Determination of the relativistic parameter gamma using very long baseline interferometry

TL;DR

This paper uses VLBI observations of radio wave deflections near the Sun to estimate the relativistic parameter gamma, finding it cannot be measured more precisely than 2×10⁻⁴ due to coordinate uncertainties and systematic errors.

Contribution

It provides a comprehensive analysis of VLBI data since 1979 to estimate gamma, highlighting the limitations imposed by coordinate uncertainties and systematic errors.

Findings

Gamma cannot be estimated better than 2×10⁻⁴

Main limitations are source coordinate uncertainties and systematic errors

Analysis strategies influence the estimation accuracy

Abstract

Relativistic bending in the vicinity of a massive body is characterized only by the post-Newtonian parameter $\gamma$ within the standard parameterized post-Newtonian formalism, which is unity in General Relativity. Aiming at estimating this parameter, we use very long baseline interferometry (VLBI) to measure the gravitational deflection of radio waves emitted by distant compact radio sources, by Solar System bodies. We analyze geodetic VLBI observations recorded since 1979. We compare estimates of $\gamma$ and errors obtained using various analysis schemes including global estimations over several time spans and with various Sun elongation cut-off angles, and analysis of radio source coordinate time series. We arrive at the conclusion that the relativistic parameter $\gamma$ cannot be estimated at better than $2\times10^{-4}$. The main factor of limitation is the uncertainty in the determination of (global or session-wise) radio source coordinates. A sum of various instrumental and modeling errors and analysis strategy defects, that cannot be decorrelated and corrected yet, is at the origin of the limitating noise.