The Cassini solar conjunction experiment: a new test of general relativity

During the Cassini solar conjunction experiment, radio-metric data have been used to measure the space components of the sun´s metric tensor. In the parameterized post-newtonian formalism, this quantity is controlled by a parameter (γ) which is unity in General Relativity but may assume slightly different values in other metric theories of gravity. The measurement of γ with Cassini is based upon a new observable quantity: the frequency shift induced on a radio wave when the spacecraft is near solar conjunction. The calibration of the large frequency fluctuations induced by the solar corona, necessary in order to guarantee the required signal stability, is performed using the novel Cassini radio frequency system, based on a multifrequency link. In this paper the operations performed and the results obtained from the preliminary analysis of the June-July 2002 Cassini solar conjunction data are described. The multifrequency link plasma calibration scheme is also illustrated along with the techniques for the reduction of the non-dispersive, tropospheric effects. The high quality of the calibrated Doppler data indicates that the experiment provides a high accuracy test of general relativity.