Applying the theory of general relativity to reducing geodetic VLBI data

TL;DR

This paper introduces an alternative gravitational time delay formula for geodetic VLBI data reduction, accounting for solar system effects and testing general relativity with high precision.

Contribution

It derives a new Taylor series-based formula for gravitational delay, linking it to light deflection and improving data reduction accuracy in VLBI.

Findings

Difference in arrival times within 1 ps at 1° from the Sun

Standard VLBI reduction is equivalent to source position displacement

New formula matches conventional results with high precision

Abstract

We present an alternate formula for calculating gravitational time delay. We use this formula to reduce geodetic Very Long Baseline Interferometry (VLBI) data, taking into account gravitational effects within the solar system, and to test general relativity. The alternate formula was obtained by expanding the conventional formula in a Taylor series. We show that the gravitational delay can be split into several terms including a term due to the coordinate transformation and terms that are explicitly linked to the light deflection angle. Our formula is compared numerically with the conventional formula, and difference in arrival times within 1 ps are found at 1$^\circ$ from the Sun for a full range of baseline lengths. We conclude that the standard reduction of geodetic VLBI data for the effects of general relativity is equivalent to displacing the reference radio sources from their original catalogue positions in accordance with the classical light deflection formula across the whole sky.