Doppler measurement of the solar gravitational deflection

Testing alternative metric theories of gravity with an accuracy much better than the present level has recently drawn great attention, in particular i n relation to the search for a very weak scalar field, a possible remnant o f an early inflationary cosmology. The gravitational deflection of electrom agnetic waves is controlled by the dimensionless post-Newtonian parameter g amma, which takes a value of unity in general relativity. In this work we c laim that the accuracy in the measurement of gamma can be substantially imp roved by measuring the Doppler frequency shift of a microwave beam transpon ded back to the ground by an interplanetary spacecraft near solar conjuncti on. In this kind of experiment, the dispersion due to the plasma in the sol ar corona is the crucial difficulty, which, however, can be essentially ove rcome using skilful combinations of carriers with different frequencies. Th e spacecraft Cassini; launched in 1997, adopts a sophisticated radio system , including a Ka-band link at 32-34 GHz, which makes this possible. We disc uss the noise budget for two experiments to be carried out with Cassini in 2002 and 2003. In particular, we consider the contribution of the solar cor ona, the non-gravitational accelerations, and thermal noise due to solar ra dio emission. We estimate that an accuracy in gamma of about 10(-5) is achi evable.