The barycentric motion of the Sun and the test of general relativity using radio links with the Cassini spacecraft
The most precise test of general relativity in the solar system has been achieved in measurement of the frequency shift of radio waves to and from the Cassini spacecraft as they passed near the Sun. The relativistic model that was used to fit the Cassini data is based on an assumption that the Sun is static but, in fact, it moves around the barycenter of the solar system. We analyze the effect of this motion on the propagation of a radio wave passing closely to the solar limb and demonstrate that it contributes in a systematic way to the measured value of the PPN parameter γ characterizing deviation of gravity from a pure geometry. This our observation should stimulate re-processing of the relativistic Cassini experiment since the numerical value of γ = 1 + (2.1 ± 2.3) × 10 obtained by Bertotti, Iess & Tortora (2003) is affected by the barycentric motion of the Sun bringing about an uncertainty in γ − 1 amounting to ∼ 10. Moreover, we emphasize that by taking into account the motion of the Sun with respect to the barycenter of the Solar System we actually measure the gravimagnetic component of the gravitational field on the Sun caused by its orbital motion.