# Observations of radio sources near the Sun

## TL;DR

This paper discusses VLBI observations of radio sources near the Sun to measure gravitational light deflection and test general relativity, achieving high-precision estimates of the PPN parameter gamma.

## Contribution

It presents new VLBI measurement techniques and results that improve the accuracy of gamma estimation near the Sun, confirming general relativity predictions.

## Key findings

- Achieved formal accuracy of gamma less than 0.01% in single sessions
- Demonstrated high-precision measurements of light deflection near the Sun
- Plan to conduct further experiments with improved conditions from 2020

## Abstract

Geodetic Very Long Baseline Interferometry (VLBI) data are capable of measuring the light deflection caused by the gravitational field of the Sun and large planets with high accuracy. The parameter $\gamma$ of the parametrized Post-Newtonian (PPN) formalism estimated using observations of reference radio sources near the Sun should be equal to unity in the general relativity. We have run several VLBI experiments tracking reference radio sources from 1 to 3 degrees from the Sun. The best formal accuracy of the parameter $\gamma$ achieved in the single-session mode is less than 0.01 percent, or better than the formal accuracy obtained with a global solution included all available observations at arbitrary elongation from the Sun. We are planning more experiments starting from 2020 using better observing conditions near the minimum of the Solar activity cycle.