The effect of the motion of the Sun on the light-time in interplanetary relativistic experiments

TL;DR

This paper examines how the Sun's motion affects light-time calculations in interplanetary experiments, finding the correction negligible compared to experimental accuracy and clarifying discrepancies with previous claims.

Contribution

It provides a detailed analysis of the Sun's motion correction in relativistic light-time measurements and refutes larger corrections claimed in prior work.

Findings

The Sun's motion correction is negligible at current measurement precision.

The analysis clarifies the discrepancy with previous claims of larger corrections.

The study confirms the validity of existing relativistic models in interplanetary experiments.

Abstract

In 2002 a measurement of the effect of solar gravity upon the phase of coherent microwave beams passing near the Sun has been carried out with the Cassini mission, allowing a very accurate measurement of the PPN parameter $\gamma$. The data have been analyzed with NASA's Orbit Determination Program (ODP) in the Barycentric Celestial Reference System, in which the Sun moves around the centre of mass of the solar system with a velocity $v_\odot$ of about 10 m/sec; the question arises, what correction this implies for the predicted phase shift. After a review of the way the ODP works, we set the problem in the framework of Lorentz (and Galilean) transformations and evaluate the correction; it is several orders of magnitude below our experimental accuracy. We also discuss a recent paper \cite{kopeikin07}, which claims wrong and much larger corrections, and clarify the reasons for the discrepancy.