Global Surveyor) and ground systems are used to produce temporal statistics of tropospheric and charged particle phase scintillation. Using the differing transfer functions to the Doppler observable, the variance of Doppler frequency fluctuations is approximately decomposed into two propagation processes. The first, associated with distributed scattering along the sight-line in the solar wind, has a smooth spectrum. The second, associated principally with localized tropospheric scattering for X-band experiments, has a marked autocormlation at the two-way light time between the earth and the spacecraft (thus a cosine-squated modulation of the fluctuation power spectrum). For the X-band data, taken in the antisolar hemisphere, the average noise levels of this process are in good agmment with average tropospheric noise levels determined independently from water vapor radiometer observations and radio interferometic data. The variance of the process having a smooth spectrum is consistent with charged-particle noise levels determined independently by dual-frequency observations of the Viking spacecraft made at comparable sun-earth-spacecraft angles. The observations reported here are used to refine the propagation noise model for Doppler tracking of deep space probes. In particular they can be used to predict propagation noise levels for high precision X-and Ka-band tracking observations (e.g. atmosphere/ionosphertAing occultations, celestial mechanics experiments, gravitational wave experiments) to be done using the Cassini spacecraft. ,.