# The impacts of source structure on geodetic parameters demonstrated by the radio source 3C371

## TL;DR

This paper introduces a new method to assess source structure effects on geodetic VLBI measurements using closure quantities, demonstrating how structure corrections improve the accuracy of station positions and Earth orientation parameters.

## Contribution

A novel structure index based on closure quantities is proposed, enabling structure effect correction directly from VLBI data without imaging, and applied to source 3C371.

## Key findings

- Structure correction reduces rms delay by 1 ps.
- Impacts on station positions up to 1.4 mm.
- Impacts on Earth orientation parameters up to 4.4 microarcseconds.

## Abstract

Closure quantities measured by very long baseline interferometry (VLBI) observations are independent of instrumental and propagation instabilities and antenna gain factors, but are sensitive to source structure. A new method is proposed to calculate a structure index based on the median values of closure quantities rather than the brightness distribution of a source. The results are comparable to structure indices based on imaging observations at other epochs and demonstrate the flexibility of deriving structure indices from exactly the same observations as used for geodetic analysis and without imaging analysis. A three-component model for the structure of source 3C371 is developed by model-fitting closure phases. It provides a real case of tracing how the structure effect identified by closure phases in the same observations as the delay observables affects the geodetic analysis, and investigating which geodetic parameters are corrupted to what extent by the structure effect. Using the resulting structure correction based on the three-component model of source 3C371, two solutions, with and without correcting the structure effect, are made. With corrections, the overall rms of this source is reduced by 1 ps, and the impacts of the structure effect introduced by this single source are up to 1.4 mm on station positions and up to 4.4 microarcseconds on Earth orientation parameters. This study is considered as a starting point for handling the source structure effect on geodetic VLBI from geodetic sessions themselves.